TECHNICAL REPORT S-74-1

AN EVALUATION OF THE. THERMOCOUPLE PSYCHROMETRIC TECHNIQUE FOR THE

MEASUREMENT OF SUCTION IN CLAY SOILS by

L. D. Johnson

January 1974

Sponsored by Office, Chief oF Engineers, U. S. Army

Conducted by U. S. Army Engineer Waterways Experiment: St:at:ion

Soils and Pavements Laboratory

Vicksburg, Mississippi

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED

THE CONTENTS OF THIS REPORT ARE NOT TO BE

USED FDR ADVERTISING, PUBLICATION, OR

PROMOTIONAL PURPOSES. CITATION OF TRADE

NAMES DOES NOT CONSTITUTE AN OFFICIAL EN­DORSEMENT OR APPROVAL OF THE USE OF SUCH

COMMERCIAL PRODUCTS.

iii

FOREWORD

The work reported herein is part of an overall investigation of

the properties of expansive clay soils sponsored by the Office, Chief

of Engineers, Directorate of Military Construction. The investigation

of thermocouple psychrometers for the measurement of soil suction was

begun in 1967 under the Operations and Maintenance Program, U. S. Army.

These studies were continued in 1971-1972 under the Permanent Construe-

tion Materials and Techniques Program. The work was performed and this

report was prepared by Mr. Lawrence D. Johnson, under the general direc-

t'ion of Messrs. J. P. Sale and R. G. Ahlvin, Chief and Assistant Chief,

respectively, of the Soils and Pavements Laboratory, U. S. Army Engi-

neer Waterways Experiment Station.

Directors of WES during the conduct of this study and the prepara-

tion of this report were COL Levi A. Brown, CE; BG Ernest D. Peixotto,

CE; and COL G . H. Hilt, CE. Technical Director was Mr. F. R. Brown.

v

CONTENTS

FOREWORD

LIST OF FIGURES AND TABLES

NOTATION

SUMMARY • •

CHAPTER I INTRODUCTION •

1 . 1 Objective of the Study • • • • • . • • • • • • • • • • • 1 . 2 Concept of Suction • •

1 . 2 . 1 Matrix suction • . 1 . 2 .2 Osmotic suction • • • • •

1 . 3 Evaluation of Suction by Thermocouple Psychrometers 1 . 3 . 1 Calibration properties • • • • • • • • • • • • • 1. 3 • 2 Range • . • • • • • • 1 . 3 . 3 Temperature Control 1 . 3 . 4 Applications • • • • • •

CHAPTER II EQUIPMENT AND PROCEDURES

2 . 1 Monitoring System • • • • • 2 .2 Temperature Control System • • 2 . 3 Psychrometers • • • • • • • 2 . 4 Laboratory Testing Procedures

CHAPTER III TESTS AND ANALYSIS

3 . 1 Calibration Properties • • 3 . 1 . 1 Calibration solutions • • • • 3 . 1 . 2 Optimum current • • • • • • • • 3 . 1 . 3 Volt age output with time • • • • • 3 . 1 . 4 Voltage output and suction • 3.1.5 Temperature errects 3 . 1 . 6 Reproducibility 3 . 1 . 1 Range • • • • • . • •

3 . 2 Laboratory Measurements of Soil Suction

CHAPTER IV CONCLUSIONS AND RECOMMENDATIONS •

LITERATURE CITED

vii

. . . . . .

v

ix

xi

xv

1

1 2 4 6 8

10 18 20 27

30

30 30 33 39

42

42 42 42 44 46 46-50 52 52

61

FIGURE

1-1

1-2

1-3

1-4

1-5

2-1

2-2

2-3

3-1

3-2

3-3

3-4

3-5

3-6

3-7

3-8

LIST OF FIGURES AND TABLES

Theoretical Calibration of Various Size Thermocouples at 25 C • • • • • • • • • • • • • • • • • • • . • • • • •

Sensitivity of Chromel-Constantan Thermocouples for 1- and 2-Mil-Diam at T = 0 • • • • • • • • . •

Optimum Cooling Current as a Function of the Length of Thermocouple Wire for Various Diameters (A�er Re ference 30) • • • • • • • • • • • • . • • • • • • •

Maximum Temperature Depres sion (Range) of Thermocouple s at 25 C for Various Diameters of Constantan Wire

Sketch of Thermocouple Psychrometer that is Useful Without Precise Temperature Control • • • • • •

Electrical Circuit for the Thermocouple Psychrometer

Monitoring System • • • • • • • •

Laboratory and Field Thermocouple Psychrometers •

Voltage Output with Time for Psychrometer No . 5 at 25 C • • • • • • • • • • • • • • • • • • • •

Calibration of 1-mil-diam Thermocouple Psychrometers at 25 C • • • • • • • • • • • • • • • • • • • • • • • •

Calibration of 2-mil-diam Thermocouple Psychrometers at 25 C • • • • • • • • • • • • • • • • • • • • •

Calibration of 4- and 10-mil-diam Thermocouple Psychrometers at 25- C • • • • • • • • • • . _ . _ . _ . _ . _

Temperature Characteristics of Thermocouple Psychrometers • • • • • • • • • • • • • • • • • • .

Calibration of P-Psychrometers, Open Chamber, 2-mil­diam Wire at 25 C • • • • • • • • • • • • •

Range of Calibration of Field Psychrometers at 25 C • •

Suction-Water Content Relationship of Weathered Upper Midway Soil on Drying at 25 C • • • • • . • • • • • • •

ix

12

15

21

22

25

31

34

36

45

47

48

51

5 3

54

FIGURE

3-9

TABLE

1-1

1-2

1-3

1-4

1-5

1-6

2-1

2-2

2-3

Suction Versus Water Content Relationship on Drying for Undisturbed Soil • • • • • • • • • • � • • • •

Description of Suction

Saturation of Soil (After Re ferences 20 ,22 ) •

Parameters for Theoretical Equations

Theoretical Accuracy of Chromel-Constantan Psychrometers • • • • • • • • • • • • • • •

Data for Calculating Optimum Current and Temperature Depression . . . . . . . . . . . . . . . . . . . . . . Criteria for Thermocouple Psychrometers Without Precise Temperature Control • • • • • • • • • •

Laboratory Psychrometers

Prototype P-Psychrometers

Type F Psychrometers

3-1 Concentration of Potassium Chloride for Certain Relative Humidities • •

3-2 Ranges of Psychrometers •

3-3 Propertie s of Boring Samples

x

59

3

7

14

17

23

26

37

38

40

43

55

57

A

A

a

B

cs T

NOTATION

2 Cross-sectional are a of wi re , cm

Complex para.meter

Circumference of wire , cm

Slope of at temperature T

Saturated concentration of water vapor at temperature T , g/cc

D D • ffu i t t f t . . 2; i s on cons an or wa er in air , cm s e c

E Volt age , mi crovolts

Et Voltage at temperature t , microvolt s

I

I 0

K w

L

p

p

Volt age at 25 C , microvolt s

2 Heat transport coe ffi cient , watts /cm -de g C

Current , coulomb s / s e c (ampere s )

Optimum c urrent , amperes

Thermal conductivity of wire one , calories -cm/sec-de g C

Thermal conduct ivity of wire two , c alories-cm/se c-deg C

Thermal conductivity of ai r , calorie s -cm/se c-deg C

Thermal conduct ivity of thermocouple wire , calorie s ­cm/se c-deg C

Latent heat of vaporization- for water-, calories/g-

Length of wire , cm

Pelt ier coe ffi cient , j oule s /coulomb (volt s )

Pres s ure of water vapor , atm

Vapor pre s sure of the pore water ext racted from the soil , atm

Pres sure of s aturat e d water vapor , atm

xi

Q

R

r c

r w

s

T

t

u a

u w

v w

a

e

µ

a

a

T m

Log10 of the negative pressure of water in centimeters at 20 c

LoglO of negative pressure of water in centimeters at temperature t

Rate of cooling, joules/sec

Universal gas constant, 82 .06 cc-atm/deg Kelvin-mole

Radius of chamber containing thermocouple junction, cm

Radius of thermocouple junction, cm

Radius of thermocouple wire, cm

Sensitivity, microvolts/atm

Absolute· temperature, deg Kelvin

Temperature, deg C

Void air pres sure, atm

Pore water pressure, atm

Volume of a mole of liquid water, 18 . 02 cc/mol

Compressibility factor

Tempe rature depres sion, deg C

Optimum temperature depression thermocouple wire one, deg C

Thermoelectric power, 60 . 5 microvolt s/deg C for chromel­constantan thermocouples at 25 C

-Electrical zpe-Cifi_c resistance of wire one� ohm-cm

Electrical specific resistance of wire two, ohm-cm

Total applied pressure, atm

6 -12 I 2 Stefan-Bolt zman constant, 1 . 3 •10 calories cm -deg Kelvin4

Total suction, atm

Matrix (soil water) suction, atm

xii

in situ T m 0 T m

T s

Matrix suction of in situ soil , atm

Matrix suction free of external pressure other than atmospheric pressure , atm

Osmotic ( solute ) suction , atm

xiii

SUMMARY

Suction or negative pore water pressure is important in control­

ling the physical properties of less than fully saturated soils.

Methods for the accurate measurement of a wide range of suctions in

such soils are not well known and are under development. This study

to evaluate thermocouple psychrometers as a technique for the measure­

ment of suction was initiated because these devices are relatively in­

expensive, simple to use, allow rapid measurements, and possess a

large range.

Apparatus was designed, constructed, and tested for the thermo­

couple psychrometric measurement of suction pressure in clay soils

without the need for precise temperature control. This report reviews

the background literature leading to the development of the apparatus,

describes the equipment, and outlines procedures for making psychro­

metric measurements.

A study of various types of thermocouple psychrometers showed that

psychrometers with 1- or 2-mil-diam chromel-constantan thermocouples

provide a range of total suction up to about 90 atm. The reproduci­

bility is about + l atm- or- less- for- suctions- less- tharr 50- atm- arra.- ab-out-

.:!:. 2 atm for suctions greater than 50 atm. Suction measurements of

Yazoo clay from Jackson, Mississippi, and clay from the weathered Upper

Midway formation, Lackland Air Force Base, Texas, support the defini­

tion that total suction is the sum of matrix and osmotic components.

xv

CHAPTER I

INTRODUCTION

1.1 Objective of the Study

Partly saturated clay soils, especially montmorillonite expansive

clays, possess an affinity for water that can lead to detrimental

changes in physical properties such as volume and strength. Differen-

tial heave of foundation soils from imbibition of moisture often leads

to considerable damages to overlying structures. The strength of

partly saturated foundation soil can also be reduced from absorption

of moisture or from increases in pore water pressure. Such changes in

soil moisture conditions o�en result from the construction of build-

ings, embankments, and other overlying structures.

A measure of the affinity or driving force for soil to retain

moisture can be given by the magnitude of the negative pressure or sue-

tion in the porewater (l ).* Experimental evidence supports the conclu-

sion that negative pore water pressures are instrumental in controlling

physical properties in less than fully saturated soils. Bearing

capacity (2,3) increases and the hydraulic conductivity (4,5) decreases

with increasing suction-. Both surface-- tens-ton- forc-e-s- and- soluble

salts in soil water influence distances between clay particles (6,7)

and interlayer expansions of the crystal lattice in montmorillonites

(8). The apparent significan_ce of negative pore water pressure in

soils has stimulated many programs to develop techniques to measure

* Numbers in parentheses refer to references listed in the LITERATURE CITED at the end of the text.

1

the se pressures (1,6). The thermocouple psychrometer is only one of

the devices that has evolved from these studies , but it has a poten­

tially large range and it i s relatively inexpens ive and simple to use.

Applications of thermocouple psychrometers are being extended to in­

clude in situ measurements (9 ,10) and the measurement of suct ion dur­

ing triaxial compression tests (11) .

This study was initiated to compare various procedures and designs

of thermocouple psychrometers use ful for suction measurements in soils

without the need for precise temperature control . Known concepts of

evaluating suction from psychrometric measurement of relative humidity

were reviewed and formed the basis for establishing equipment, tech­

niques , and many of the experiments . A simple and economical system of

measuring suction of undi sturbed soil· specimens in the laboratory was

developed . , The calibration properties of a variety of thermocouple

psychrometers were investigated to determine design parameters adequate

for the measurement of �uction with the suggested system . Some of

these devi ces are adaptible to in situ measurements .

1 . 2 Concept of Suction

The ability of soils to attract water and the driving force for

moisture flow can be expressed quantitatively from the laws of thermo­

dynamics . A fundamental me asure of the energy or condition of pore

water in soils is given as the free energy of the soil water relative

to free pure water . The free energy i s commonly converted to a total

suction de fined in table 1-1 and for isothermal conditions given by

(1):

2

Suction

Total

Osmotic (solute )

Matrix (soil water suction )

Mechani sm

Water fixation by polar adsorption

Surface tension

Osmotic imbibition

Table 1-1

Description of Suction

Definitions of Suction (1) Symbol Definition

'[

'[ s

'[ m

Negative gage pressure relative to ex­ternal gas pressure on soil water to whi ch a pool of pure water must be subj ected in order to be in equilib­rium through a semipermeable (perme­able to water molecule s only ) membrane with the soil water .

Negative gage pre ssure to which a pool of pure water must be subj ected in order to be in equilibrium through a semipermeable membrane with a pool containing a solution identic al in composition with the soil water .

Negative gage pressure relative to ex­ternal gas pressure on soil water to which a solution identic al in compo­sition with the soil water must be subj ected in order to be in equilib­rium through a porous permeable wall with the soil water .

Mechanisms of Suction (12 ,13 ) Description

Bipolar water mole cules are attracted to plate­like surfaces of clay particles because of negative electromagnetic charges concentrated in the parti cles .

Surface tension forces arise from an attempt of surface liquid molecules at the air-liquid in­terface- to- draw together becaus-e- of the- dif-­ference in interaction forces between surface and body molecules and produces a tension of the liquid divided by radius of curvature of the meniscus .

Pressure arises when solutions exist at differ­ent concentrations separated by a semiperme­able membrane . The semipermeable membrane in clays constitutes "the strongly held cations near the surface of the clay particle .

3

Where:

1" = - RT ln E.... v w

T = total suction, atm

R =universal gas constant, 82 . 06 cc - atm/deg Kelvin-mole

T = absolute temperature, deg Kelvin

v = volume of a mole of liqui d water, 18 . 02 cc/mole w

p/po = relative humidity

p = pres sure of water vapor

p0 = pressure of saturated water vapor

( 1-1)

The thermocouple psychrometer measures the total suction, which is the

sum of matrix and osmotic components . The effect of atmospheric pres-

sure variations i s ignored because these differences cause no percep-

tible change in suction (14 ) .

Some mechanisms that contribute to total suction are given in

table 1-1 . Contributions of mechanisms such as polar adsorption, sur-

face tension, and osmosis to the suction in pore water is not clear .

Water in free contact with air, for example, will cavitate when tension

in the pore water exceeds one atmosphere . Instruments capable of mea-

___s_u:ci.ng _suctions .exc..e.e.ding .one atmo_sphe_re .may be monitoring the nature

of water films within osmoti c and adsorptive fields of force from the

clay particle s (12 ) . These water films may be in a state of

compression.

1 . 2 . 1 Matrix suction . The matrix suction is related to negative

pore water pressure in soils by (15-17 ) :

4

Where :

l" = m

u = a

u = w

matrix suction

void air pressure

pore water pressure

l" = u m a u w (1-2 )

Void air pressure is set equal to zero for atmosphere pressure . Matrix

suction is usually determined from pressure membrane tests in which air

pressure is applied to balance the pore water pressure in the specimen .

The positive quantity of suction o�en makes this term more convenient

than negative pore water pressure .

The matrix suction in a soil under a load simulating the in situ

soil conditions can be determined from (2 ,18 ,19 ) :

Where :

in situ o l" = l" - aa m m

in situ -r = matrix suction of in situ soil m 0 l" = matrix suction free of external pressure m

a = compres sibility factor

a = total applied- pres.sure..

The compressibility factor a is the fraction of applied pressure

(1-3 )

which is effective in changing the pore water pressure . It is given by

multiplying the unit weight of water in g/cc by the slope of a curve

relating the reciprocal of the dry density in cc/g (specific total vol-

ume (20 ) ) to water content in percent of dry weight (21 ) . This factor

5

will be near zero for incompressible soils such as clean sands at low

degrees of saturation . The compressibility factor will be equal to one

for all fully saturated or quasi-saturated soils ; thus, the applied

pressure is effectively trans ferred to the pore water (2 ,18 ) . If the

applied pressure is caused by a gas, then thi s pressure i s transferred

to the pore water regardless of the degree of saturation and the com-

pressibility factor is also equal to one . Descriptive terms for di f-

ferent degrees of saturat ion and the corre sponding states of pore water

and pore air pressure are given in table 1-2 (20 ,22 ) .

1. 2.2 Osmotic suction. The osmoti c suction is determined by the

concentration of soluble salts in the soil suction and can be computed

by (1 ) :

Where:

T s (1-4 )

pe = vapor pressure of the pore water extracted from the soil

Soluble salts in pore water influence the di stance between clay par-

ticles ( 6 ,7) and interlayer spacings in the crystal lattice (8 ).

Increasing concentrations of soluble salts in the external water avail-

able to the soil decre ase both the amount of swell and the swell

pre ssure (23 ,24 ) . Experiments have also shown that the efficiency of

o'smot i c suction to move external water into a soil depends on the type

of dis solved salt s in the soil water (25 ) . Movement of moi sture such

as distilled water into a soil by osmosis is possible because of the

smaller free energy of water in the soil solution compare d to the free

energy of pure water .

6

Table 1-2

Saturation of Soil (A�er References 20 ,22)

Degree of Saturation

Descript ion Percent

Fully saturated 100

Quasi-saturated 100

Partially Less than saturated 100

Uns'aturated Les s than 100

Unsaturated Less than 100

Unsaturated Less than 100

7

Pore Water Pressure

(+z Oz - ) +, 0

+, 0

Pore Air Pressure Relat ive to Atmospheric Pressure

No air

No air

+

+

Air drained to atmosphere

Trapped air

1 .3 Evaluation of Suction by Thermocouple Psychrometers

The thermocouple psychrometer measures the relative humidity of

an air space . Relative humidit ies for practical applications with

soils are usually greater than 95 percent . When the relative humidity

of the air space is in equilibrium with the vapor pressure of the pore

water in a soil spe cimen or of an aqueous solution, the suction may be

evaluated by equation (1-1 ) or by equat ion (1-4 ) . The ambient temper­

ature T substituted into equations (1-1 ) or (1-4 ) only needs to be

known within 3 C to infer suction to the nearest percent (26) . The

relative humidity must be known very accurately, especially for humid­

ities exceeding 95 percent, because small errors in relative humidity

amount to large errors in the natural logarithm of the humidity .

Large errors in suction will follow from equations (1-1 ) or (1-4 ) .

The thermocouple psychrometer can accurately determine the di f­

ference in temperature between the dewpoint (wet bulb ) and ambient

(dry bulb ) temperature s . The relative humidity, and thus the suction,

is evaluated from the temperature di fference and the ambient tempera­

ture . One method of determining the tempe rature di fference with

thermocouple psychrometers is to deposit a drop of distilled water on

one (wet bulb ) of two thermocouple junctions . Evaporation of water

into.the atmosphere from the wet bulb cools thi s junction until the

vapor pressure of the free water on the wet bulb is lowered to that of

the ambient atmosphere . The maximum temperature depression will never

be obtained in practice due to he at trans fer from the surroundings to

the wet bulb . The voltage output developed between the two

8

thermocouple junctions is measured and can be calibrated in terms of

relative humidity ( 14 ,27 ,28) , or in terms of suction from equations

(1-1) or ( 1-4 ) . This method is not readily adapt able to in situ or

long-term measurements of relative humidity in soils because the water

drop cannot be easily replenished.

A technique of evaluating suction with thermocouple psychrometers

investigated in this study and originated by Spanner ( 29) is based on

the Peltier effect in which a single thermocouple junction is cooled

by causing a direct current to flow through the junction in the proper

direction (26 ,29-31) . A bead of water starts to condense on the ther­

mocouple junction (wet bulb ) when the temperature reaches the dew­

point, inhibiting further cooling below the dewpoint temperature .

A�er the cooling current is terminated, the bead of water on the wet

bulb junction begins to evaporate, but a difference in temperature from

the re ference junction ( dry bulb ) will be maint ained until the bead has

evaporated. The temperature difference causes a volt age of magnitude

on the order of microvolts determined from the sum of Peltier and

Thomson effects ( 32) . The Thomson voltage arises in a closed circuit

of dis similar metal wires when a temperature gradient exists between

the junctions- of the wires- . The- voltage- output- from- t-he Pelt1er and­

Thomson effects can be related to total suction from calibration

curves . The calibration relationship, microvolts versus suction, is

determined by measuring the voltage output of psychrometers placed in

air above salt solutions of known concentration . The relative humidity

of the air must be in equilibrium with the pressure of water vapor

9

cause d by the salts in the solution . The vapor pressure is known ( 33 )

and permits computation of the total suction by equation ( 1-1 ) .

Chromel - P and const antan thermocouples are chosen because these

yield relatively large voltage outputs for small temperature di ffer-

ences and they also resist corrosion . A further advantage of chromel-

constantan thermocouples is that direct currents sufficient to heat

the junction in excess of 500 C may be applied to burn off residual

moi sture prior to making any readings .

1 . 3 . 1 Calibration properties . The calibration characteristics of

thermocouple psychrometers can be determined by consideration of the

heat energy required to vapori ze liquid water and the trans fer of heat

between the thermocouple and its surroundings . The volt age output of

the thermocouple is given by:

E = µ8

Where:

E = voltage, microvolt s

µ = thermoelectric power, 60 . 5 microvolts/deg C for chromel­const antan thermocouples at 25 C

e = temperature depression, deg c

( 1-5}

The temperature depression of the thermocouple can be related to the

suction of equation ( 1-1} by assuming a steady state condition in which

the heat gained by the wet junction from radiation of the surroundings

and conduction through the wire equals the heat lost by evaporation of

water from the wet junction . The relationship i s expres sed by ( 31 ):

10

Where:

p/p0 = relative humidity

p = pressure of water vapor

p0 = pressure of saturated water vapor

A = a complex parameter

V = volume of a mole of liquid water,_18 . 02 cc/mole w

� = total suction, atm

R = universal gas constant, 82 . 06 cc - atm/deg Kelvin-mole

T = absolute temperature, deg Kelvin

The parameter A is a function of many variable s given by (31 ):

Where:

- s rDLCT A = 2 3 2 16,g_rjT + 2r K y + R ( K + DBL ) WW a

2 D = diffusion constant for water in air, cm /sec

L = latent heat of vapori zation for water, calories/g

(1-6 )

(1-7 )

c; = saturated concentration of vapor at temperature T g/c c

a = Stefan - Bolt zman constant, calories/cm2-deg Kelvin4

rj = radius of the wet junction, cm

T = absolute temperature, deg Kelvin

r = radius of the thermocouple wire, cm w

K w = thermal conductivity of the

a c w w [2K /ln (r /r ) + 8r T3]112

11

wire, calories-cm/se c-deg C

1--' I\)

/ /

00�------�2�0����-4�0

��------L����---L--��-----1.����__J--���--L-��----J 60 80 100 1 20 140 160

TOTAL SUCTION, ATM

Figure 1-1 . Theoretical Calibration of Various Size Thermocouples at 25 C

K = thermal conductivity of air , calorie s-cm/sec-deg c a

r = radius of chamber containing wet junction , cm c

B = slope of cs vs T at temperature T T

Some other assumptions required to derive equation (1-6 ) are that the

emissivity for the wet junction and sample is unity , yi > 5 where i

is the length of the wire , and r >> r • The solution of equations c j

( 1-5 ) and (1-6 ) at 25 C for 1 , 2 , 4, and 10-mil-di am chromel-constantan

thermocouples is shown in fig . 1-1 . The constants required for this

solution are given in table 1-3 . The figure shows that the voltage is

essentially a linear function of total suction up to about 80 atm . The

output s of the large diameter thermocouples are less for a given sue-

tion , which implies that small diameter thermocouples should be more

sens itive and accurate than large diameter thermocouples .

The resolution or measurement accuracy is greater for greater

slopes of the voltage output versus total suction curves and thi s sen-

sitivity can be computed by:

v - w ( S = A RT exp -VwT/RT)

Where:

S = sensitivity ,- microv:olt s/atm-

(1-8 )

The sensitivities as a function of tempe rature for zero suction and

thermocouple diameters of 1 , 2 , 4, and 10 mils are given in fig . 1-2 .

The data for solution of equation ( 1-8 ) were taken from table 1-3 . The

radii of the junctions were measured from commercially available

thermocouples . The results from fig . 1-2 show that greater sensitivity

13

Table 1-3

Parameters for Theoretical Eg,uations

Function of temEerature � 31 ) Quantit;t

Parameter Units 0 c 10 c 20 c 25 c

D

L

cs T

B

a

K a

cm2/sec 0 . 219 0 . 233 0 . 248

calories/g 595 . 4 590 . 2 584 . 9

g/cc•l06 4 . 85 9 . 41 17 . 30

g/cc -deg C•106 0 . 33 0 . 60 1 . 02

mi crovolts/ 58 59 60 deg C

calories- 5 . 68 5 . 85 6 . 01 cm/sec-deg c·10 5

IndeEendent function of temEerature ( 31 )

0 . 255

582 . 3

23 . 04

1 . 29

60

6 . 09

Const ants Units Qtiantity

r c K w

ThermocouEle mil

1

2

4

10

. I 2 . 4 -12 calories cm -deg Kelvin •10

cm

calories-cm/sec-deg C

ThermocouEle s i ze diameter Junction

cm ·mil

0 . 00127 4

0 . 00254 5

0 . 00507 7

0 . 0127 20

14

1 . 36

1 . 0

0 . 024

diameter cm

0 . 005

0 . 0065

0 . 009

0 . 025

30 c

0 . 263

579 . 6

30 . 35

1 . 63

61

6 . 17

9' >­t-> t= Cl)

I MIL

� 0.21--�....,,._�--.._..�--���-+-����--,._.����-+-����--4 Cl)

o ._���� ...... �����--�����.._����_.....����__,

0 10 20 30 40

TEMPERATURE, DEGREES C

Figure 1-2. Sensitivity of Chromel-Constantan Thermocouples for 1- and 2-Mil-Diam at T = 0 •

15

50

is obtained from smaller wire diameters and that the sensitivity sub-

stantially incre ases with increasing temperature . The sens itivity only

slightly decreases with increases in suction because the exponent in equation (l-8) remains nearly constant for suctions in the practical

range of thermocouple psychrometers. If it is supposed that measure-

ments of the voltage output can be reproduced to within 0.2 mi crovolt ,

then the suctions can theoretically be measured to within the limits

indicated in table 1-4 for 1-, 2-, and 4-mil-diam chromel-constantan

thermocouples .

Use ful approximations to correct a reading taken between 10 and

30 C to the temperature of a known calibration curve for 1- and 2-mil-

diam thermocouple wires are given by:

one mil: E25

Et = 0.31 + o.0276t (l-9a )

two mil: E25

Et =

0.21 + o.0316t ( l-9b )

Where:

E25 = corre cted reading at 25 C

Et = reading at temperature t deg C

The equation for one-mil wires i s found from fig . 1-1 assuming that:

(1-10)

and from fig . 1-2 assuming that:

f1E = 0.0125t.tT (1-11)

The sensitivity i s also assumed to vary linearly with temperature

16

Table 1-4

Theoretical Accuracy of Chromel-Constantan Psychrometers

Plus-Minus Deviation in Atmospheres Suction Pre ssure

Diam of Wire for ±.0. 2 Microvolt Errors mils li 15 c 25 c

1 1.0 0. 7 o . 4

2 1.5 o. 8 o.6

4 2.0 1.0 0 . 8

17

between 10 and 30 C . The relationship for temperature correction given

by (34,35):

E25 = 0.325 + 0.027t (1-12)

for one-mil thermocouples agrees closely with equation (l-9a ) derived

from figs . 1-1 and 1-2. The above voltage correction for temperature

variations simplify data analysis by permitting consistant comparisons

at a single standard temperature such as 25 C .

1.3.2 Range . The range of Peltier thermocouple psychrometers can

be computed by consideration of the cooling ability of the thermocouple

junction and heat trans fer characteristics between the junction and the

surroundings . The cooling ability of the junction is given by (31,32):

Q = PI (1-13) Where :

Q = rate of cooling , joules/sec

P =Peltier coefficient , joules/coulombs (volts )

I = current , coulombs/sec (amperes )

The magnitude of the Peltier coefficient is determined by (31,32):

P = T ( dE/dT ) (1-14) Where :

T = absolute temperature , deg Kelvin

E = output voltage , volts

The actual amount of cooling at the junction is determined by the

rat e of cooling less the heat flow into the junction . Heat sources

18

include ohmic heating of the thermocouple wires from the applied cur-

rent , trans fer of he at from the he at sinks through the wires to the

thermocouple junction , and trans fer of he at from the surrounding air .

The optimum temperature depression of the therm.oc.ouple junction in

terms of the physical parameters of a single wire can be determined

by ( 30 ) :

( 1-15 )

Where :

801

= optimum temperature depression of wire one , de g C

:\1

= P1VK;_/(P1� + P2� ) pl

= electrical specific resistance of wire one , ohm-cm

S = therm.al conductivity of wire one , calories-cm/sec-deg C

p2 = electrical specific resistance of wire two , ohm-cm

K2 = therm.al conductivity of wire two , calories-cm/sec-deg

p = Peltier coe fficient , volts

IO = optimum current , amperes

A cross-sectional area of wire , 2 = cm

a-1 = ( Ha/SA )1/2

H = heat transriort coefficient,_ watts/cm2 -deg_ C_ ·

a = circumference of wire , cm

t =length of wire , cm

The optimum cooling current is given by ( 30 ) :

PTrr3/2 tan a1 i tanh a2 t

�o

19

c

( 1-16)

Where:

r =

(l2 =

K2 =

<l>o

=

radius of wire , cm

(Ha/K2A )1/2

thermal conductivity

1/2 2 (Kl K2 ) [ <l>l - <I> 2]

of wire two , calories-cm/sec-deg C

The optimum currents as a function of length for 1- , 2- , and 4-mil

chromel-constantan thermocouples from equation (1-16 ) are shown in

fig. 1-3 for 25 C. The optimum temperature depression calculated for

known optimum currents of the constantan wire by equation (1-15 ) are

shown in fig. 1-4. The data required for these computations are given

in tables 1-3 and 1-5. These results show that shorter lengths and

larger diameter thermocouples should yield greater range . The maximum

theoretical range of suction should be about 200 atmospheres.

1. 3. 3 Temperature Control. Early studies had required a constant

temperature within :!_0. 001 C , whi ch was o�en achieved with water or

other fluid baths ( 27 , 36-38 ). Precise temperature control was neces-

sary because small thermal gradients across the thermocouple psychrom-

eter can cause large measurement errors between ambient and dewpoint

temperatures , particularly at high relative humi dities , leading to

large errors in suction. Later work had shown the satisfact ion of

20

en la.I a: la.I Q.. � c( J � :J

.. ... z la.I a:: a:: :::> u C> z _J 0 0 u :J :::> :l: � 0.. 0

40

35

30

Z5

zo

15

10

0 0

\ '

\ '

\ � 1,

I \ \

I \ I I \

I �If MIL (0.00507 CM) I '

I ........ I � - -

' , . - -i. 1 ' \ I I

\ \ � \ \ .

\ \

q� ,_2 MIL (0.0025.t CM)

q 1) 'o--o >-----� r-----o --c

\ I MIL(0.00127 CM) _/

'(_ - -- -

o.� 1.0 1.5 2.0 z.� LENGTH PER THERMOCOUPLE WIRE, CM

Figure 1-3 . Optimum Cooling Current as a Function of the Length of Thermocouple Wire for Various Diameters ( After Reference 30)

21

2 .5 286 0 fl) w w 4 MIL {0.00507 CM) a:: � w 0

... z.o 236 z 0 fl) fl) w a:: Q. w 0 w a:: :::> � a:: w Q. � L&J .... � :::> � .... Q. 0

2 MIL (0.00254 CM)

1.5 173

I MIL (0.0012 7 CM}

1.0 113

0.5

o --����_...����� ...... ����----����-6 0 --'> -0.-� H;> L !> Z.O

LENGTH PER THERMOCOUPLE WIRE, CM

Figure 1-4 . Maximum Temperature Depres sion ( Range} of Thermocouples at 25 C for Various Diameters of Constantan Wire

22

� .... <

... z 0 .... u :::> Cl)

Table 1-5

Data for Calculating Optimum Current

and Temperature Depression

Constant

p

H

P constantan

Pchromel

I. constant an

A chromel

K constant an

K chromel

suantit;y:

18 . 03•10-3

83 . 21•10-4

49 . 06·10-6

70 . 6·10-6

o . 422

0 . 578

0 . 22

0 . 20

23

Units

volt s , 25 C

watts/cm2 - deg C

ohm - cm

ohm - cm

watts/cm - deg C

watts/cm - deg C

certain criteria will minimize the effect s of changing ambient tempera­

ture on thermocouple psychrometer readings .

A device designed and built by Rawlins and Dalton (26) shown in

fig . 1-5 is useful without precise temperature control . Main features

of this device include a one-mil-diam chromel-constantan thermocouple

enclosed in a porous cerami c bulb . This design is meant to fulfill the

criteria given in table 1-6 . The soil s ample should cover the ceramic

bulb and the thermocouple should have a long length to reduce measure­

ment error caused by the di fference in temperature between the chamber

and heat sinks ( re ference junction or dry bulb ) . The purpose of the

he at sinks i s to absorb or emit heat with a minimal temperature change

of its mass .

Later studies (39) have shown that errors from thermal gradients

c an be practically eliminated by simply enclosing the entire sample and

psychrometer insi de massive heat sinks with good thermal conducting

properties . A commercial devi ce of thi s design manufactured by

Wescor, Inc . , Logan, Utah, includes ni ckel-plated bras s heat sinks .

The sample chamber is about 4 in . hi gh by about 3 in . di am, requires

little temperature control, and readings can usually be made a few

minutes a�er inserting the sample .

The adverse effects of existing thermal gradients may also be re­

duced by including nonevaporative materials within the psychrometer

chamber or by a temperature compensation des ign . The failure of some

psychrometers to yield reproducible readings was attributed to the

sorption of water vapor on the walls of the chamber ( 40 ) . Some

24

I. COPPER LEAD WIRES 2. PLAS T IC BODY 3. COPPER HEAT SINK_S 4. I MIL-DIAM CROMEL­

CONSTANTAN THERMOCOUPLE �- CERAMIC POROUS BULB

Figure 1-5 . Sketch of Thermocouple Psychromet er that is Useful Without Precise Temperature Control

25

Table 1-6

Criteria for Thermocouple Psychrometers·Without

Precise Temperature Control

Criteri a

1 . Determine temperature o f sample to within 3 C to infer suct ion to the nearest percent

2 . Determine microvolt output as a function of temperature

3 . Use large he at sinks

4 . Cover psychrometer chamber completely with soil

5 . Provide for the free flow of water vapor into and out of the chamber

6 . Use nonevaporative materi als for the chamber , i . e . brass , copper , te flon , paraffin

26

Reference

26

26

26 ,39

26

26

39 ,40

desirable chamber materi als include brass, copper, stainless steel,

paraffin, sili cone-on-bras s, and teflon ( 39,40), although copper may

oxidize a�er several tests ( 41,42) . An alternative approach is to re­

duce thermal gradients between the reference (dry bulb) and the thermo­

couple (wet bulb) junctions by placement of both junct ions in the same

environment within the chamber ( 4 3) . Neither junction i s required as a

heat sink, but an additional thermocouple may be necessary to correct

for the di fference in temperature between the thermocouple junct ions

and the soil sample .

1 . 3 . 4 Applications . Thermocouple psychrometers have found num­

erous applications in the measurement of water pot ential in soils and

plants ( 27, 36, 37, 44) including some in situ measurement s ( 9,10,11 ,44) .

The commercially available Wescor psychrometer had been successfully

inst alled in desert soils for field measurements from 1 to 50 atms of

suction (10,39) . The optimum cooling current for this type of psy­

chrometer is about 8 to 10 milliamperes ( 45)� The range of thermo­

couple psychrometers has been ext ended in the laboratory to 1000 atms

by cooling the thermocouple junction over a 10 atm potass ium chloride

solution for 10 to 20 minutes, then immediately replacing the solution

with a sample and measuring the voltage output ( 45) . The soil tempera­

ture can be found by a copper-constant an thermocouple attached to the

psychrometer .

Practical experience gained from working with psychrometers has

shown that cleanliness i s import ant for successful applications . Min­

ute particles of matter adhering to the thermocouple often renders the

27

psychrometer inoperable . Sufficient cleanliness can be achieved by

care ful rinsing in solvent s such as acetone followed by distilled

water (39) . Permanent protect ion can be provi de d by forming a chamber

with porous cerami c bulbs (26 , 39 ,46 ,47 ,48) or fine-mesh wire cages (11 ,

44) that enclose the thermocouple .

Reproducibility between calibrat ion curves o f individual psychrom­

eters can be achieved by arc welding the thermocouple junction in an

inert atmosphere to a consistent size and shape (47) . The best type of

weld appears to be one in which the wires proj ect from opposite sur­

faces of the bead .

An interesting adaptation has permitted independent measurements

of both matrix and osmotic suct ion as well as the total suction . The

independent measurement of matrix and osmotic suct ion is possible with

a psychrometer similar to that in fig . 1-5 except that the ceramic bulb

i s capable of a bubbling pressure such as 15 bars (46) . The chamber

interior is vented to atmospheric pressure . This psychrometer is part

of an apparatus in which air pressure can be applied to a soil sample

that surrounds the ceramic bulb . The air pressure will reduce the

matrix contribution to the total suction similar to the pressure mem­

brane technique . When the air pressure exceeds the matrix suction ,

soil water will be forced into the bulb chamber . The measured suct ion ,

which remains constant during this time, i s equal to the osmot ic com­

ponent . Platinum electrode s were also inserted inside the porous bulb

to measure electri cal conduct ance in the ceramic . This measurement

28

confirms the di ffusional equilibrium of the soil solution between the

bulb and sample (46) .

Psychrometric measurements of suction at different temperatures

had shown negligible differences for most cases ( 28 ,48) . Temperature

effects may become significant , especially in fine textured soils , at

high suctions (48) . The suction usually decreases slightly for higher

temperatures . This result compares favorably with others ( 2) who found

small di fferences approximated by :

p� - P�o + 0 . 017 - o . ooo86t (1-17)

Where :

F loglO of the negative pressure of water in centimeters at pt =

the temperature t in deg C

F logia of the negative pres sure of water in centimeters at P20 =

20 c

Bulk density effects were usually negligible except in a swelling clay

in which suction decre ased for gre ater bulk densities at constant water

content ( 48) .

29

2 . 1 Monitoring System

CHAPTER II

EQUIPMENT AND PROCEDURES

The monitoring system must include provis ion for applying a vari­

able milliampere cooling current to the psychrometers with the capa­

bility of immediate switching to the voltage readout circuit on

termination of the current . The circuitry in fig . 2-1 includes a

Keithly model 148 nanovoltmeter (1), two Leeds and Northrup enclosed

silver switches (2), and the cooling circuit . The voltmeter has a min­

imum full-scale range of 0 . 01 mi crovolt with a battery system adaptable

to field applications . The 12-position rotary selector switches allow

up to 12 simultaneous connections to psychrometers . The 0-25 milli ­

ammeter (3) , two 1 . 5-volt dry-cell batteries (4) , and the variable po­

tentiometer (5) form the cooling circuit .

The voltage output can be immediately read by switching from the

cooling circuit to the voltage meter by one incremental change of the

proper rotary switch . Thi s circuit is stable with unperceptible dri�,

noi se, or waver in the .entire mi crovolt range of interest when subj ect

to normal temperature variations and laboratory operating conditions .

Field conditions such as extreme temperature variations that might oc­

cur from direct exposure to sunlight can induce several microvolts in

the circuit, but these outputs are independent of psychrometric read­

ings and can be suppressed through controls on the nanovoltmeter.

2 . 2 Temperature Control System

Several different enclosure systems, composed of combinations of

30

'

w I-'

\ 0

0 o r ol 0

Q 0 0 I -

9 01 o r o' I 0 0 0 I

I

0 9 0 0

0 0 0

0 0 0 0 0

0 0

0 0

0 0 0

0 0 0

I. KEITHLEY MODEL 148 NANOVOLTMETER

2. LEEDS- NOR THRUP ENCLOSED SILVER SWITCH

3. MI LLIAMMETER (DC) RANGE 0-25 MA

4. SIZE 0 DRY CELL BAT TERIES

�. BOURNS TEN TURN PO TENTIOMETER

Figure 2-1 . Electrical Circuit for the Thermocouple Psychrometer

plastic insulation and metal containers, were studie d for maintaining

temperature control . Exce ssive ambient temperature variations were

indicated by microvoltage outputs from the psychrometers prior to ap­

plicat ion of the cooling current . One system that proved unsatisfac­

tory relied entirely on plastic insulation . This system included six

1-in . -diam by 5-in . -long plastic psychrometers inserted within styro­

foam matching 1-in .-diam insulated chambers . The psychrometers were

sealed to polystyrene test tubes cont aining the test specimens . Ini­

tial voltages were usually one or more microvolts, indicating lack of

uniform temperature di stribution . These psychrometers could not be

properly calibrated and readings were not reproducible . Sample sizes

were also small prohibiting accurate measurements of soil water con­

tents and the testing of undisturbed specimens . The adopted system

consi sts of metallic sample containers placed within a compact commer­

cially available polystyrene insulated dry i ce chest .

Proper temperature control was first maintained with a 3-ft by

1-1/2-ft by 1-� plywood box insulated with two inches of styrofoam .

Calibration solut ions and soil samples were places inside a metallic

pint-size steel paint can that had the internal surfaces sprayed with

a plastic to retard corrosion . The test psychrometers were inserted

inside the cans with cables conducted through No . 13-1/2 rubber stop­

pers . The pint cans were placed ins ide a 3-lb-size coffee can in­

sulated with packing paper and the coffee can was inserted inside the

insulated box . Thi s apparatus was operated in a thermostatically con­

trolled room that could provide temperatures from 18 to 26 C .

32

Temperatures within the insulated box normally varied :!:_0 . 5 C for a par­

ti cular room temperature setting throughout an 8-hour period. A chart

recorder was connected to the circuit during initial tests to permit a

record of voltage outputs . Occasional excessive room-temperature vari­

ations were responsible for initial voltages of a few microvolts pro­

hibiting accurate readings during these periods . The apparatus was

als o bulky and inconvenient to handle . An improved system shown in

fig . 2-2 includes a commercially available 1-ft by 1-1/4-� dry ice

chest insulated with 1-1/2 in . of foamed polystyrene . At le ast six

metallic pint-size sample containers can be placed inside the chest and

psychrometers inserted into the containers . The le ad wires are shunted

through a 1/2-in . -di am hole centrally located in the chest cover . The

temperature within the chest i s measured by a thermometer inserted

through the hole in the cover . Temperature equilibrium is attained

several hours after placing the cover .

2 . 3 Psychrometers

Four succes s ful thermocouple psychrometric designs invest igated in

this study can be briefly des cribed as (a) a plastic �ylindrical body

enclosing the heat sinks, (b) external plate heat sinks separated by a

sheet plastic insert, (c) metal tubing containing a thermocouple, and

(d) a commercially available in situ psychrometer made by Wescor, Inc .

of Logan, Ut ah . All o f these des i gns are adaptable to field installa­

tion . Detailed descriptions of the first three types are given i n the

following paragraphs .

Chromel-constantan thermocouples were obtained preas sembled from

33

2-2. .Monitorin,g

the manufacturer . Laboratory psychrometers constructed o f a plexiglass

body enclosing the heat sinks are similar to that shown in fig . 2-3a .

A few units containing only two heat sinks and one thermocouple were

also built . Each coppe r heat sink ( 3 ) is 1/8-in . diam by 1 in . long .

The thermocouples may be either long (1 ) or short ( 2 ) and were made

with 1- , 2- , or 10-mil-diam chromel-constantan thermocouple wires . The

thermocouples we re soldered to the heat sinks of the reference junc­

tion . A plastic coating was ·sprayed on the thermocouples in early ex­

periments for the purpose of reducing corrosion , although thi s proced­

ure was later abandoned because it was not found necessary for

laboratory appli cations . Screens or cage s may be placed around the

sensors to form a chamber for field studies . Seven of the se psychrom­

eters were built with properties given in table 2-1 . Optimum cooling

currents found for these psychrometers are discussed later .

A design that should help maintain the temperature of the heat

sinks , chambers , and soil at the same temperature i s shown in

fig . 2-3b . The external heat sinks ( 1 ) will be in close contact with

the soi l . The chamber is located between the he at sinks and protected

from soil entry by a coarse , porous stone ( 2 ) permitting free acces s to

soil water . The he at sinks are connected to the center plastic sheet

( 3 ) and several coats of epoxy coating material are painted on the body

to protect the copper from corrosion . Several of these prototype P­

psychrometers were built and are de scribed in t able 2-2 . Three psy­

chrometers were also made with an open chamber (112 , 212 , and 312 ) in

35

I. LONG L E A D THE RMOCOUPLE NO. 8 2. SHORT L E A O T H E R MOCOUPLE NO. �

3. HEAT S I N l< S

4. COPPER L E A D W I R E

!!t. PLAS T I C BOD Y

a . L A B O R ATORY DOUBLE

T H E R M OCOUPLE PSYC H R O M E T E R

I . COPPER TUBE 2. ZOO MESH STAINLESS

SHEL SCREEN 3. C E R A M I C INSULATOR

4. THERMOCOUPLE �

5. COPPER LEAD WIRE

c. F I E L D T H E RMOCOUPLE PSYC H R O M E T E R TYPE F

I . COP P E R S I N K S

Z. P O R O U 5 STONE

3. PL A ST I C S P A C E R

4, T H E RMOCOUPLE �. C O P P E R L E A D W I R E

b . F I E L D T H E R MOCOUPLE PSYC H R O M E T E R P ROTOTYPE p

4

I . POROUS C E R A M I C

2 , 1 - M l L D I A M THE RMO­

COUPLE

3 , T£FLON PLUG

4 , COPPER LEAD W I R E

d . C O M M E R C I A L PSYC H ROMETER (AFTER W E SC O R , INC., LOGA N , UTA H )

Fi gure 2-3 . Laboratory and Field Thermo couple Psychrometers

Nurrmer

1

2

3

4

5

6

7

5

6

Table 2-1

Laboratory Psychrometers

Optimum Cooling Current

5

5

5

5

12-1/2

12-1/2

200

8

8

Description

5/8-in . -diam body containing two heat sinks and one short lead 1-mil-di am thermocouple of .2-mm total length

Similar to No . 1 except that the thermo­couple length is 4 mm

Part of a 3/4-in . -diam double sensor composed of two heat s inks and one short lead 1-mil-diam wi re of 4-mm total length

Part of the double sensor adj acent to No . 3 with one long lead wire of 1-cm total length, 1-mil diam

Part of a 3/4-in . -diam double sensor composed of one short lead 2-mil-diam wire of 4-mm length

Part of the double sensor adj acent to No . 5 composed of . one long lead 2-mil­diam wi re of 3-cm length

5/8-in . -diam body with 2 heat sinks and one short lead 10-mil-diam wire of 4-mm total length

Rebuilt with 2-mil-diam wire of 4-mm length

Rebuilt with 2-mil-di am wire of 1 . 5-cm length

37

Number

8

9

10

11

28

112

212

312

Table 2-2

Prototype P-Psychrometers

Optimum Cooling Current , ma

20

35

35

30

10

8

8

8

Des cription

3/8- x 3/4- x 1/16-in . heat sinks sepa­rated by 1/16-in . plexiglass with 2-mil-diam wire of 1/16 in . total length

.3/4- x 1-1/2- x 1/16-in . heat sinks separated by 1/8-in . plexiglas s with 4-mil-di am wire of 1/8 in . length

Same as No . 9 but with 1/4 in . length wire

Same as No . 9 , but with 5/16 in . length wire

No . 9 but 1-mil-diam wire , 1/8 in . length wire

3/4- x 1-1/2- x 1/16-in . heat sinks separated by 1/8-in . plexiglas s with 2-mil-di am wire of 3/16 in . length open chamber

Same as 112 , except 1/8 in . length

Same as 112 , except 1/4 in . length

which the copper had been filed to conform to the exact shape of the

plastic spacer .

The field type F design given in fig . 2-3c may be useful for suc­

tion me asurements in partly saturated specimens during triaxial

strength tests as well as in field inst allations . The psychrometer can

be made small and inserted into a hole drilled in the specimen. Psy­

chrometers with stainless steel and copper tubing were constructed with

properties given in table 2-3 . A sketch of the commercial psychrometer

is shown in fig . 2-3d . These psychrometers are built with 1-mil ther­

mocouples and require an optimum current of about 8 mA .

2. 4 Laboratory Testing Procedures

Calibrat ion salt solutions and soil specimens were placed in pint

paint cans leaving sufficient space to insert the psychrometers . The

cables of one or more psychrometers were guided through holes in a

No . 13-1/2 rubber stopper . The psychrometers and rubber stopper were

inserted into the cont ainer to form an airtight seal . Large soil

specimens were tested by inserting the psychrometer directly into a

hole bored in the sample and sealing the as sembly with a wax coating

or plastic membrane . The specimen and psychrometer as sembly were

placed inside the insulated chest ; cable wires were passed through the

centrally located opening in the cover ; and the cover ' was placed on the

chest . The cable le ad wires are connected to the control box and a

thermometer passed through the opening in the cover to register the

ambient temperature .

Prior to any suction readings , a reversed direct current that

39

Optimum Cooling Item Current , ma

SSl 3

SS2 8

CUl 5

CU2 5-15

Table 2-3

TyPe F Psychrometers

Descript ion

3/16-in . -O . D st ainles s steel tubing , 1-1/2 in . long , 1-mil wire , 1/16 in . length

1/4-in . -O . D stainless steel tubing 1-1/2 in . long , 2-mil wi re 1/8 in . length

3/16-in . -O . D copper tubing , 1-1/2 in . long , 1-mil wire , 1/16 in . length

1/4-in . -O . D copper tubing , 1-1/2 in . long , 2-mil wire , 1/8 in . length

4o

exceeded the optimum current was applied to the psychrometer for 30 sec

to heat up the thermocouple and to vaporize residual moisture from the

junction . A�er about five minutes or after the voltage output had

subsided to 0 . 2 microvolts or less , the polarity was switched and a

cooling current was applied for 30 sec . The heating current was re­

applied i f additional suction readings were necessary .

41

CH.APTER III

TESTS AND ANALYSIS

3 . 1 Calibration Properties

Thermocouple psychrometers are calibrated with the assistance of

salt solutions whi ch cause a vapor pressure lowering or relative hu­

mi dity in the atmosphere that can be converted to suction by equat ion

(1-1) . The calibration studies with the salt solut ions determined

bas i c operat ing charact eri stics , such as opt imum current , voltage out ­

put with time , voltage output with suction , temperature effects , re­

producibility , and range . Calibration properties were investi gat ed for

psychrometers made from 1- , 2- , 4-, and 10-mil chromel-constantan

thermocouples .

3 . 1 . 1 Calibration solut ions . Potas sium chloride (KCl ) was chosen

as the salt in the calibration solutions because the relative humidity

of the se solut ions is independent of temperature for the normal range

of operating temperature . Table 3-1 speci fies the gram formula weight

of KCl per 1000 grams of distilled water (the molality M) for some

relative humidities in equilibrium with the solutions (33 ) . Equilib­

rium voltage outputs from the psychrometers can be attained within the

metal pint container in 48 hours or les s with 0 . 05M solutions and

12 hours or less with 2 . 0M solutions . The t ime required to reach

equilibrium can be reduced by about 1/2 by lining the sample container

with filter paper saturated with solution .

3 . 1 . 2 Optimum current . Initi al psychrometric tests had shown

42

Gram-Formula Weight per

Table 3-1

Concentration of Potas sium Chloride for

Certain Relative Humidities

Grams of KCl per Relative 1000 Grams Water 2 M 1000 ml Water Humidit;y 2

0 . 05 3 . 728 99 . 83

0 . 20 14 . 91 99 . 36

0 . 50 37 . 27 98 . 42

1 . 00 74 . 5 5 96 . 84

2 . 00 149 . 10 93 .68

43

% Suction at

25 C 2 atm

2 . 3

8 . 8

21 . 6

43 . 4

88 . 5

that the cooling currents applied for 30 se conds given in tables 2-1 ,

2-2 , and 2-3 are opt imwn . These values agree well with theoretical

optimwn currents given in fig . 1-3 . It was also found that current

variations of several milli amperes usually had little effe ct on voltage

outputs as indicated in the literature (49 ) . A rebuilt laboratory

double psychrometer containing Nos . 5 and 6 tested with a l .OM KCl

solution, for " example, did not yield dis cernible di fferences in voltage

outputs for cooling currents of 8 and 12 milliamperes . Consistent

voltage outputs may also be obtained from the F field psychrometers

with cooling currents from 5 to 15 milliamperes .

3 . 1 . 3 Voltage output with time . Example outputs of volt age with

time observed after 30 se conds of cooling at the optimwn current are

illustrated in fig . 3-1 for 2-mil diam thermocouples . Cooling times

longer than 30 seconds did not appreciably extend the range of the psy­

chrometer . Shorter cooling t imes were avoided to ensure adequate mois­

ture condensation on thermocouple juncti ons . It was found , however,

that di fferences in cooling time would cause slight changes in the

voltage output for a particular psychrometer and suction ; therefore,

the cooling time should be maintained const ant for greater accuracy .

Impulses were sometimes observed at low voltage output s immedi -

. ately after switching from the cooling circuit to the voltage readout

system as shown in fig . 3-1 . These impulses were more noticeable with

psychrometers containing thermocouples of di ameter greater than one

mil, but they usually di d not interfere with low readings be cause the

true maximwn continues for many seconds after switching from the

44

1 2

.... ::>

--< � 0. 5 M KCl

I°'

a. 8 ..... ::> 0

!J 0 > 0

� 4 �

.......

0 0

I / �o. 05 M KCt

3 0 6 0 T I M E , SEC

-

90

Figure 3-1 . Voltage Output with Time for Psychromete_r_ No_._ 5- at- 25- C -

45

1 20

cooling current to the voltmeter readout circuit .

3 . 1 . 4 Voltage output and suct ion. Calibration curves of psy­

chrometers at 25 C made for 1- and 2-mil thermocouples show in fig . 3-2

and 3-3 that the voltage output is almost a linear function of the suc­

tion and approximately that predi cted from fig . 1-1 of the theoretical

analysi s . The body design does not appear to have appreciable influ­

ence on calibrat ion properties . Rebuilt laboratory psychrometer No . 6

yielded a slightly greater sensitivity with a 2-mil thermocouple length

of 1 . 5 cm that was substanti ally shorter than the original length of

3 cm.

F-type psychrometers with stainles s steel tubing would not operate

properly . Psychrometer SSl (one mil) would not yield reproducible

readings and psychrometer SS2 (two mil) indi cated about 4 microvolt s

output independent of the concentration of the calibration · solution.

Substitution of copper for stainle ss steel provided satis factory re­

sults as shown in figs . 3-2 (CUl) and 3-3 . (CU2) .

Psychrometers constructed with a thermocouple diameter greater

than two mils were found to lack sensitivity and had unusually large

ordinate intercepts at zero suction pressure . Laboratory psychrometer

No . 7 (ten mil) possesses a sensitivity of only 0 . 15 microvolts/

atmosphere , compared to a theoretical value of 0 . 32 microvolt s/

atmosphere as shown in fig . 3-4 . �he ordinate intercept s of prototype

P psychrometer Nos . 9 to 11 (four mil) became larger with longer

thermocouple lengths .

3 . 1 . 5 Temperature effect s . The voltage output of psychrometers

46

I-:::> Cl. I-:::> 0 I-

.i=- ..J --.::i � 0 0: u

�

5 0

4 0

3 0 ,,,.. .,,.. ,,,.. .,,.. .,,.. ,, ,, .,,.. .,,.. ,,,..

... " - ,,,.. ,,

2 0 ... _.. ,,,.. .. � - sL OPE = 0. �3 µv/A TM ; "'

_.. " _.. .,, • .,, ' • ..,. .,,

• � ... , �

1 0 .,,..

_.. .,, - .,, .,,

, .,,.. • J- ·

r • .,,..

\... " " ! ,.. 0 0 1 0 z o 3 0 4 0 5 0 6 0 7 0

S U C T I O N PRE SSURE , ATM

Figure 3-2 . Calibration of 1-mil-diam Thermocouple Psychrometers at 25 C

--

8 0 9 0

.... :> � .... :> 0 .... ...J

+:""" 0 (X) >

0 0:: u j

5 0

4 0

3 0

z o

1 0

�1 .,.. ..... 0

0

..... ..... -- · ... .....

..... ..... ... ..... -

..... ..... ...... .....

� ..... < .. -..... - SLOPE = 0. 4 0 µ V/ A TM

... .... I' ..... ......

_ ...... .

- ..... .. ... ...... .. .. -...... ......

.. ... ..... ..... ......

...... l ...... ... .....

1 0 2 0 3 0 4 0 5 0 6 0 7 0

S UC T I O N P R E SSUR E , AH A

Figure 3-3 . Calibration of 2-mil-diam Thermocouple Psychrometers at 25 C

• .,.. .,.. ... ..... ..... .

- ... ..... ' •

•

8 0 90

3 0

4 0

.... ::> 0.. 3 0 .... ::> 0 .... -'

+ 0 > '° 0 a:: 2 0 u 2

L E GE N D

N O . MIL L E NG T H , I N. • 7 1 0 0 9 4 I

8

a 1 0 4 .!. 4

A 1 1 4 2-1 6

S U C T I O N P R E SSURE , AT t.4

Figure 3-4 . Calibration of 4- and 10-mil-di am Thermocouple Psychrometers at 25 C

place d in the equilibrium atmos phere of the KCl solutions will decreas e

with lower tempe ratures , although the relative humi dity does not

change , becaus e the amount of the temperature depress ion to the dew­

point varie s with amb ient air temperature . Temperature characteristi cs

of psychrometers wi th 1- and 2-mil di am thermocouples are shown in

fig . 3- 5 for some of the KCl s olut i ons shown in table 3-1 . Theoreti cal

outputs computed from e quat i on ( 1-5 ) and ( 1-6 ) for a chamber radius o f

1 c m are shown b y the soli d line . Experimentally determi ne d output s ,

part i cularly th e slopes of thes e curves , are in good agreement with

cal culate d values and show that us e ful approximations of unknown volt­

age outputs of another temperature can be made from known outputs and

temp eratures by e quati ons ( 1-9 ) or ( 1-12 ) . Psychrometers with thermo­

couple di ameters of 4 and 10 mil s did not ob ey theoreti cal mi crovoltage

versus t emperature relati onships . These psychrometers were not in­

vestigated furthe r because of their poor operat i ng properti es .

3 . 1 . 6 Reproduc ibi lity . Once equilibrium i s attaine d , reproduc­

ib ility between conse cutive readings of a parti cular psychrometer with

1- or 2-mil thermocouples was normally .:!:. 0 . 2 mi crovolts or .:!:. 0 . 5 atm

up t o 4 3 atm of suct i on pressure . Reproducibility between consecu­

tive readin__gs at 88 atm suction _pressure was ab out _.:!:. 0 . 5 mi crovolt s

(.:!:. 1 atm ) .

Reproducib ility of readings between different psychrometers of

i dent i cal body des ign , i dent i cal wire di ameter , and i dent i c al calibra­

tion solutions s ometime s show appre c i able s catter . Readings from s ix

of th e Wes cor 1-mil psychrometers were found reproducible within .:!:. 5%

50

1 0

0. 2 M KC�

z o 3 0

T E M P ER AT U R E , DE G R E E S C

8. T WO - M I L - D I A M W I R E S

4 0 5 0

Figure 3-5 . Temperature Charact erist i c s of Thermo couple Psychrometers

51

of 0 . 48 mi crovolt/atmospheres as claimed by the manufacturer . Three

P-psychrometers with open chambers were calibrated in Jun of 19 70 be­

fore and after being cleaned by soaking for 20 minutes in acetone fol­

lowed by a rins e in di stilled water ( fi g . 3-6 ) . The cle aning obvious ly

improved the sens itivity of the psychrometers and thus the reproduci­

bility of the data , whi ch illust rates the importance of cleanlines s .

Prototyp e P psychrometer 112 in fig . 3-6 also demonstrates that the

lapse of time need not have appreciable influence on calibrat i on be­

havior . The calibration curves of s ix prototype P and s i x type F field

psychrometers yield a range i n the calib ration properties shown by the

b andwidth in fig . 3 . 7. Individual calibrations are there fore re com­

mended to achi eve great e r accuracy in suction readings .

3 . 1 . 7 Range . The length and di ameter of the thermocouple wire

appear t o be dominating factors in determining range . Short e r lengths

did incre as e the range of 2-mil-diam wires shown in table 3-2 . The

range of most 1-mil- di am thermocouple psychrometers was les s than

88 atm except for the Wes cor psychrometer whi ch had a very large bead

diameter of 7 . 5 times the wire diameter , and the CUl type F psychrom­

eters , which had a very short the rmocouple length . The maximum range

of the 2-mil psychrometers was not much more tnan 88 atm o f suction .

3 . 2 Laborat ory Measurement s of Soil Suct i on

Suction versus wat e r content relati onships of two 1-in . -diam un­

disturbed pieces of clay soi ls on drying were measured t o help evaluate

the use fulnes s of the rmocouple psychrometers as ac curate indi cators of

suct ion . The results of these tests were compared with thos e from a

52

50

40

I-:::> ll. I-

30

::> 0

� 0 \.J1 > w 0 a: u 20

�

L E G E N D

JUN 197 1

PSYC HROMETER B E FORE A F T E R NUMBER CL E A N ING C L E A N I N G

I L 2 0 • 2L2 a • 3 L 2 v •

1 0 20 30

JAN 1 969

BEFORE C L E AN I N G

fl

40 50 SUCTION PRESSURE , ATM

60 70 80

Fi gure 3-6 . Calibration of P-Psychrometers , Open Chamber , 2-mil-diam Wire at 25 C

90

� ::::> Q.. .... ::::> 0

.... ..J 0

Vl > +:="' 0

a: u ::t

3 0

2 0

1 0

O L-----------L------------'-----------..._ ________ __., __________ _,_ ____________________ __

0 I 0 ZO 3 0 4 0 50 60 7 0

T O TA L S U CT I O N, AT M

Figure 3-7 . Range of Calibration o f Field Psychrometers at 25 C

Table 3-2

Range s of Ps;ychrometers

Experiment� Wire Bead Total Range Ps;ychrometer Diam Diam 1ength 2 cm atm

No . 3 1 4 0 . 3 43-88

No . 4 1 4 2 43-88

No . 5 2 5 o . 4 >88

No . 6 2 5 3 43-88

No . 6 Rebuilt 2 5 1 . 5 >88

No . 9-11 4 8 0 . 3-0 . 8 >88

No . 7 10 20 o . 4 >88

Wescor 1 7 . 5 1 >88

112 , 212 , 312 2 5 1 > 88

CUl 1 4 0 � 16 > 88

CU2 2 5 0 . 3 > 88

55

pressure membrane apparatus whi ch measures the matrix suct ion of

4 . 5 -in . di am by 0 . 5-in . -thi ck undi sturbed spe cimens ( 50 ) . Results from

the pre s sure membrane include the e ffe ct of the overburden load on suc­

tion . The suction of the psychrometer result s may be correct e d to in­

clude the e ffect of the surcharge pre s sure by equat ion (1-3 ) .

Propert ies of the tested soils are given in table 3-3 . The

we athere d Upper Mi dway spe cimens were t aken from s ample s of borings

8A6C-49 and PU-7 of the test· pier site at Lackland Air Force Base ,

Texas ( 51 , 5 2 ) . The unwe athere d Yazoo specimens were t aken from sample s

of borings PU-6 and PU-2 , which were drilled at an expansive soil test

site ne ar Jackson , Mi s sissippi . The Yazoo clay is le ached and does not

pos se s s much osmot i c contribut ion to the total suction pressure ( 52 ,

5 3 ) . Analys i s of the Upper Mi dway specimen had shown an osmot ic com­

ponent contribut ing 5 atm to the total suct ion pres sure ( 54 ) .

Suction pres sure results for weathered Uppe r Midway specimens from

boring 8A6C-49 on drying from natural water content are shown in

fig . 3-8 . The se dat a frqm a variety of psychrometers , which do not in­

clude a surcharge pres sure , agree within seve ral atmosphe re s of each

other . Compari son of suction dat a from the pre s sure membrane and ther-

_moc_ouple ._psy...chrometers -a.re shown in fi-g . 3-9 a.11d includ-es a surcharge

load equivalent to the in situ depth of the soil . The pressure plate

dat a of an undi sturbed specimen of the we athered Upper Midway clay on

drying from natural water content are from 5 to 10 atm le ss than the

psychrometric ally determined suct ion pressure , even after adj usting for

the effe ct of the overburden load . The di fference can be attributed to ·

Boring Sp� cimen SamEle No .

\.J1 PU-2 26 --;i

PU-6 26

T;m clay

PU-7 36

8AGC-49

Table 3-3

ProEerties of Boring SamEle s

Att erberg Natural Limit s Water

DeEth 2 ft LL PL PI Cont ent 2 %

Unweathered Yazoo clay 2 Jackson s ite

31 . 0-32 . 1 103 30 73

31 . 0-32 . 1 100 28 72

shale , weathered Uwer Midway

46 . 9·-48 . 4 81 23 58

35 . 0-40 . 0 92 22 70

4 3 . 8

46 . o

format ion ,

29 . 1

29 . 5

Unit Weight 2 Ec f Spec ific � Moi st Gravity --

77 . 3 111 . 2 2 . 71

75 . 8 110 . 8 2 . 71

Lackl and site

96 . 3 124 . 4 2 . 78

91 . 5 118 . 5 2 . 73

1 0 0

90

8 0

7 0

6 0

5 0

� 4 0 ..... < .. z Q 1-u :::> Cl) 3 0

...J � 0 I-

2 0

1 0 2 0

L EG E ND

P S YCHROMETER

0 I - • 2

� • 3 , 4

"'� v 5

" 8

� a 8

"' � 0

�o TOTAL SUCTION

I - I N . U N DISTURBED SAMPLES BO R I N G 8 AGC -49

3 5 - TO 4 0 - F T DEPTH

0\ I\:

2 2

, DY

2 4 2 8 2 8 WATER C O N TE N T 1 % ORY WEIGHT

N O.

•

0

3 0

Fi gure 3-8 . Suction-Water Cont ent Relat ionship of Weathered Upper Midway Soil on Drying at 25 C

3 2

50

1 0

5

1 .0

0.5

\ TOTA L SUCTION OF' BORING \ 6 A G C - 4!1, SPECIMEN FROM \ JS- 40 F'T ��������

\

\ � TOTA L SUCTION OF' BORING • y.-PU- 6 , SPECIMEN 26, 31.0 FT

\ \

\ '

\ \

\• TOTAL SUC TION IN SITU---\�

LE G E N D

MA TRIX SUC TION OF BORING PU- 1, SPECI­MEN 36, 4 6. fl - 4 6. 4 FT

e PSYCHROMETER 5 Y PSYCHROMETER 8 I PSYCH ROMETER 28 l!i. PRESSURE MEMBRANE

APPARATUS

\ '\ \

TOTA L SUCTION IN \ SITU OF BORING � \ P U - 6, SPECIMEN 26, � Jl. O FT .,

\

\

\ MA TRIX SUC TION OF' \ BORING PU- 2, SPEC/-MEN 26, Jl.0 - 32. I F T

I

� y

WEATH ERED

U PPER M I D WAY

UNWEATHERED

YAZOO C LAY

4

3

O . IL-������-L-�������.i.-������--J1--������_._������--- 2 0 I 0 20 30 40 50

WATER CONTENT, 0/o DRY WEIGHT

Figure 3-9 . Suction-Water Content Relationship on Drying for Undisturbed Soil

59

k. 0.. z 0 � I) :::> II)

the osmotic component of suction pressure . Suction pressure data from

the Yazoo clay given in fig . 3-9 ·compare closely with the pressure mem­

brane data , subst antiating the leached nature of this soil . These

findings support the de finition that the total suction is the sum of

matrix and osmotic components .

CHAPTER IV

CONCLUSIONS AND RECOMMENDATIONS

Suction in soil spe cimens not subj ect to overburden loads can be

me asured from zero to more than 88 atms with the present apparatus .

Reproducibility is about + 1 atm or less up to 50 atms of pressure and

about + 2 atms from 50 up to about 88 atms . Samples may be remoulded

or undisturbed soils of almost any size .

Psychrometers made from 1- and 2-mil-diam chromel-constantan ther­

mocouples are most suitable for the me asurement of suction . Calibra­

tion curves of these devices are in re asonable agreement with calcula­

tions made by theoretical equations and are es sentially independent of

the design . Individual calibration curves are required for maximum ac ­

curacy . The sensitivity of psychrometric readings i s greater with

smaller diameter thermocouples , such as psychrometers with the 2-mil

or slightly more sensitive one mil thermocouple . The suct ion range

c an be extended by using shorter thermocouple lengths and larger ther­

mocouple and junction (bead ) diameters .

Sufficient temperature stability can be reali zed by enclosing the

soil spe cimen in a metallic- he.at_ sink_,__ such_ aa_ a metal cont ainer placed

within an insulated chest . The apparatus should be operated in a room

not subj ect to rapidly changing temperatures for best results .

Suction measurement s of soil spe cimens support the definit ion that

tot al suction i s the sum of matrix and osmoti c components . Total suc­

tions measured from psychrometers of various de signs were consistent

61

with matrix suctions from pressure membrane apparatus and osmotic suc­

tions from electrical conductivity measurements .

Further work would be useful in which these devices are used in

measurement of suction in soils subject to overburden loads and shear

stresses . Information regarding suction pressure is applicable to the

evaluation of in situ heave behavior of expansive clays , rates of

heave , and computation of effective stresses in partly saturated soils

for the design of foundations .

62

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2 . Croney , D . , Coleman , J . D . , and Black , W. P. M . , "Movement and Distribution of Water in Soil in Relation to Highway Design and Performance , " Highway Research Bulletin Report No . 40 , NAS-NRC Publication 629 , 195 8 , pp 226-251 . ·

3 . Black , W. P . M . , "A Method o f Est imating the Cali fornia Bearing Rat io of Cohesive Soils from Plasticity Data , " Geotechni que , Vol 12 , 1962 , pp 271-282 .

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6 . Richards , B . G . , "Moisture Flow and Equilibria in Unsaturated Soils for Shallow Foundations , " Permeability and Capillarity of Soils , ASTM STP 417 , Aug 1967 , pp 4-34 .

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8 . Foster , W . R . , Savins , J . G . , and Waite , J . M . , "Lattice Expansion and Rheological Behavior Relationships in Water-Montmorilloni te Systems , " Clay and Clay Minerals , Proceedings of the Third Na­tional Conference , NAC-NRC Publication 395 , 1955 , pp 296-316 .

9 . Moore , R . T . , and Caldwell , M. M . , "Field Use of Thermocouple Psy­chrometers in Desert Soils , " Symposium on Thermocouple Psychrom­eters 2 17-19 March 1971 , Utah State University , Logan , Utah .

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63

11 . Richards , B . G . , "Psychrometric Techniques for Measuring Soil Water Potential , " Division of Soi l Mechanics Technical Report No . 9 , 1969 , Commonwealth Scientific and Industrial Research Organization , Australia.

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64

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37 . Hoffman , G . J . , and Splinter , W . E . , "Instrument ation for Measur­ing Water Potenti al of an Intact Plant-Soil System , " Transact ions of the ASCE , Vol 11 , No . 1 , 1968 , pp 38-40 , 42 .

38 . Klute , A . , and Richards , L . A. , "Effect s of Temperature on Rela­t ive Vapor Pressure of Water in Soi l : Apparatus and Preliminary Measurement s , " Soil Science , Vol 93 , 1962 , pp 391-396 .

39 . Campbell , E . , "Vapor Sink and Thermal Gradient Effect s on Psy­chrometer Calibrations , " Symposium on Thermocoupls Psychrometers 2 17-19 March 1971 , Utah State Univers ity , Logan , Utah .

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41 . Hoffman , G . J . , and Herkelrath , W . N . , 'iDesign Features of Int act Leaf Thermocouple Psychrometers for Measuring Water Potential , " Trans . Amer. Soc . Agr . Engrs . , Vol 11 , 1968 , pp 631-634 .

42 . Mohsin , M . A . , and Ghildyal , B . P . , "Thermocouple Psychrometer -ror -water Pot-ential -Measurement-s i:n Paddy Plant s -: I , Principle and Design Problems , " Estratto da , Rivi sta IL Rl50 - Ann XIX , N . 4 , Dicrembre 1970 , pp 293-307 .

43 . Hsieh , J . J . C . , and Hungate , F . P . , "Temperature Compens ated Peltier Psychrometer for Measuring Plant and Soil Water Pot en­tial , " Soil Science , Vol 110 , No . 4 , 1970 , pp 253-257 .

44 . Lang , A . R . G . , "Psychrometri c Measurement of Soil Wat er Potential In Situ Under Cotton Plants , " Soil Science , Vol 106 , No . 6 , Dec 1968 , pp 460-464 .

66

45 . Campbell , G . S . , "Water Potential Measurements of Soi l Samples , " Symposium on Thermocouple Psychrometers 2 17-19 March 1971 , Utah State University , Logan , Utah .

46 . Oster , J . D . , Rawlins , S . L . , and Ingvalson , R . D . , "Independent Measurement of Metric and Osmotic Potential of Soil Water , " Soil Science Society of America Proceedings , Vol 33 , 1969 , pp 188-192 .

47 . Lopushihsky , W . , "Microwelder for the Construct ion of the Thermo­couple Junction of Peltier Type Thermocouple Psychrometers , " � posium on Thermocouple Psychrometers 2 17-19 March 1971 , Utah State Univers ity , Logan , Utah .

48 . Campbell , G . S . , and Gardner , W . H . , "Psychrometric Measurement of Soil Water Potential : Temperature and Bulk Density Effects , " Soil Science Society of Ameri ca Proceedings , Vol 35 , 1971 , pp 8-12 . ��

49 . Rawlins , S . L . , "Water Trans fer from Soil to the Atmosphere as Related to Soil Properties , Plant Characteristics , and Weather , " Research Report No . 404 , De c 1968 , U . S . Salinity Laboratory , Agricultural Research Series , U . S . Department of Agriculture , Riverside , Calif .

50 . Johnson , L . D . , "The Influence o f Negative Pore Water Pre ssure on the He ave Behavior of Foundation Expansive Soils , " Miscellaneous Paper S-73-17 , Apr 1973 , U . S . Army Engineer Waterways Expe riment Station , Vicksburg , Miss .

51 . U . S . Army Engineer District , Fort Worth , "Investigat ions for Building Foundations in Expansive Clays , " Vol 1 , Fort Worth , Tex . , Apr 1968 .

52 . Johnson , L . D . , "Properties of Expansive Clays - The Jackson Field Test Section Study , " Interim Report 1 , Mi scellaneous Paper S-73-28 , May 1973 , U . S . Army Engineer Waterways Experiment Station , Vicksburg , Mi ss .

5 3 . Sowers � G . F . , "Higll Volume Chan�e ClaJ'"S o f the Southeastern Coastal Plain , " Proceedings of the Third Panamerican Conference on Soil Mechani cs and Foundation Engineering , Caracas , Venezuela , Vol 2 , Jul 1967 .

54 . Carlson , C . A . , "Evaluation of Influence of Negative Pore Pressure Development in Expansive Des i c cated Clays on Building Behavior -Soil Solut ion Ex.traction from Lackland City , " Unpubli shed Progress Report No . 6 , U . S . Army Engineer Waterways Experiment Station , Vicksburg , Mi ssissippi , Jan 1970 .

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1. O R I GIN A T I N G AC: T I V I TY (Corpor•I• eulhor) Uo. R E P O R T S E C U R I T Y C: L ASSI F I C: A TI O N

u. S. Army Engineer Waterways Vicksburg, Mi s s i s s ippi

Experiment Station TT- � 1 � - - � f'i <>n 2b. GR OUP

J. R E P O R T T I T L E

AN EVALUATION OF THE THERMOCOUPLE PSYClffiOMETRIC TECHNIQ.UE FOR THE MEASUREMENT OF SUCTION IN CLAY SOILS

•• D E S C RI P T I V E N O T E S (T)'p• ol r•potl •nd lnc/ue1 ... . , •• , Final report

1. AU T H O R ( S J (Flr•t IMlm•, middle lnllle/, /ael n•m•)

Lawrence D. Johnson

a. R E P O R T DA T E 7e. T O T AL N O . O F P A G E S rb. 54· O F R E F S

January 1974 77 la. C ON T R A C T O R G R A N T N O . ea. O R I G I N A T O R ' S R E P O R T NU"'8E RtSJ

b. " RO.IEC: T N O . Technical Report S - 74-1

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d.

1 0. DI S T RI BU T IO N S T A T EM E N T

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1 1 · S U P P L EM E N T A R Y N O T E S 1 2. S P O N SO RING M I L I T A R Y A C TI V I T Y

Office , Chief of Engineers , u. S . Army Washington , D . c .

1 3 . A B S T R A C: T

Suction or negative pore water p re s sure i s important in controlling the physical properties of le s s than fully saturated soils . Methods for the accurate measurement of a wide range of suctions in such soils are not well known and are under develop-ment . This study to evaluate the thermocouple psychrometers as a technique for the measurement of suction was initiated because the se devices are relatively inexpens ive , simple to use , allow rapid measurement s , and po ssess a large range . Apparatus was desi gned , constructed , and tested for the thermo couple psychrometric measurement of suction pres sure in clay so ils without the need for preci se t emperature control . This report reviews the background literature leading to the development of the appa-ratus , de scribes the equipment , outlines procedures for making psychrometric measure-ment s . A study of various types of thermocouple psychrometer s showed that psychrom-eters with 1- or 2-mil-diam chromel-constantan thermo couple s provide a range of total suction up to about 90 atm. The reproducibility is about .:!: l atm or le ss for sue-tions less than 50- atm ami about- .:!: 2 atm for- suctions- grea�er- than 50 atm� Suction._ measurement s of Yazoo clay from Jackson , Mi s s i s s ippi , and clay from the weathered Upper Midway format ion, Lackland Air Force Bas e , Texas , support the definition that total suction i s the sum of matrix and osmotic component s .

DD .'!': .. 1 473 RE .. LACES DD FORM 1•71, I .IAN ••. WHICH 1 9 OBSOLETE F O R ARMY UIE. Unclassified

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' ' · L I N K A L I N K 8 L I N K C K IE V WO ll D I

llO L IE W T R O L E W T ll O L E W T

Clayey so ils

Negative pore water pre s sure

Psychrometers

Soil properties

Soil suction

Thermocouple psychrometers

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Unclassified Security Claaaiflcation