thermocouples: capabilities & challenges

Department of Fire Protection Engineering

A. JAMES CLARK SCHOOL of ENGINEERING ● UNIVERSITY of MARYLAND

Thermocouples: Capabilities & Challenges

Ajay V. SinghGraduate Student

Department of Fire Protection EngineeringUniversity of Maryland College Park, MD


Introduction

2Dynamics of Boundary Layer FlamesApril 19, 2023

The basis of thermocouples was established by Thomas Johann Seebeck in 1821 when he discovered that a conductor generates a voltage when subjected to a temperature gradient.

To measure this voltage, one must use a second

conductor material which generates a different voltage under the same temperature gradient. The voltage difference generated by the two materials can then be measured and related to the corresponding temperature gradient.

It is thus clear that, based on Seebeck's principle, thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings

Thermocouple is a relative not an absolute temperature sensor. In other words, a thermocouple requires a reference of known temperature

The above formula is effective only if the reference temperature TRef in the experiment is kept the same as the reference temperature specified on the data sheet.

The temperature at the probe tip can then be related to the voltage output as,


Types of thermocouplesType Materials

Typical range (deg

C)Suitable environment Comments

TCopper (Cu) vs

Constantan-270 to 400

Vacuum, oxidizing, reducing, and inert

High stability at sub zero and cryogenic temperatures

JIron (Fe) vs Constantan

-210 to 1200Vacuum, oxidizing, reducing, and inert

Heavier gauge wire is recommended for long term life above 540 degC since Iron oxidizes at high temperatures

K Chromel vs Alumel -270 to 1370 Oxidizing or inertShould not be used in alternating reducing or oxidizing

atmospheres

EChromel vs Constantan

-270 to 1000 Oxidizing or inertNot recommended for alternating oxidizing or inert

atmospheres

S (Pt-10% Rh) vs Pt -50 to 1768Oxidizing or reducing Relatively strong. Stable calibration. Very accurate at

high temperatures

B(Pt-13% Rh) vs (Pt-

6% Rh)0 to 1820

Oxidizing or reducingRelatively strong. Stable calibration. Very accurate at

high temperatures

R (Pt-13% Rh) vs Pt -50 to 1768Oxidizing or reducing

Relatively strong. Stable calibration. Very accurate at high temperatures

N(Ni-Cr-Si) vs (Ni-

Si-Mg)-270 to 1300

Oxidizing, dry reducing or inert

Very reliable and accurate at high temperatures. Can replace type K thermocouples in many applications.


How to make a thermocouple: Best Practice

Thermocouple wire extension in a flame [13]

Probably, the best way to goVery common design. However, errors become substantial due to wire extension in flames and

other combusting environments [13]


Thermocouples: Data Acquisition

NI-9214, 16 channel, 24 bit ADC

Mettler Toledo load

cell

2D traverse

mechanism

Stepper Motor

Controller

NI cDAQ 9171

NI 9214 has in-built signal conditioner and Cold-junction compensation (CJC) module


Thermocouple: Concepts Support wire diameters should be at least 4 times the wire diameter, so that the

resistance per unit length of the fine wires is significantly higher than that of support wires

Bead diameters and shape vary widely, depending on the technique and care used to join the wires

In practice, most thermocouples have bead diameters in the range Formed junction is not a “sphere” and resembles a “truncated sphere”

(left) Schematic of a typical thermocouple arrangement for combustion-system measurements. Relatively large lead wires are placed in a protective, ceramic rod and then bent outward in order to provide tension for the thermocouple wires themselves and to allow for sufficiently long thermocouple wire segments to reduce conductive heat transfer back to the leads [1]. (right) Electron micrograph (180 X) of the Pt/Pt-13%Rh thermocouple bead. Dark patches are the remnants of soot. [2]

wbw ddd 5.25.1


Thermocouples: Conduction loss

1) Conduction loss 2) Radiation loss3) Uncertainties due to catalytic effects

Heat conduction to support wires is assumed to have negligible effect on the temperature of the junction if , where L is the length of the fine wire [3]. Can be quantified and is negligible for

It is reasonable to assume that there are no radial temperature gradients in the wire, since it can be shown that radial thermal diffusion is always at least one order of magnitude less than the time constant for the wire diameters of 50 [3].

Conduction losses can be reduced by placing the thermocouple along an isotherm. However, in flames it’s a difficult task to achieve (especially in turbulent flames)

wdL 200

ww kr 2

w m

wdL 200


Thermocouples: Catalytic effects Errors in thermocouple measurements due to catalysis can be reduced by coating the

wires with a thin layer of non-catalytic coating Often, silicon dioxide (SiO2) or yttrium oxide (Y2O3, toxic in nature) coatings are used

to prevent exothermic reactions on the possibly catalytic platinum surface [2] Although coating reduces the catalytic effect, it increases the thermocouple diameter,

the response time of the thermocouple, and the radiation losses. It also changes the convective and conductive heat transfer of the thermocouple. Since the coating changes the heat transfer properties of the thermocouple, it is difficult to quantify the effect of catalysis.

SiO2 coatings in a reducing atmosphere can lead to the formation of silicon solid solution in the legs of a thermocouple. This would shift the Fermi-energy levels of the two components of the junction and can de-calibrate the potential difference output [2].

Thermocouple coating microscopic images []


Thermocouples: Radiation Loss

4 8 21.507 10 1.596 10T T

)(),( 44surrb

bgbg TT

dt

dTUTTT

b

pb

hA

cm

dt

dTcV

dt

dTcmQQQQ b

pbbb

pbcondradconvcat 0 0

)()( 44surrbb

bpbbgb TTA

dt

dTcmTThA

An energy balance on the thermocouple takes the following form [1]:

For an unsteady system, it becomes

where

h

(1)

(2)

(3)

For a steady system, it becomes

)( 44surrbbg TT

kNu

dTT

Emissivity of Pt can be represented as a function of absolute temperature [2]

(4)

Appropriate Nusselt number correlation should be used to model the convective heat transfer about the thermocouple.

k

hdNu

Here ‘d’ represents the diameter of fine wire or thermocouple bead depending on whether Cylindrical or Spherical Nu assumption is

used

Can be quantified precisely and accurately.


Spherical Nusselt number Correlations

For spherical bead approximation,

31

21

, PrRe60.00.2 dsphdNu Ranz and Marshall (1952) [4]

41

4.032

21

, Pr)Re06.0Re4.0(0.2

sddsphdNu

Whitaker (1972) [5]

For Reynolds number between 0 and 200. Properties evaluated at

(1)

(2)

380Pr71.0 76000Re5.3

Properties evaluated at . is the gas viscosity evaluated at the surface temperature

T

T s

(3) 33.06.0, PrRe37.02 sphdNu [2]

For all Re and Pr numbers of interest in low flow velocities


Cylindrical Nusselt number CorrelationsFor cylinders, many Nusselt number correlations have been introduced,

(1) For the low Reynolds numbers applicable for fine-wire thermocouple measurements in combustion systems, the Collis and Williams (1959) correlation is most commonly used (Cylindrical Nu number correlation preferably that of Collis and Williams should be used) [6],

(2) Another widely quoted correlation is that due to Kramers (1946) [7]

(3) Andrews et. al. (1972) [8] evaluated the following expression for ,

17.0

45.0, )Re56.024.0(

T

TNu m

dcyld 44Re02.0

With the Reynolds number evaluated at the so-called “film temperature”, 2

TTT b

m

21

312.0

, RePr57.0Pr42.0 dcyldNu 10000Re01.0

With the gas properties evaluated at 2

TTT b

m

45.0, Re65.034.0 dcyldNu

2

TT

T bmGas properties evaluated at

20Re02.0


Conclusions

A cylindrical Nu correlation (preferably that of Collis and Williams) should be used, with estimation of the local flow velocity when it is known.

It is better not to coat the thermocouple wires with a non-catalytic coating as it increases uncertainties and errors that cannot be quantified.

Conduction losses can be avoided if fine wire thermocouples are used (50-75 ).

Radiation loss from thermocouples at high temperatures is a major source of error in thermocouple measurements.

In turbulent flames, unsteady term comprising the time constant cannot be neglected if we wish to capture temperature fluctuations at high frequency.

Emissivity of thermocouples varies with temperature and surface characteristics including roughness, coating of the metal. Variation of emissivity with absolute temperature can be estimated for certain thermocouples.

m


References

[1] C.R. Shaddix, Proceedings of the 33rd National Heat Transfer Conference (1999) Aug 15-17, pp. 1-9[2] J.A. Ang, P. J. Pagni, T.G. Mataga, J.M. Margle and V.L. Lyons, AIAA Journal 26 (1988), No. 3, pp. 323-329[3] Yule, A. J., Taylor, D. S., and Chigier, N. A. (1978), J.Energy 2:223.[4] Ranz, W. E., and Marshall, W. R., Jr. (1952), Chem.Engr. Progress 48:141-146 and 173-180.[5] Whitaker, S., AlChE Journal, Vol. 18, No. 2, pp. 361-371, 1972[6] D.C. Collis, M.J. Williams, J. Fluid Mech. 6 (1959), pp. 357-384[7] Kramers, H. (1946), Physica 12:61.[8] Andrews, G. E., Bradley, D., and Hundy, G. F. (1972), Int. J. Heat Mass Transfer 15:1765-1786.[6] Ballantyne, A., and Moss, J. B. (1977), Combust. Sci.and Tech. 17:63-72.[7] Bradley, D., and Mathews, K. J. (1968), J. Mech. Engr.Science 10:299-305.[8] Bradley, D., Lau, A. K. C., and Missaghi, M., Combustion Science and Technology, Vol. 64, pp. 119-134, 1989[9] Heitor, M. V.; Taylor, A. M. K. P.; Whitelaw, J. H. (1985), Experiments in Fluids 3, 109-121[10] Lockwood, F. C.; Moneib, H. (1980), Comb. Sci. Tech. 22, 63-81[11] Lockwood, F. C.; Moneib, H. (1981), Comb. Sci. Tech. 26,177-181Miles, P. C., and Gouldin, F. C. (1993), Combust. Sci.and Tech. 89:181-199.[12] Yule, A. J.: Taylor, D. S.; Chigier, N. A. (1978), AlAA paper 78-30[13] R. Ghoddoussi, An Investigation of Thermal Characteristics of Premixed Counter flow Flames using Micro thermocouples, MS thesis, University of Maryland College Park, 2005.[14] M. Jakob, Heat Transfer, Vol. 1, Wiley, New York, 1949.[15] C. D. Hodgeman (ed.), handbook of Chemistry and Physics, 42nd ed., CRC Pess, Cleveland, 1960[16] G.G. Gubareff, J.E. Janssen, R.H. Torborg, Thermal Radiation Properties Survey, 2nd ed., Honeywell Research Center, Minneapolis-Honeywell Regulator Co., Minneapolis, 1960.

Questions?

14Dynamics of Boundary Layer FlamesApril 19, 2023

thermocouples: capabilities & challenges

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rh vs pt

known reference temperature

reference of known temperature

alternating reducing

dry reducing

si vs nisi

high temperatures rpt