industrial instrumentation module 2

Industrial Instrumentation

Lecture 2

8/13/2009 2

Measurement of Pressure

Module II

Units of pressure – manometers – different types – elastic type pressure

gauges –Bourdon tubes– bellows – diaphragms – Electrical methods – elastic

elements with LVDT and strain gauges –capacitive type pressure gauge –

Piezo resistive pressure sensor – capacitive type– resonator pressure

sensor.

Measurement of high pressure. Low pressure measurements – McLeod

gauge – thermal conductivity gauges –Ionization gauge– hot cathode and

cold cathode types – testing and calibration of pressure gauges – dead

weight tester– Differential pressure transmitter.

8/13/2009 3

Pressure

� Pressure = Force / Area

� Stress ( Solids )

� Pressure ( Liquids and Gasses )

8/13/2009 4

Measurement of Pressure

� Not an independent variable

� Derived from force and area

� Depends on other factors like elevation,

density, temperature, flow etc

� Usually expressed in terms of atmosphere

� Height of barometric column at 0º C at sea

level

� 76 cm of mercury

8/13/2009 5

Units

� SI unit – Pascal – Metric Units

� 1 psi = 6894.76 Pa

� 1 Pa = 1 Newton/m2

� 1 atm = 10,332 Kg/m2

� 1 Torr = 1 mm of Hg

� Pounds per square inch (psi) –

British Units

� Pressure = Height x Density x g

8/13/2009 6

Units

8/13/2009 7

Terms and definitions

� Density : Mass per unit volume of a material (Kg/m3)

� Specific weight : Weight per unit volume (N/m3)

� Specific gravity : Ratio of the density of a material to that of

water/air ( dimensionless )

� Static Pressure : Pressure of fluids and gasses when not moving

� Total vacuum : which is zero pressure or lack of pressure, as

would be experienced in outer space

� Vacuum : Pressure measurement made between total vacuum

and normal atmospheric pressure (14.7 psi)

8/13/2009 8

Terms and definitions

� Absolute pressure is the pressure measured with respect to a

vacuum and is expressed in pounds per square inch absolute

(psia)

� Gauge pressure is the pressure measured with respect to

atmospheric pressure and is normally expressed in pounds per

square inch gauge (psig)

� Differential pressure is the pressure measured with respect to

another pressure and is expressed as the difference between

the two values

8/13/2009 9

Manometers

� Low range pressures

� Simplest and accurate

� Within 2 Kg/cm2

� Differential pressure

� U-tube

� Well type

8/13/2009 10

Manometers

� Inexpensive

� Clear plastic / Glass

� Inclined manometers –

increased sensitivity and

resolution

8/13/2009 11

Manometers

8/13/2009 12

Manometers

8/13/2009 13

Ring balance manometer

•Hollow ring ( transparent polythene tube)

•Manomeric fluid ( paraffin or kerosene )

•∆pαr=RMg sinθ

•∆p= kθ

•50 Pa to 125 KPa

8/13/2009 14

Manometer fluids

•Should have high density ( not for low p)

•Low vapour pressure

•Freely movable

•Incompressible

•Chemically inert

•Non sticky

•Surface tension errors

8/13/2009 15

Elastic type pressure gauges

•Bourdon tubes or pressure springs

•Bellows

•Diaphragms

•3 to 6 MPa bellows and diaphragms

8/13/2009 16

Bourdon tube pressure gauge•Eugene Bourdon ( 1849 )

•Changes shape when pressure is applied

•Bellows

•Diaphragms

•3 to 6 MPa bellows and diaphragms

•C-type, spiral type, twisted tube type and helical

•The free end of bourdon tube produces

displacement or rotation on application of pressure

•Can be converted to electric signals using LVDT

•Usually called as bourdon gauges

8/13/2009 17

Bourdon tube pressure gauge

•Elliptical cross section

•C shaped ( Arc length of 27º )

8/13/2009 18


•(a) C-type

•(b) spiral type

•(c) twisted tube type

•(d) helical type

8/13/2009 19


8/13/2009 20


8/13/2009 21

Bourdon tube pressure gauge•Tip movement is nonlinear ( compound stress )

•Linear – for small tip movements

•Tip movement must match rotational movement

•Multiplication, angularity adjusted by the lever length

•Rotation inversely proportional to lever length

•Good elasticity gives better repeatability

•Elasticity sensitive to temperature

•Phosphor-bronze, silicon-bronze, beryllium-copper, Nickel

alloys, chromium alloys

8/13/2009 22


∆Ф = KФP f(αααα,β,t,R)/E

∆Ф = Tip deflection

Ф = Arc Length

P = Applied Pressure

α = Width of bourdon element

β = Height of bourdon element

t = Thickness of bourdon element

R = Radius of the bourdon arc

E = Elastic modulus of the element

Sensitivity (S) = ∆Ф / ФP

8/13/2009 23

Diaphragm pressure gauge

•Used for low pressure measurements

•A thin disk of material, Blows outwards

•Converted to twisting or rotational motion

•Mostly metals with spring type

•Cascading capsules increases sensitivity

8/13/2009 24


•Slack diaphragms ( less elastic )

•Used along with coil spring or elastic elements

•For very low pressure

•Animal skin, impregnated silk, Teflon, Polythene etc

8/13/2009 25


•Corrugations improves strength and linear deflection

•Deflection depends on diameter, number, depth and thickness

•Doubling diameter increases deflection 16 times

Deflection (d) = kN(∆P)Dntm/E

N = Number of corrugations

∆P = Applied pressure

D = Diameter of the element

T = Thickness of the element

E = Modulus of elasticity

Usually n=4 and m= -1.5

8/13/2009 26

Bellows Element gauge

•Thin walled cylindrical shells with deep convolutions

•Cascaded capsules sealed at one end, with axial displacement

•Made from continuous piece of thin metal

•Fastening several individual diaphragms

8/13/2009 27


•Turning from a solid stock of material ( low hysteresis )

•Soldering or welding stamped annular rings ( high hysteresis )

•Rolling a tubing

•Hydraulically forming a drawn tubing

•Maximum stroke length and maximum pressure

•Stroke length can be increased by increasing diameter and folds

•Phosphor bronze, silicon bronze, beryllium copper, stainless steel

•Low corrosion, hysteresis loss, pressure range, ease of fabrication

8/13/2009 28


•Used with separate

calibrating springs

•Compression type

•Expansion type

•Receiver elements

•Used in pneumatic loops

d = P1αe/(kb+ks)

αe= effective area

kb and ks are spring constants

8/13/2009 29

Electrical methods

•Resistive, Inductive and capacitive

•Pressure sensitive resistance ( Piezoresistive )

•Elastic elements with strain gauges

•Stretched or compressed

•Resistance changes ( length )

•Primary and secondary

8/13/2009 30

Resistive type

•Silicon wafer as diaphragm and gauge

•Excellent mechanical property (linear)

•Non chemical compatibility

•Isolating diaphragm needed

8/13/2009 31

Resistive type

•Arrangement for high sensitivity

•Temperature compensation

8/13/2009 32

Resistive type

•Bourdon tube, Diaphragm (elastic elements) as primary sensing element

•Potentiometer as the secondary sensing element

8/13/2009 33

Resistive type

•Bellows ( elastic element ) and manometer tube as primary sensing element

•Potentiometer as the secondary sensing element

8/13/2009 34

Resistive type

•Resistance ratio element gauge

•Bellows as primary and strain gauge as the secondary sensing element

8/13/2009 35

Resistive type

•Bridgman type ( primary )

•Pressure from all sides, changes the resistance

•Rp = R0(1+β∆P) , where β = pressure coefficient of resistance

•Used at high pressures ( small β ) and constant temperature (temperature coefficient

of resistance)

8/13/2009 36

Inductive type

•Elastic elements with LVDT

•Deflection of the diaphragm/bellows moves the core

8/13/2009 37

Inductive type

8/13/2009 38

Inductive type

•Reluctance of the coil is altered

•Coils have equal number of turns

•When P1 = P2, e0 = 0

•P1 ≠ P2,

pe

)KdR(2

Kxee

0

0

i0

∆∝

+=

•R0 = Initial reluctance of the coil

•x = small displacement of diaphragm

•d = Initial distance between coil and diaphragm

8/13/2009 39

Capacitive type

•Movable steel diaphragm

•Spherical depression of 0.0025cm

diaphragmof diameter d

diaphragmof ntdisplaceme x

d

Exe~ee 210

=

=

==

8/13/2009 40

Capacitive type

8/13/2009 41

Capacitive type

•Cylindrical capacitor

•Very small change in

capacitance

•0.5% change for 10Mpa change

•Only used at high pressures

•Used along with resonance ckts

8/13/2009 42

Piezoelectric type

•Discovered in 1880 by the Jacques and Pierre Curie brothers

•Certain crystals produce e.m.f when deformed

•Applying pressure along certain axes

•Charge asymmetry within crystal

•Inverse piezoelectric effect ( electric -> mechanical )

•Natural ( Quartz, Tourmaline )

•Synthetic ( Barium titanate, Lead Zirconate )

•Natural crystals are polarized

•Synthetic crystals baked under strong D.C field for polarization

•Direction sensitive

8/13/2009 43

Piezoelectric type

8/13/2009 44

Piezoelectric type

•Electric axes (X), Mechanical axes (Y)

•Synthetic crystals can be moulded into any shape or size

•Mostly used for dynamic pressure measurements

•Surface charge proportional to applied force

•Q = d F , d is the charge sensitivity of the crystal

8/13/2009 45

Piezoelectric type

ysensitivit Voltageg

crystal theof Thicknesst

force Applied F

ysensitivit Charge d

AreaA

gptE

t)A

dF(

C

QE

t

AC

0

r0

0

r0

=

=

=

=

=

=

εε==

εε=

8/13/2009 46

Piezoelectric type

•Quartz is the most stable

•Lower temperature stability, good sensitivity, linearity

•Low hysterisis, chemically inert, elastic nature

•PZT are small in size, light weight and rugged

•Pressure range 1:105

•Temperatures upto 350°C

•Smart sensors ( amplifiers and signal conditioner packed together )

•Not suitable for steady state pressures or slow varying pressures

8/13/2009 47

Comparison of pressure sensors

Made LinearUpto 10KHz0 to 70MPaPiezo electric

Non-LinearUpto 3KHz0 to 10MPaCapacitance

LinearUpto 400Hz0 to 70MPaLVDT with elastic

element

LinearUpto 2KHz0 to 200MPaStrain gauge on

diaphragm

Approx-Linear1 to 100Hz2 to 700MPaElastic Element

Non-Linear0.1 to 1HzUp to 0.2MPaRing Balance

Linear1 to 2HzUp to 0.2MPaManometer

LinearityFrequency rangePressure rangeType

8/13/2009 48

Low pressure measurement

•Conventional methods unable to measure very low pressures

•Low pressures are measured in torr

•Classified into mechanical and electrical types

10-3Diaphragm

10Bourdon tube

0.1Bellows

0.1Manometer

Lowest measurable pressure (Torr)Type

8/13/2009 49

Mechanical Type•McLeod gauge

•Compress a small quantity of low

pressure

•P = K h2

•Independent of gas composition

•Error if condensable gas present

•Not suitable for continuous

measurement

•Non linear scale

•Lowest pressure 10-4 torr

•Mercury vaporizes below this value

8/13/2009 50

Thermal Type

•At normal pressure heat conductivity is independent of pressure

•At low pressures, heat conductivity starts falling

•Due to low availability of gas molecules

•Lesser number of molecules take part in carrying away the heat

•Thermal gauges utilizes this lowering of heat conduction

•Heat loss due to conduction, convection and radiation must be small

•Convection loss is negligible

•Conduction loss due to lead wires minimized by proper construction

•Radiation loss minimized by low emissivity metal

•Surface deterioration due to oxidation and carbonization

8/13/2009 51

Pirani gauge•Coiled tungsten or platinum wires

•Current of about 10 to 100 mA

passed ( 70 to 400°C )

•Gauge is connected to one arm of

bridge circuit

•At low pressures, the heat

conduction is low, increasing the

temperature of the filament (PTC).

• Three modes- Constant voltage,

constant current and null balance

8/13/2009 52

Pirani gauge•In constant voltage mode, the

current versus pressure is obtained

•In the constant current mode,

variation of the resistance with

pressure is obtained

•Voltage or current is adjusted using

potentiometer for a null reading and

this change in voltage or current is

the measure of the pressure

•Pirani gauge is composition

dependent and require calibration

•Can measure up to 10-3 torr

8/13/2009 53

Thermocouple gauge•Temperature of the hotwire (H) is

now directly measured by

thermocouple (C)

•A constant current is passed

through the heating element

•High impedance voltmeter gives

the reading directly

•Sensitivity is proportional to the

current and pressure

•Mainly used for comparison

purposes

•Composition dependent

8/13/2009 54

Thermocouple gauge

Sensitivity maximum at 10-2 torr for I=22.3 mA

8/13/2009 55

Ionization gauge

•Hot cathode type and cold cathode type

•A potential difference is impressed across a column of gas

•The applied voltage is kept above ionization potential of gas

•Free electrons from cathode are accelerated

•The electrons collide with the gas molecules creating +ve ions

•At low pressures more than one collision is unlikely

•Rate of production of positive ions is proportional to pressure

•Sensitivity of the gauge is given by

g

p

Pi

iS =

8/13/2009 56

•The grid is maintained at higher positive potential

w.r.t cathode and plate

•Plate is kept negative w.r.t cathode

•Positive ions generated between plate and grid are

collected by the plate

•And positive ions between plate and cathode are

collected by cathode

•Internal control type and external control types

•Plate and grids interchanged

Ionization gauge

8/13/2009 57

•Pressure ranges between 10-8 to 19-3 torr

•Negligible response time

•Sensitivity higher for external control type

•Internal control type have more linear range

•Gases with higher molecular weights yields more

ion currents

•More than 10-3 torr, positive ions make greater

impact on cathode and heats up

•Internal control type is more temperature stable

than External type, due to cancellation by

secondary electrons

Ionization gauge

8/13/2009 58

•Cold cathode type

•Philips cold cathode ionization gauge

( PIG)

•Pair of cathodes and a hollow anode

•Magnetic field helps ionization

•Potential above 2KV applied

•Electric field initiates electron emission

•The current produced is not linear with

pressure, due to interaction of +ve ions

•Pressures upto 10 -5 torr

Ionization gauge

8/13/2009 59

•Rectangular box type insulated titanium anodes

•Electrodes are very thin and arranged as cells

•Separation between crate and cathode is small

•Potential in the range of 5KV is applied

•Electrons are emitted from cathode and move

towards anode, creating positive ions ( collision

with gas molecules)

•These created +ve and –ve ions create large

current in external circuit

•Pressures as low as 10 -9 torr can be measured

Ionization gauge

8/13/2009 60

•Radioactive ionization gauge

•Alphatron

•10 -13 to 10 -9 A flows through R

Ionization gauge

•Radioactive ionization gauge uses alpha particles to ionize the gas

•Ions formed are collected by the electrode is propotional to the pressure

•A high impedence meter is used to measre the voltage

•Pressures from 103 to 10 -3 torr can be measured

8/13/2009 61

Testing and calibration•Dead weight pressure gauge

•Used for testing and calibrating pressure gauges

•Pascal’s law ”A change in the pressure of an enclosed incompressible fluid is

conveyed undiminished to every part of the fluid and to the surfaces of its

container ”

•It consist of a handle operated piston, hollow cylinder with measuring piston, oil

reservoir and needle valves

•The cylinder is filled with oil

•Handheld piston is drawn out and oil is allowed to fill the system

•Suitable weights are placed on the piston

•Gauge for testing is connected and piston is moved so that the weight just floats

8/13/2009 62

Dead weight pressure gauge

•p – pA is gauge pressure

•Accuracy is 0.01%

8/13/2009 63

Differential pressure

Transmitter

industrial instrumentation module 2

Documents