Chapter 4 Temperature Measurement

CHAPTER 4

TEMPERATURE TEMPERATURE TEMPERATURE TEMPERATURE MEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENT

ObjectivesAfter finish this lesson, you will be able to:

Define the temperature

Convert temp unit

Realize the categories of temp device

Understand the important of thermowells Understand the important of thermowells in temp device

Apply the principle and operation of temp device (RTD. TC, filled-system, bimetallic)

Introduction TemperatureTemperatureTemperatureTemperature is a physical property of

matter that quantitatively expresses

the common notions of hot and cold.

most widely used since 1959 by most widely used since 1959 by

Galileo

technology is continuously improved

typical assembly consists of a

thermowell, temperature element,

extension, temperature transmitter

Measurement Unit

common: Fahrenheit & Celsius

F, C & K recognized internationally

F & C developing from 2 fixed point: ice

& steam, at atmospheric pressure& steam, at atmospheric pressure

conversion

=

59CF 15.273= KC

used to protect the element protect the element protect the element protect the element

If well not required, clear label attached to

element to indicate no well present

T/Ws create time delay. Without well has

Thermowells

T/Ws create time delay. Without well has

1-10s time delay, a well 20-50s delay

Used in most cases, where temperature elements are installed.

There are exceptions to this rule, such as in

Internals of some equipment (compressors, turbines)

Bearings, where space is very limited

Surface temperature measurement

Fast response applications

Air-space temperature measurements

T/W construction & material must carefully matched with process requirement

Material vary with the application & required speed of responsespeed of response

- metal: max varies from 800F (iron) - 2300F (inconel)

- ceramic: max 1900F (fused silica) - 3000F (silicon carbide)

TEMPERATURE MEASUREMENT DEVICES

Principles

every metal unique composition & has a

different resistance to flow electrical

Resistance Temperature Detector (RTD)

different resistance to flow electrical

current

most metal - changes in electrical resistance

directly proportional to change in

temperature; linear

it is called temperature coefficient of

electrical resistance (TCR)

RTD is regarded high precision wire

wound resistor; resistance varies with

temperaturetemperature

by measuring resistance, temperature can

be measured

RTD

ConstructionsConstructionsConstructionsConstructions

pure metals pure metals pure metals pure metals (platinum, nickel, copper)

typical probe contains a coil coil coil coil of very fine metal wire; allowing large resistance change without great space requirementchange without great space requirement

common: platinum RTD - accuracy & linearity

Accurate sensor - can measure

temperature change of 0.00001 C

usually protected from the environment

by a sheathsheathsheathsheath made of stainless steel or

another temperature & corrosion

resistance materialresistance material

element fits snugly inside sheath to

produce high rate of heat transfer

fine powder fine powder fine powder fine powder used to eliminate air pockets

ceramics insulators ceramics insulators ceramics insulators ceramics insulators used to isolate

internal lead wires.

At the end of tube a hermetic seal, a hermetic seal, a hermetic seal, a hermetic seal, to

protects the element

assembly may be terminated with lead

wires/ may supplies with an appropriate

terminal block similar to TC assembly

Operations

To detect small variations of resistance, temperature transmitter in form of Wheatstone bridge Wheatstone bridge Wheatstone bridge Wheatstone bridge is used

the circuit compared RTD value with three the circuit compared RTD value with three known & highly accurate resistors

Wheatstone; available 2 wire, 3 wire, 4 wire elements

Wheatstone bridge consist: 3 resistors, voltmeter, voltage source

when current flow in the meter is zero (voltage point A = voltage point B); null balance

this is set point on RTD temperature this is set point on RTD temperature output.

As RTD temp increase, voltage increase

voltage transducer replaces voltmeter, 4-20 mA signal can be monitored

when RTD is install some distance away some distance away some distance away some distance away

from transmitter, it caused problem

long connecting wires; resistance of wires

changes as ambient temperature fluctuates

variations in wire resistance would variations in wire resistance would

introduce errorerrorerrorerror in the transmitter

eliminate problem; 3-wire RTD is used

(impedance in wires will cancel because

they are in opposite legs of the bridge)

RTD

RTD Advantages

most stable, more accurate at moderate

temperature

less susceptible to electric noise less susceptible to electric noise

operate higher level of electrical signal

response time very fast compared

thermocouple (faction in sec)

more sensitive & more linear than TC

not experience drift problems because not

self-powered

not required special extension cable not required special extension cable

radioactive radiation has minimal effect

RTD Disadvantages

more expensive (purest metal)

not capable measuring as wide temp range as TC

power supply failure, cause erroneous power supply failure, cause erroneous reading

small changes in resistance (vibration, not tight, corrosion) create error

resistance curve vary from manufacturer

accuracy & service life are limited at high

temperature

RTD can found in reactor area temperature

measurement & fuel channel coolant measurement & fuel channel coolant

temperature

Principles

consist 2 pieces of dissimilar metals with their ends joined together (twisting, soldering, welding)

Thermocouple

soldering, welding)

when heat applied, voltage (mV) is generated

the joined produce a thermal electromotive force (emf) when junctions at different temperature

TC is self-powered

Typical response time of bare TC

(0.2 -12s)

Constructions

TC wires are manufactured to close

tolerances & tend to be expensive

their limited is to probe itself

TC extension wires, used as a link between TC extension wires, used as a link between

TC & measuring device/transducer

Thermocouple

3 basic types of TC construction

Ceramic beaded

Insulated (plastic, glass, ceramic fiber)

Metal-sheathed mineral-insulated

(MSMI); extruded (sheath stainless (MSMI); extruded (sheath stainless

steel/inconel & mineral Al2O3/MgO)

TC can be constructed to be protected/

exposed

protected; can be grounded/ungrounded

grounded: give faster response, but

susceptible to electrical noise

ungrounded: slower response, electrically

isolated

TC may be spring-loaded, so the tip & well

surface remain in contact to ensure good

heat transfer

if exposed, the faster response is provided,

but the wires are totally unprotected

when TC get thinner:

the recommended upper temp limit is

reducedreduced

the error decreases & the response is

faster to temp changes

the element becomes more fragile

at high temp, accuracy is more sensitive to

material (wire impurities)

OperationsTC: 2 junctions for measuring.

hot junction: the end inserted in the medium to measured temperature

Hot junction

cold junction: connected to measurement device (e.g. milimeter, potientiometer, galvanometer)

~ voltage generated depends on temperature

Cold junction

in a circuit, loop current

depends on relative

magnitude of voltage.

Detect by galvanometer

to measure temp, one

end contact with end contact with

process, other end kept

at const temp

emf thermocouple

increases when the

difference in junction

temp increases

relationship between total circuit voltage

(emf) & emf at the junction is:

if circuit emf & reference emf known,

Circuit emf = Measurement emf Reference emf

if circuit emf & reference emf known,

measurement can be calculated

convert to 4-20 mA signal, transmitted is

needed: temperature transmitter

the temp measurement circuit consists a TC connected directly to temperature transmitter.

hot & cold junction can located wherever required to measure temp diff

Need monitor the temp rise to ensure the safe operation

T rise of a device is the operating T using ambient or room temperature as the reference.

Types of TC

Copperconstantan (Type T)

pure copper (+ve) element &

constantan (-ve) elementconstantan (-ve) element

Constantan: group of alloys, contain

approximately 55% copper & 45%

nickel.

Iron constantan (Type J)

- iron (+ve) element & constantan (ve)

element

- can be applied in oxidizing / reducing - can be applied in oxidizing / reducing

atmospheres.

Chromel Alumel (Type K)

- Chromel (+ve) element & alumel (-ve)

element

- Chromel is an alloy with a nominal

composition of 90 % nickel & 10 % composition of 90 % nickel & 10 %

chromium

- Alumel contains 95 % nickel plus

aluminum and silicon with manganese

- Chromel alumel TC must be used in

oxidizing / neutral atmospheres

Chromel constantan (Type E)

- Chromel (+ve) element & Constantan (-ve) element

- highest emf per degree change of temp of any commonly used TC

- suitable for oxidizing atmospheres & - suitable for oxidizing atmospheres & not corrode at sub-freezing temp

- operating limits of -180oC and +870oC when protected and is available in wire

Advantageso resistance to corrosion in moist atmosphere,

limits error below 0oC, suitable for

subfreezing temperature measurement.

o can be furnished higher degree of accuracy o can be furnished higher degree of accuracy

for temp between -270 & +400oC than any

other commonly used thermocouple

o can be applied in either oxidizing /reducing

atmospheres between stated temp

o used on most transformers

(hot junction inside the transformer oil & cold junction at the meter mounted on the outside).

o used exclusively around the turbine hall because of their rugged construction and because of their rugged construction and low cost.

o capable of measuring a wider temperature range than an RTD

o self-powered

o simple & rugged

o inexpensive (half-price of RTD)

o wide choice of physical forms

can be calibrated to generate a specific o can be calibrated to generate a specific

curve & easy to interchange

o fast response & measurement at one

specific point

Disadvantages

TC located some distance away from the

measuring device, expensive extension

wires / compensating cables have to be

used.used.

TC are not used in areas where high

radiation fields are present.

generate non-linear output & low voltage

required a reference junction

have a low sensitivity

limited in accuracy

need type-matching extension wiresneed type-matching extension wires

slower response than RTDs

susceptible to stray electrical signal

Principles

metallic assembly consists a bulb, small-diameter tubing (capillary) & Bourdon spring

Filled-Systems

Bourdon spring

indicator linked to Bourdon tube indicates temp

sometimes bellows & diaphragm are used

system filled with a liquid / gas that expands & contract as the temp sensed at the bulb increased & decreased

expansion / contraction translated to mechanical translated to mechanical motion

liquid causes volume changes & gas causes press changes

this device generally used for local indication

Filled-systems

Operations

an improvement

liquidin-glass

thermometer

need no power to need no power to

function, simple,

rugged, self-

contained, accurate

over narrow temp

span

bulb may be too large for existing

application & on system failure, the whole

system must be replaces

capillary tubing is limited to a distance of

250 ft

slow to respond & relative expensive slow to respond & relative expensive

it is susceptible to ambient temp changes

around the capillary & ambient temp

compensation is often required

occasional checking & testing required to maintain accuracy

capillary tubing should be continuously supported & protected against damage

capillarys construction material capillarys construction material should be compatible with the surrounding environment

bulb must be immersed sufficiently to ensure that the actual temp is being measured

Vapor Liquid

Principles

a spiral made of two different metals, having different coefficient of expansion;

Bimetallic

having different coefficient of expansion; expands as the temp increase

movement by expansion drives an indicator on a scale

industrial bimetallic use a helical; coil to fit inside a stem

most temp

switches operate

on this principle;

but guides

provide min of

friction for the friction for the

moving

component

Operations

generally used in local temp gages & switches

To facilitate reading all-angle types usually are selected with a 5 in. diameter usually are selected with a 5 in. diameter dial

capillary type is sometimes used for operating visibility

if vibration exist; thermometer may be

filled with a dampening fluid (compatible

with the process fluid, in case of leakage)

simple construction, has few moving parts

& required little maintenance& required little maintenance

lowest cost among temp device, low

accuracy & provides no remote indication

To calibrate; must be immersed in a bath of

known temp

They are simple,

robust and inexpensive.

Their accuracy is

between +or- 2% to 5%

of the scale.

They are not

recommended for

temperature above

4000C.

When regularly used,

DISADVANTAGES ADVANTAGES

of the scale.

They can with stand

50% over range in

temperatures.

When regularly used,

the bimetallic may

permanently deform,

which in turn will

introduce errors.

Coefficient of expansion.

The following are the important properties a

material should have to be selected for bimetallic

thermometers.

Coefficient of expansion.

Modulus of elasticity.

Elastic limit after cold rolling.

Electrical conductivity.

Ductility.

Metallurgical ability.

Thermometer, resistance bulb, TC & bulb systems are placed in thermowell.

Thermowell protects the measuring device from corrosive and erosive effect of measured

Temperature Measurement Safety

corrosive and erosive effect of measured medium.

Bare TC is install to increase measurement speed and sensitivity.

Primary temp measuring devices can be connected to indicator, recorder & controller.