CLD 20202.:Karte 04 :.

Temperature Measurement

By:MOHD SYAZWAN B MOHD GHAZALI

(521801)SECT. OF CHEM. ENG. TECH. (SCET)

UniKL MICET

Objectives

After finish this lesson, you will be ableto:

• Define the temperature• Convert temp unit• Realize the categories of temp device• Understand the important of thermowells in temp

device

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

Keywords

• Temperature• Thermowells• Resistance temperature detector (RTD)• Wheatstone bridge• Thermocouples (TC)• Type J• Type K• Type E• Type T

• Filled-system• Bimetallic• Mercury thermometer

Introduction

• 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• conversion

5

9CxF

15.273 KC

Categories

• physical properties that change with temperature are used. eg: material expansion when heated used in bimetallic & filled-system measurement

• electromotive force used in thermocouple & electric resistance change in RTD

Thermowells

• used to 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 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 limitedSurface temperature measurementFast response applicationsAir-space temperature measurements

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

• material vary with the application & required speed of response

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

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

RTDPrinciples

• every metal – unique composition & has a different resistance to flow electrical current

• most metal, changes electrical resistance directly proportional to change of temperature; linear

• it is called temperature coefficient of electrical resistance (TCR)

• RTD is regarded high precision wire wound resistor; resistance varies with temperature

• by measuring resistance, temperature can be measure

Constructions

• pure metals (platinum, nickel, copper)• typical probe contains a coil of very fine metal wire; allowing

large resistance change without great space requirement• common: platinum coz accuracy & linearity

• can measure temperature change of 0.00001C• usually protected from the environment by a sheath

made of stainless steel/another temperature & corrosion resistance material

• element fits snugly inside sheath to produce high rate of heat transfer

• fine powder used to eliminate air pockets• ceramics insulators used to isolate internal lead

wires; at the end of tube a hermetic seal protects the element

• assembly may be terminated with lead wires/ may supplies with an appropriate terminal block similar to TC assembly

Operations

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

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

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

• 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• temp increase, voltage increase • voltage transducer replaces voltmeter, 4-20 mA

signal can be monitored

• Problem: RTD install some distance away from transmitter

• connecting long wire, resistance of wires changes as ambient temperature fluctuates

• variations in wire resistance would introduce error in the transmitter

• eliminate problem; 3-wire RTD is used (impedance the wires will cancel because they are in opposite legs of the bridge)

RTD Advantages

• most stable, more accurate at moderate temperature

• less susceptible to electric noise• operate higher level of electrical signal• response time very fast compared thermocouple

(fraction in sec)

• more sensitive & more linear than TC• not experience drift problems because not self-

powered• not required special extension cable• radioactive radiation has minimal effect

RTD Disadvantages

• more expensive than TC (purest metal)• not capable measuring as wide temp range as TC• 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 temperature

ThermocouplePrinciples

• consist 2 pieces of dissimilar metals with their ends joined together (twisting, 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 &

measuring device/transducer

• 3 basic types of TC constructionCeramic beadedInsulated (plastic, glass, ceramic fiber)Metal-sheathed mineral-insulated (MSMI)

• 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 HT

• if exposed, the faster response is provided, but the wires are totally unprotected

• when TC get thinnerthe recommended upper temp limit is reducedthe error decreases & the response is faster to

temp changes

the element becomes more fragileat high temp, accuracy is more sensitive to material

(wire impurities)

Operations

• in TC: 2 junctions for measuring.

Hot junction

Cold junction

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

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

• voltage generated depends on temperature

• in a circuit, loop current depends on relative magnitude of voltage. Detect by galvanometer

• to measure temp, one 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, measurement can be calculated

• convert to 4-20 mA signal, transmitted is needed: temperature transmitter

Circuit emf = Measurement emf – Reference emf

• the temp measurement circuit consists a TC connected directly to TT

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

• monitor temp rise; safe operation

Types of TC

• Copper–constantan (Type T)

– pure copper (+ve) element & constantan (-ve) element– Constantan: group of alloys, contain approximately

55% copper & 45% nickel.

– advantage: resistance to corrosion in moist atmosphere, limits error below 0oC, suitable for subfreezing temperature measurement.

– can be furnished higher degree of accuracy for temp between -270 & +400oC than any other commonly used thermocouple

– can be applied in either oxidizing /reducing atmospheres between stated temp

• Iron – constantan (Type J)

- iron (+ve) element & constantan (–ve) element- 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 % 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 & not corrode at sub-freezing temp- operating limits of -180oC and +870oC when protected and is available in wire

TC Advantages

• used on most transformers(hot junction inside the transformer oil & cold junction at the meter mounted on the outside)

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

• capable of measuring a wider temperature range than an RTD

• self-powered• simple & rugged• inexpensive (half-price of RTD)• wide choice of physical forms• can be calibrated to generate a specific curve &

easy to interchange• fast response & measurement at one specific point

TC Disadvantages

• TC located some distance away from the measuring device, expensive extension wires / compensating cables have to be 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 wires• slower response than RTDs• susceptible to stray electrical signal

Filled-Systems

Principles

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

• an 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 motion

• liquid cause's volume changes & gas causes press changes

• this device generally used for local indication

Operations

• an improvement liquid –in-glass thermometer

• need no power to function & simple, rugged, self-contained, accurate over narrow temp

• the unit’s bulb may be too large for existing application & no syst failure the whole syst must be replaces

• capillary tubing is limited generally to a distance of 250 ft

• the syst as a whole is 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; continuously supported & protected against damage

• capillary’s construction material should be compatible with the surrounding environment

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

Bimetallic

Principles

• a spiral made of two different metals, having different coefficient of expansion; expands as the temp increase

• movement 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 provide the min of friction for the moving component

Operations

• generally used in local temp gages & switches• facilitate reading. “all-angle” types usually are

selected with a 5 in. diameter dial• capillary type is sometimes used for operating

visibility

• 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

• lowest cost among temp device, low accuracy & provides no remote indication

• calibrate; must be immersed in a bath of known temp

Mercury thermometer

• Common types .• Has a reservoir or bulb

- very large compare to the volume of the capillary or bore of the column, which rises in the stem.

• mercury or alcohol expands and rises in column - as heat is applied

• height of expanding material rises can be calibrated to indicate temperature.

• used for occasional local temperature measurement where continuous remote measurement is not required