Temperature Measurement

CHAPTER 4

TEMPERATURE MEASUREMENT

Objectives After 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 in temp device • Apply the principle and operation of temp device (RTD. TC, filled-system,) • • • •

Introduction • Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. • 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 9   F   C   5 

C  K  273.15

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 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 response - metal: max varies from 800F (iron) - 2300F (inconel) - ceramic: max 1900F (fused silica) - 3000F (silicon carbide)

TEMPERATURE MEASUREMENT DEVICES

Resistance Temperature Detector (RTD) Principles • every metal – unique composition & has a 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 temperature • by measuring resistance, temperature can be measured

RTD

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 RTD - accuracy & linearity

• Accurate sensor - can measure temperature change of 0.00001 C • usually protected from the environment by a sheath made of stainless steel or 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, 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 is used • 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 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 from transmitter, it caused problem • long connecting wires; 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 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 • 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 • 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 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

Thermocouple Principles • 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

Thermocouple

• 3 basic types of TC construction

Ceramic beaded Insulated (plastic, glass, ceramic fiber) Metal-sheathed mineral-insulated (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 reduced 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)

Operations TC: 2 junctions for measuring.

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

• 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: 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 •

Copper–constantan (Type T) – pure copper (+ve) element & constantan (-ve) element – Constantan: group of alloys, contain approximately 55% copper & 45% nickel.

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Iron – constantan (Type J) - iron (+ve) element & constantan (–ve) element - can be applied in oxidizing / reducing atmospheres.

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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

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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

Advantages o resistance to corrosion in moist atmosphere, limits error below 0oC, suitable for subfreezing temperature measurement. 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 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 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. 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 • metallic assembly consists a bulb, small-diameter tubing (capillary) & Bourdon spring • indicator linked to Bourdon tube indicates temp • sometimes bellows & diaphragm are used

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Operations 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 causes volume changes & gas causes press changes this device generally used for local indication

Filled-systems

Advantages • an improvement liquid–in-glass thermometer • need no power to function, simple, rugged, selfcontained, accurate over narrow temp span

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DisAdvantages 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 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 • 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

Vapor

Liquid

Temperature Measurement Safety • Thermometer, resistance bulb, TC & bulb systems are placed in thermowell. • Thermowell protects the measuring device from 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.