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DKK2413 Process Instrumentation & ControlFaculty of Chemical & Natural Resources Engineering

CHAPTER 4Temperature Measurement

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Topic Covered

Temperature Measuring Devices

Criteria For Selecting A Suitable Temperature-

measuring Instrument

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OBJECTIVES

After studying this chapter, you will be able to

1. Differentiate between absolute and relative

temperature scale

2. Transform a temperature reading among the Kelvin,Rankine, Celcius and Fahrenheit.

3. Explain the operation thermocouple and relationship

between emf and temperature.4. Explain the operation of RTD and relationship

between metal resistance and temperature.

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Introduction

Temperature is a measure of the amount of

thermal energy (heat) possessed by an object.

Temperature control is important for

separation and reaction processes, and

temperature must be maintained within limits

to ensure safe and reliable operation of

process equipment.

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Temperature Scale

There are three temperature scales in use

today, Fahrenheit, Celsius and Kelvin.

1. Fahrenheit (F)

~ British Imperial system

~ Water boiling point: 212o

~ Water freezing point: 32o

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2. Celcius (C)

~ Water boiling point: 100o

~ Water freezing point: 0o

3. Kelvin (K)

~ S.I unit

~ Water boiling point: 373.15

~ Water freezing point: 273.15

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Relative and absolute temperature

Absolute Relative

-Assign a zero temperature to a

material that has no thermal energy.

-These scales have negative and

positive numbers-The temperature scale differ by the

quantity of energy represented by

unit of measure.

-Common use Rankine (oR) and Kelvin

(K)

-When the scale indicate zero,

the thermal energy is not zero.

-These scales only have positive

numbersCommon use Celcius, oC (Related

to Kelvin)and Fahrenheit, oF

(related to Rankine)

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Temperature Conversion Formula

From To Fahrenheit To Celsius To Kelvin Rankine

Fahrenheit (oF) F (F - 32) * 5/9(F - 32) * 5/9 +

273.15F + 459.67

Celsius (oC) (C * 9/5) + 32 C C + 273.15(C + 273.15)

95

Kelvin (K)(K - 273.15) *

9/5 + 32K - 273.15 K K 95

Rankine (oR) R 459.67(R 491.67)

59R 59 R

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Convert 172.9 0F to degrees Celsius.

0C = x (0F 32)95

0C = x (172.9 32) = 78.395

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Convert 78.3oC to degrees Rankine

R = (oC + 273.15) x5

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R = (78.3oC + 273.15) x = 632.6159

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Thermocouple

Consist of two dissimilar metal and joined at both ends ~ produces a voltagerelated to a temperature difference

Hot junction ~ measure temperature (attached to a probe)

Cold junction ~ reference (maintained at known temperature at 0oC)

Output voltage depends on temperature difference between the two junctionand thermal properties of the metals used in the circuit

E1 = voltage generated by T1 (from hot junction)

E2 = voltage generated by T2 (from cold junction)

Et = E1 E2

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T1T2

E1E2

Et

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Thermocouple

Thermocouple are often insulated electrically with ceramic material(high temperature) and sheathed in stainless steel

Used thermowell for effectively seal off the process fluid or gas-temperature sensor isprotected from the process materials to

prevent interference with proper sensing and to eliminate damageto the sensor.

An additional advantage of such a thermowell is the ability toremove, replace, and calibrate the sensor without disrupting the

process operation.

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Temperature sensor with thermowell

Temperature sensor without thermowell

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Commercial Available Thermocouples

________________________________________________

Type Materials Normal Range

J Iron-constantan -190oC to 760oC

T Copper-constantan -200 oC to 371 oCK Chromel-alumel -190 oC to 1260 oC

E Chromel-constantan -100 oC to 1260 oC

S 90% platinum + 0 oC to 1482 oC

10% rhodium-platinumR 87% platinum + 0 oC to 1482 oC

13% rhodium- platinum

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Advantages and Disadvantages

Advantages Low cost

No moving parts, less likely to be broken

Wide temperature range

Reasonably short response time

High realibility Disadvantages

Sensitivity is low, usually 50 V/C (28 V/F) or less

Accuracy, usually no better than 0.5 C (0.9F), may not be highenough for some applications

Requires a known temperature reference, usually 0C (32F) ice water.Modern thermocouples, on the other hand, rely on an electricallygenerated reference

The relationship between the process temperature and thethermocouple signal (millivolt) is not linear.

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Resistance Temperature Detectors

(RTD)

RTD is wire and thin film devices that measure temperaturefrom the change in the electrical resistance of the metal wire.

The electrical resistance of many metals changes withtemperature; metals for which resistance increases with

temperature are used in RTDs. Linear relationship using equation

RT= Ro(1+aT)RT = the resistance at temperature, T

R0 = the resistance at base temperature of 0 CT = the temperature of the sensor (to be determined from RT)

a = the temperature coefficient of the metal

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RTD

metals selected should that have high

resistivity, good temperature coefficient of

resistance, good ductile or tensile strength,

and chemical inertness with packaging andinsulation materials

Most common RTD are made from platinum.

For some application nickel is used

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RTD

R100 = Resistance at 100oC (steam point)R0 = Resistance at 0

oC (ice point)

R100/ R0 - 1

100a =

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RTD

RTD sensitivity, a can be noted from typical

value of metal used,

Platinum = 0.00385 / oC

Nickel =0.005 / oC

The effective range of RTDs principally depend

on the type of wire used

Platinum RTD = -100 to 650 oC

Nickel RTD = -180 to 300 oC

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Example 1

A nickel resistance thermometer has a

resistance of 114 at 0oC and the value of the

temperature coefficient of the nickel, a is

0.005/oC. In operation, the resistance is 125 .Calculate the temperature

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RTD

They are slowly replacing theuse of thermocouples inmany industrial applications

below 600 C, due to higheraccuracy and repeatability.

RTDs are commonly used forapplications in which higheraccuracy than provided bythermocouples is required.

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Advantages & Disadvantages of RTD

Advantages:

Stable output for long period of time

Provide excellent accuracy and repeatability

Ease of recalibration

Disadvantages, compared to the

thermocouples, are:

Smaller overall temperature range

Higher initial cost

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Criteria For Selecting A Suitable

Temperature-Measuring Instrument

Operating range

The operating range and type of function are theimportant considerations when selecting a measuringinstrument.

Scale: RTD thermometer have linear scale. Thederived electrical signal (analog or digital) is availablefor indication and transmission.

Remote reading instrument: Thermocouple and RTD

have better capability for remote readout. Directconnection of the sensing element and remote displayinstrument.

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Criteria For Selecting A Suitable

Temperature-Measuring Instrument

Sensing elements For the fastest measurement response, the sensing

element itself should ideally be directly exposed to theprocess fluid.

But it always not possible because of the followingreasons: contamination of sensing element by the process fluid.

contamination of the process fluid by the sensing element.

aggressive attack on the sensing element by the process fluid.

high pressure in the line or vessel containing the process fluid.

a process fluid that is dangerous to handle.

Most element are located within wells or pockets toseparate the sensor and the fluid.

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