Download - CHAP04 - Temperature Measurement

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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

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    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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