Temperature Measuring Instrumentation. M H Joshipura Chemical Engineering Department IT, NU

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1 Temperature Measuring Instrumentation M H Joshipura Chemical Engineering Department IT, NU

2 Why Measuring Temperature in Chemical Industry is Important?

3 Temperature A measure of a substance s internal kinetic energy The degree of hotness or coldness of a substance (as measured on a thermometer)

4 Temperature Scale Absolute zero is the temperature at which the movement of molecules completely stops Temperature value on one scale can be converted to other

5 Temperature measurement Following Characteristics can be used to measure temperature: A change in dimensions - expansion or contraction of material w.r.t. Temp change A change in electrical resistance of metals and semi-conductors w.r.t. Temp change A thermo-electric emf produced due to temp difference at two junctions of two metal alloy joined together A change in intensity and colour of radiation emitted by the hot body w.r.t to change in temp

6 Temperature Sensing Techniques Changes in Physical Dimensions Bimetallic Thermometers Filled-Bulb and Glass-Stem Thermometers

7 Temperature Sensing Techniques Changes in Electrical Properties Integrated Circuitry (IC) Transistors and Diodes Resistance Temperature Detectors (RTDS) Thermistors Thermocouples

8 Temperature Sensing Techniques Changes in Chemical Phase Quartz Crystal Thermometry Changes in Emitted thermal radiation Radiation and Infrared Pyrometers

9 Expansion thermometers (1)Bimetalic thermometer -solid expansion (2)Liquid in glass thermometers

10 (1)Bimetallic Thermometer Two basic principles of operation is to be followed in the case of a bimetallic sensor. A metal tends to undergo a volumetric dimensional change (expansion/contraction), according to the change in temperature. Different metals have different co-efficient of temperatures. The rate of volumetric change depends on this co-efficient of temperature. Frequently used in Thermostats (On/Off control) Range of Use -65 to 430 C Advantages: Low Cost, Negligible Maintenance, Stability

11 Liquid In Glass Thermometers The volume of mercury changes slightly with temperature. The space above the mercury may be filled with nitrogen or it may be at less than atmospheric pressure, a partial vacuum Mercury filled thermometers Range= -37 to 320 C, Best Accuracy ± 0.02 C Alcohol filled thermometers. Range = -75 to 120 C, Best Accuracy ± 0.6 C Advantage : Low cost, simple Disadvantage: No remote measurement, No Recording

12 Problems Associated with Thermometers Mercury solidifies at -40 C. Alcohol boils at around 120 C. Accurate manufacture is needed and this makes accurate ones expensive. It is easy for people to make mistakes reading them.

13 Pressure spring thermometers Vapour pressure thermometers

14 A fluid filled bulb is connected to a pressure measuring device via a capillary tube. As fluid is heated it expands increasing the pressure. Pressure is linked to temperature. Accuracy and range depends on fluid. Advantages/Disadvantages Low cost, Stable in operation, Remote readings are possible. Transient response is a function of bulb size and capillary tube length.

15 Resistance Temperature Detectors (RTD) RTD (resistance temperature device or detector) is a metal conductor, has a positive coefficient of resistance. R increases with T Resistance change with temperature change should be as linear as possible Platinum has been accepted as the material, which best fit all the criteria and has been generally accepted for industrial measurement between 300 and 1200 F (-150 and 650 C ) Thermistors have large Negative Temperature Coefficient

16 RTD Response Linear verson is often used over smaller temperature interval R R 1 a T o T o Coefficients a depend on the wire material and its purity. Example: Platinum, a= E -03 RTDS are commercially available with resistances from 50 to 1000 ohms Platinum RTD known as Pt100, because it has 100 Ohm resistance at 32 F ( 0 C)

17 RTD Characteristics Low resistance 100 (most common) to 1000 Wide operating range (-200 C to 850 C) High sensitivity High accuracy (up to 0.1 C or better 0.01 C ) High Repeatability and Stability Low drift ( C/year) Industrial models drift < 0.1 C/year

18 Thermocouples Ta V Ta B A T X A thermocouple consists of two pieces of dissimilar metals with their ends joined together (by twisting, soldering or welding). SEEBECK EFFECT

19 In order to use a thermocouple to measure process temperature, one end of the thermocouple has to be kept in contact with the process while the other end has to be kept at a constant temperature. The end that is in contact with the process is called the hot or measurement junction. Circuit emf = Measurement emf - Reference emf If circuit emf and reference emf are known, measurement emf can be calculated and the relative temperature determined

20 Any time a pair of dissimilar wires is joined and a junction is formed an emf voltage will be generated. Voltage or EMF produced depends on: Types of materials used, A and B Temperature of the junctions, measuring and reference

21 Types of Thermocouples Thermocouples exist in many different types, each with its own color codes for the dissimilar-metal wires. Indicated by letters like Type J, Type K, Type E, Type N.. They all differ in Metal composition Seebeck coefficient Applicability

22 Types of Thermocouple Assembly Some thermocouple assemblies are manufactured as follow Ground junction Ungrounded junction Exposed junction

23 The exposed junction is often used for the measurement of static or flowing non-corrosive gas temperatures where the response time must be minimal The ungrounded junction often is used for the measurement of static or flowing corrosive gas and liquid temperatures in critical electrical applications The grounded junction often is used for the measurement of static or flowing corrosive gas and liquid temperatures and for high-pressure applications

24 Thermowells Thermowells are used to protect the detector and so that the detector can be changed without interrupting the process. One downside of using a thermowell is the time delay it introduces into the measurement system due to thermal lag Thermowlls should be installed where a good representative sample of the process fluid temperature can be measured. The optimum immersion length of a thermowell depends on the application If the well is installed perpendicular to the line, the tip of the well should be between one half and one third of the pipe diameter If the well is installed in an elbow, the tip should point towards the flow The speed of response of a sensor in a thermowell will be slower than that of an unprotected buib. Keeping the clearance between bulb and pocket down to an absolute minimum and filling the space with oil or glycol (antifreeze) can reduce this effect.

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