lecture notes 13.ppt

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Temperature Sensors

Temperature Sensors

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Warm-upsWarm-ups

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At the end of this chapter, the students should be able to:

describe the principle of operation of various sensors and transducers; namely..

Resistance Temperature Detectors. Thermocouples Thermistors Photodiode Phototransistors

ObjectivesObjectives

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Temperature TransducersTemperature Transducers• The temperature transducers can be divided

into four main categories:

o Resistance Temperature Detectors (RTD)o Thermocoupleso Thermistorso Ultrasonic transducers

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Resistance Temperature Detectors (RTDs)Resistance Temperature Detectors (RTDs)

• Detectors of resistance temperatures commonly employ platinum, nickel, or resistance wire elements, whose resistance variation with temperature has a high intrinsic accuracy.

• They available in many configurations and sizes and as shielded and open units for both immersion and surface applications.

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• Some examples of RTDs are as follows:

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• The relationship between temperature and resistance of conductors can be calculated from this equation:

where;R= resistance of the conductor at temp t (oC)Ro=resistance at the reference temp.= temperature coefficient of resistance= difference between operating and reference

temp.

)1( TRR o

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Example:A platinum resistance thermometer has a resistance of 220Ω at 20oC. Calculate the resistance at 50oC? Given that 20oC=0.00392.

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ThermocouplesThermocouples• A thermocouple is a sensor for measuring temperature. It consists of two dissimilar / different metals, joined together at one end, which produce a small unique voltage at a given temperature. This voltage is measured and interpreted by the thermocouple.

•The magnitude of this voltage depends on the materials used for the wires and the amount of temperatures difference between the joined end and the other ends.

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ThermocouplesThermocouples• Some examples of the thermocouples are as follows:

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ThermocouplesThermocouples• Common commercially available thermocouples are specified by ISA (Instrument Society of America) types.

• Type E, J, K, and T are base-metal thermocouples and can be used up to about 1000°C (1832°F).

• Type S, R, and B are noble-metal thermocouples and can be used up to about 2000°C (3632°F).

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ThermocouplesThermocouples• The following table provides a summary of basic thermocouple properties.

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ThermocouplesThermocouples•Calibration curves for several commercially available thermocouples is as below:

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• The magnitude of thermal emf depends on the wire materials used and on the temperature difference between the junctions.• The effective emf of the thermocouple is given as:

•Where; c and k – constant of the thermocouple materialsT1 - temperature of the ‘hot’ junction.T2 - temperature of the ‘cold’ or

‘reference’ junction.

)()( 22

2121 TTkTTcE

ThermocouplesThermocouples

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ThermocouplesThermocouplesExample

During experiment with a copper- costantan thermocouple, it was found that c= 3.75x10-2 mV/oC and k = 4.50x10-5 mV/oC. If T1= 100oC and the cold junction T2 is kept in the ice, compute the resultant electromotive force, emf?

lecture notes 13.ppt

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