Temperature Sensors

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Temperature Sensors
2
Warm-ups

3
Objectives

• 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

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Temperature Transducers

• The temperature transducers can be divided into
  four main categories
• Resistance Temperature Detectors (RTD)
• Thermocouples
• Thermistors
• Ultrasonic transducers

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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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Resistance Temperature Detectors (RTDs)

• Some examples of RTDs are as follows

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Resistance Temperature Detectors (RTDs)

• The relationship between temperature and
  resistance of conductors can be calculated from
  this equation
• where
• R resistance of the conductor at temp t (oC)
• Roresistance at the reference temp.
• temperature coefficient of resistance
• ? difference between operating and reference
  temp.

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Resistance Temperature Detectors (RTDs)
Example A platinum resistance thermometer has a
resistance of 220? at 20oC. Calculate the
resistance at 50oC? Given that
?20oC0.00392.
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Thermocouples

• 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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Thermocouples

• Some examples of the thermocouples are as
  follows

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Thermocouples

• 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 1000C (1832F).
• Type S, R, and B are noble-metal thermocouples
  and can be used up to about 2000C (3632F).

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Thermocouples

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

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Thermocouples

• Calibration curves for several commercially
  available thermocouples is as below

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Thermocouples

• 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 materials
• T1 - temperature of the hot junction.
• T2 - temperature of the cold or
  reference junction.

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Thermocouples
Example 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?