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SENSORS
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2. Observable effects of temperature change
Change in physical state
Change in chemical state
Change in dimension
Change in electrical properties
Change in radiation properties
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Change in physical state (boiling and freezing point)
Last three type of change are used for the temperature measurement.
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4. Various temperature measuring devices
Mechanical Methods
Electrical methods
Based on radiation properties
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Mechanical Methods
Expansion of solids
Solid Rod thermometer
Bimetallic thermometer
Expansion of liquids
Liquid in glass thermometer
Liquid in metal thermometer
Expansion of gases
Gas thermometer
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Electrical Methods
Electrical resistance thermometer
Thermistors
Thermocouples
Thermopile
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Radiation Methods
Total radiation pyrometer
Optical pyrometer
Infra red pyrometer
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Solid rod thermometer
The change in the length due to change in temperature change is
Δl = l *α *Δt
The expansion of a single metal is widely used in the rod type thermostat as switch
But not used for indicating thermometer
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Thermostat
Metals expand at different rates/ The major element is two strips of different metals bonded together. As the bimetal strip is heated, the two metal strips begin to expand. Because of expanding difference, the strip bends away from the metal with the higher coefficient of expansion.
The strip is used as a part of the switching circuit, which contact if the strip bends
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Bimetal thermometer
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When two metal strips having different coefficient expansion are bonded together, an increase of temperature causes the deflection of the free end of the strip.
The defection is nearly linear.
One metal with low thermal expansion is used and the other one with high expansion like brass is used
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Bimetal thermometer
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14. Liquid in Glass thermometer
It works on the principle of volume expansion It consists of Alcohol and mercury are two Liquids most commonly used
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Electrical methods
Thermo resistive or variable resistance
Thermo electric or e. m. f. generating
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16. Resistance Temperature Detector (RTD)
The Resistance Temperature Detector (RTD) or resistance thermometer uses the fact that the resistance of metals increases with temperature. Examples are RTD's are shown schematically below.
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Thermometer
typical devices use platinum wire (such a device is called a platinum resistance thermometers or PRT)
linear but has poor sensitivity
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RTD
Pros
Stable and accurate
Linearity is better than thermocouples
Higher signal to noise ratio
Cons
More Expensive
Self heating
Requires a current source
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19. Bulk Semiconductor Sensor (Thermistor)
Similar to Resistance Temperature Detectors (RTD), the Thermistor (Bulk Semiconductor Sensor) uses resistance to detect temperature. However, unlike an RTD's metal probe where the resistance increases with temperature, the thermistor uses ceramic semi conducting materials which respond inversely with temperature. Examples of thermistors are shown in the following schematic
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20. Bulk Semiconductor Sensor Thermistors
use materials with a high thermal coefficient of resistance
sensitive but highly non-linear
A threaded thermistor
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Pros and Cons
Pros
High accuracy
High sensitivity
Small size compared to thermocouple
Response time shorter than RTD
Cons
Limited temperature range ( )
Non linear resistance temperature relationship
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22. Generating e m f (Thermocouples
Thermocouples operate on the principle that two wires made of dissimilar materials connected at either end will generate a potential (a difference in voltage) between the two ends that is a function of the materials and the temperature difference between the two ends.
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Seebeck effect: current will flow through a junction of disimilar metals if there is a temperature difference
millivolts and non linear
varies from 6 uV/C to 90 uV/C
Thermocouple
See beck effect: current will flow through a junction of dissimilar metals if there is a temperature difference
mill volts and non linear
varies from 6 u V/C to 90 u V/C
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The temperature difference between the two junctions is detected by measuring the change in voltage (electromotive force, EMF) across the dissimilar metals at the temperature measurement junction
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Thermocouples
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27. Thermocouples circuit
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29. Thermocouple compensation
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Pros and Cons
Pros
Low cost
No moving parts, less likely to be broken
Wide temperature range
Reasonable short response time
Reasonable repeatability and accuracy
Cons
Low sensitivity
Requires a known temperature reference, usually 0°C (32°F) ice water
Nonlinearity could be bothersome
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Radiation Pyrometer
A Pyrometer, or radiation thermometer, is a non-contact instrument that detects an object's surface temperature by measuring the temperature of the electromagnetic radiation (infrared or visible) emitted from the object.
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Pyrometer
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33. The Electromagnetic Radiation Spectrum
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Pros and Cons
Pros
Non contact Measurement
Fast response time
Good stability
Cons
Expensive
Accuracy may be affected by surrounding
( dust, smoke, thermal radiation)
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