Specifications

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commercial units, vehicles as a digital or analog display, and thermometers (to
read a person‟s internal temperature or for instance; to read the temperature of a
swimming pool). These sensors are linearly proportional because as the
temperature changes the resistance changes as well.
The different varieties of temperature sensors are resistance temperature
detector, thermistor, and a thermocouple. A resistance temperature detector or
RTD is a type of temperature sensor that has an electrical resistance change
when the temperature changes as well. RTD‟s in today‟s industry are used in
many household appliances because of their dependable, very good accuracy
and precision, and maintainability. Most HVAC system controls use RTD‟s even
though they are more expensive then thermistors and thermocouples.
Thermistor is a type of temperature sensor (which acts like a resistor) that is
occasionally used in HVAC system controls whose resistance is directly altered
with the initial temperature. A thermocouple is a type of thermometer that
contains two wires of dissimilar metals. A junction exists between these metals
and in return generates a voltage to a change in temperature. Thermocouples
produce small current when heated and are not very precise or accurate in a
standard HVAC system control. Figure 27 is a picture of thermocouple sensor
used in electronic appliances around the house.
4"
3.5"
Figure 27 Thermocouple Sensor (Permission granted under the GNU Free
Documentation License http://en.wikipedia.org)
Bead type thermistors are composed of two wires. The sensor is at the end of
the wires denoted by the black area. The wire is approximately 2 inches in
length. Figure 28 shows a bead type of thermistor which is a type of temperature
sensor still being used today.