Temperature sensor-device used to sense temperature
The device which sense the variation in the temperature is referred as temperature
sensor. The Temperature sensors can be divided into four main categories as mentioned below.
• Resistance Temperature Detectors (RTD)
• Thermocouples
• Thermistor
• Ultrasonic transducers
Resistance Temperature Detector(RTD):
The materials such as platinum and nickel is commonly used.
The resistance of these wire elements will vary with temperature and
they will have high accuracy.
They are available in different configurations for immersion as well as surface mount
applications.
The equation below mentions relationship between temperature and resistance of
conductors.
R = Ro* ( 1 + αΔT)
Where,
R = Resistance of conductor at temperature T
Ro = Resistance at reference temperature usually 23 DegreeC
α = temperature co-efficient of the resistance
ΔT = Difference between operating and reference temperature
Thermocouple:
This type of temperature sensor is most popular and consists of two wires of different
metals which are joined together at one end.
The difference between the temperature at one end and the other end
of wires produces a voltage between the wires.
The amplitude or magnitude of this voltage depends on the materials
used for the wires and the amount of temperature difference produced
between the joined ends and the other ends.
The emf of the thrmocouple is expressed as follows:
E = c *(T1-T2) + K *(T12 - T22)
Where, c and K are the constants of thermocouple materials
T1 = Temperature of the hot junction
T2 = Temperature of the cold junction or reference junction
The table below covers thermocouples of various code types such as type E, Type J, Type K, Type N, Type T and Type U. It mentions conductors used as well as sensitivity limits.
Thermocouple sensor color codes
| Thermocouple code type | Conductors(+/-) | Sensitivity |
|---|---|---|
| Type E | Nickel Chromium/Constantan | -200 to +900 DegreeC |
| Type J | Iron/Constantan | 0 to 750 DegreeC |
| Type K | Nickel Chromium/Nickel Aluminium | -200 to 1250 DegreeC |
| Type N | Nicrosil/Nisil | 0 to 1250 DegreeC |
| Type T | Copper/Constantan | -200 to 350 DegreeC |
| Type U | Copper/Copper Nickel compensating for "S" and "R" | 0 to 1450 DegreeC |
Thermistor:
A thermistor is a semiconductor made by sintering mixtures of metallic oxide e.g.
oxides of nickel, manganese, copper, cobalt and uranium.
Thermistors have negative temperature coefficient (NTC) and positive temperature coefficient (PTC).
In the case of NTC, resistance decreases as their temperature increases.
In the case of PTC, resistance increases as the temperature increases.
There are various types of thermistors as mentioned below with their
resistance values:
Disc- 1 to 1 MΩ
Washer- 1 to 50 KΩ
Rod- High resistance
Refer thermistor page for thermistor basic details as well as thermistor manufacturers.
Resistive Position Transducer:
The principle of the resistance transducer is that the physical variable under
measurement causes a resistance change in the sensing element.
A common requirement in industrial measurement and control work is to be able to
sense the position of an object or distance it has moved.
R = ρ*L/A
The equation is of simple potentiometer, here R is resistance change,
ρ is density , L is length and A is the Area.
Sensor RELATED LINKS
Also refer IoT compliant sensors which includes ambient light sensor, optical sensor, gesture sensor, proximity sensor, touch sensor,
fingerprint sensor etc.
Refer Sensors and transducers article which covers types, basic functions and provide links to sensors and transducers.
It include temperature sensor, humidity sensor, strain sensor, proximity or occupancy sensor, force or load sensor, voltage and current sensor, pressure sensor, speed sensor, resistance sensor, power sensor, level sensor,
event or state sensor etc.