Temperature sensors detect the ambient temperature and convert it to a readable data depending on the conversion mechanism they are further divided as follows
Mostly temperature sensors are used as a means of preventive maintenance and alarm system.
Electrical temperature sensor
Thermistors are temperature dependent resistors. While all conductors are temperature dependent, the materials used as thermistors show large change in resistance for a small change in temperature. Usually made of ceramics or polymers, they give high precision over a small temperature range. There are two types
a) Positive temperature coefficient or PTC
Resistance increases as temperature rises.
Used as self-resetting overcurrent protectors, and self-regulating heating elements.
B) Negative temperature coefficient or NTC
Resistance decreases as temperature rises.
Commonly used as sensors and inrush current limiters
Thermistor international standard symbol
A thermocouple produces a voltage proportional to the temperature difference between its two junctions at different temperature by thermoelectric effect. Seebeck effect is most commonly used principle in thermocouples.
There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility.
Type J, K, T, & E - “Base Metal” thermocouples, (most common ).
Type R, S, & B - “Noble Metal” thermocouples, (high temperature applications)
Standards and identification of thermocouple types
Advantages of thermocouple
- low cost
- high temperature limits
- wide temperature ranges
Factors affecting Thermocouple output
- alloy manufacturing uncertainties
- aging effects
- circuit design mistakes/misunderstandings.
Temperature - voltage characteristics
3. Resistance Thermometer
Metals usually change in resistance when temperature changes. They are not as sensitive as thermistors but are industrially used upto 600 °C. Platinum is highly preferable due to its sensitivity but using beyond 600 °C makes it open to contamination from lamination material (Ceramic or polymer) . Tungsten is used at high temperature applications only because it is very brittle. Silver and Gold are not used because of their small resistivity and Copper can only be used until 120 °C. Nickel and Nickel alloy is also used in low temperature applications.
Advantages of Platinum RTD
- High accuracy
- Low drift
- Wide operating range
- Suitability for precision applications.
4. Silicon bandgap Temperature Sensor
The silicon bandgap temperature sensor is BJT that is easily included in a silicon integrated circuit at very low cost. the forward voltage of the base-emitter junction of a bipolar junction transistor (BJT) made of silicon diode is temperature-dependent
Selection of Temperature Sensor
|Parameters||Thermistors||Platinum RTD||Thermocouple||Semiconductor Based|
|Temperature Range||-50 to 250 °C||-200 to 600 °C||-200 to 1750 °C, (type dependent )||-70 to 150 °C|
|Power Source||Any constant voltage or current||Any constant voltage or current||Self-powered||4-30 V DC|
|Output||-4.4%/°C, flexible as output can be used as resistance or voltage||0.00385 ohm/°C||10 to 40 mV/°C||Various (since output is digital, it can be anything), Common nominal resistance|
|Response Time||Fast: 0.12 to 10 s||Very Slow: 1 to 50 s||Slow: 0.2 to 20 s||Very Slow: 5 to 60+ s|
|Accuracy||0.05 to 1.5 °C||0.1 to 1 °C||0.5 to 5 °C||1 to 5 °C|
|Electrical Noise||Not sensitive||Less sensitive||Extreme sensitivity at cold junction||Dependent upon board layout|
|Stability||Epoxy Coated: 0.2 °C/year Hermetically Sealed: 0.02 °C/year||Film: 0.05 °C/year Full wire: 0.002°C/year||>1 °C/year||2 °C/year|
|Output characteristics||Exponential,||Fairly linear||Non-linear||Linear|
|Effect of lead resistance on Accuracy||Very low||Highly susceptible, especially for 3- and 4-wire configurations||None||n/a|
|Cost||Low to moderate,||High||Medium||Medium|