Temperature Sensor

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

1. Thermistor

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 Image image

Temperature-Resistance chracteristics image

Thermistor international standard symbol image

2. Thermocouple

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
  • durability

Factors affecting Thermocouple output

  • alloy manufacturing uncertainties
  • aging effects
  • circuit design mistakes/misunderstandings. Thermocouple symbol image 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.

Temperature-Resistance chracteristics

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