A thermal sensor is an electrical sensor which measures temperature and produces an electrical signal whose magnitude depends on the change in temperature.
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Resistance thermometer (RTD)
RTD or resistance thermometer consists of a long fine wire (made of copper, nickel or platinum) that is usually wrapped around a ceramic or glass core.
Working principle: Based on the fact that resistance changes with the change in temperature.
Features of RTD
- RTD’s provide a high level of accuracy
- They are linear devices
- Electrical resistance is utilized
- Are precise devices
- Comes in 2 wire, 3 wire, or 4 wire package
- Can work up to 600°C temperature
The figure below displays PT100 resistance thermometer which is widely employed in various electrical engineering Instrumentation, Biomedical Engineering and control/measurement industries.
Thermistor
A thermistor is a two terminal transducer whose resistance changes with the change in temperature. Thermistor are often classified into two types:
- NTC thermistor: Resistance decrease with increases in temperature
- PTC thermistor: Resistance increase with increases in temperature
Working principle: The change in temperature produces a change in temperature.
Features of thermistors
- More accurate than RTDs
- Nonlinear devices
- Can work up to 130°C temperature
- Made of ceramic/polymer materials
Both thermistors and RTDs are similar in terms of working. However, the major difference is their working ranges and design features. An RTD can deal with high temperatures such as 600°C while Thermistors can’t deal above 130°C. RTD’s are metal made, while thermistors are usually made from polymeric materials. The figure below displays a PTC Thermistor.
Thermocouples
A thermocouple contains two wires that are joined together on one end. Using Seebeck effect it responds to temperature change by generating a voltage across it.
Features of thermocouples
- Fast reacting device
- Non-linear resistance vs temperature curve
- Fast device
- Contains two wires on the output side
The video below briefly illustrates the working of thermocouples:
Radiation thermometers
The amount of thermal radiation emitted by any material increases with the increase in temperature. Radiation thermometers detect the temperature of a body by measuring the radiations exposed by it. Radiation thermometers are widely used in industries for monitoring process and control. The video below illustrates an IR thermometer. Such thermometers are also known as Laser thermometers since they use a Laser for aiming target:
Acoustic thermometers
The acoustic thermometer is used for measuring temperature using temp-dependent transit time of sound in gases, liquids or solids. Such temperatures are useful in harsh conditions and high temperatures where ordinary sensors can’t be employed. Examples include nuclear reactors and similar environments.
Others
- CMOS Thermometers: These are the complementary metal oxide semiconductor sensors.
- Gas thermometers: It measures temperature by measuring variation in volume or pressure of a gas.
- Diode temperature sensors
- Liquid thermometers
- Bimetallic thermometers