Understanding exactly how a temperature sensor works would require knowledge of thermodynamic law. Still, it is important to be aware that different types of thermometers and heat sensors work in different ways. The key to both is the ideal of a bimetal thermometer.
Electronic or battery-powered thermistors, thermocouples and temperature sensors - including digital thermometers - rely on an electrical signal. A digital sensor probe contains two metals (bimetallic), which generate altering voltage or resistance at different temperatures. An onboard heat sensor chip in the device can read these signal changes as accurate heat readings in Fahrenheit or Celsius (often both).
Analogue dial temperature gauges work on a similar principle. However, instead of the heat change generating an electrical signal, it physically alters the positions and properties of small metal components in the instrument sensor. The dial needle is tuned to display these changes as a number value.
Infrared temperature sensors work quite differently again, but these tend to be used in more specialist devices and settings. They are not typically found in standard, low-cost temperature gauges for traditional air or fluid heat measurement.
What are the Symptoms of a Bad Thermostat or Temperature Sensor?
Most types of thermostats and thermocouples have a shelf life. This is because electrical current or physical movement can eventually wear out the metal components of a heat sensor.
Dirt or moisture ingress can also cause problems for certain thermometer types. However, most models designed for outdoor use will be IP rated against this.
A thermostat or temperature gauge not working reliably will need to be tested for accuracy. If you suspect this is the case, the faulty unit may need cleaning, calibration, repair or replacement. In most cases, it is best to contact a qualified engineer for this, particularly when dealing with temperature-critical applications such as sanitary and cleanroom environments, sensitive electronics, food preparation and storage, or industrial and laboratory processes.