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Whether it’s compensating for temperature-dependent electrical changes, or ensuring equipment doesn’t overheat, there’s almost always a need to know whether your system is blowing hot or cold. The temperature sensor market is, therefore, substantial, and growing healthily – predicted to top $7 billion by the year 2023.

There are multiple different types of temperature sensor, including thermocouples, thermistors, resistance temperature detectors and infrared sensors. Let’s look at four typical applications, and how one or more of these sensor types can be used for them.

Thermocouples: Keeping Gas Safe

A thermocouple is based on two different metals touching each other to form two junctions. It produces a voltage (due to the thermoelectric effect) when one of the junctions is exposed to heat and the other is kept at a reference temperature. Thermocouples are cheap and rugged and can measure over a very wide range of temperatures up to 2000º or more. Although, they can be lacking in accuracy.

They are perhaps the most popular temperature sensors that are used in many applications in industry and elsewhere. For example, they are often used in gas-powered boilers or ovens, to sense if a pilot flame is burning. If the thermocouple detects a drop-in temperature, the resulting drop in voltage will close a valve to turn off the gas supply, ensuring there is no risk from unburnt gas.

Thermistors: Automotive Warnings

Often made of nickel and a ceramic material, thermistors are basically resistors whose value depends on temperature. Thermistors have a fast response, are rugged, and are low cost. They do provide a non-linear output, so extra circuitry is required to generate a voltage that is proportional to temperature.

The automotive industry is a major user of thermistors, which measure the temperature of oil and coolants. They can therefore be used to trigger warning lights to alert the driver to a problem, to avoid engine damage.

Resistance Temperature Detector (RTD): Long-Term Stability

RTDs are similar to thermistors in that they measure temperature by how it changes their resistance, but they are made of metal. Their advantage is their output is linear and can provide accurate measurements.

Compared to thermocouples, RTDs are typically more accurate, are stable over time, and each component provides the same readings – so a faulty RTD can be replaced by a new part without recalibration. This makes them ideal for use in many industrial applications, where long-term reliability and accuracy are essential.

Infrared Sensors: Medical and More

For this kind of non-contact application, an infrared sensor is often the best choice. This determines temperature by measuring the infrared energy emitted by an object and converting that into an electrical signal. This is then corrected for the influence of the ambient temperature, 

Typical applications for an infrared sensor vary widely, from measuring the temperature of patients, to checking oven temperature in food production. The medical application is familiar to many of us from its use in home temperature readers, which are held in the ear for a few seconds and much easier when used with a crying toddler than the conventional thermometers they’ve replaced.