A Guide to Temperature Sensing Technologies

There are numerous ways to measure temperature. Selecting a sensor type will obviously depend on the requirements of the application in terms of temperature range, accuracy, speed, size and cost. Here’s a rundown of how some of the most popular sensor types work, and the differences between them.

 

Thermocouples

Thermocouples use the thermoelectric effect; simply put, any conductor exposed to a temperature gradient will produce a voltage, with the amount of voltage produced being a characteristic of the material. A thermocouple in its simplest form is two wires made from different materials, with the insulation stripped from the ends and twisted together. The wires have to be made of different materials because if they were made from the same material, the voltages produced would cancel each other out. With two different materials, a net voltage is produced, which can be measured.

There are various popular combinations of metals that are used, with the most common types given a letter name. Different types have different properties, whether it is suitability for different temperature ranges, sensitivity, corrosion resistance or even cost. Type K is one of the most popular general-purpose types, as it is fairly inexpensive. In this type of thermocouple, the wires are made from chromel (nickel-chromium alloy) and alumel (nickel-aluminium alloy). Outside the lettering convention, there are also very specialist combinations for extreme applications like the deep cold of outer space, or the heat from nuclear reactors.

It’s not necessary to know which type of thermocouple you want; it can be chosen based on temperature range, response time and probe size/type required. Tolerance is not as good as some other types, and thermocouples also don’t necessarily have great drift characteristics. However, thermocouples are commonly used in industry for measuring temperatures above 600°C, or where a really fast response is required.

This Type K thermocouple from RS is a typical example of the basic, twisted wire format. They also come inside probes, like this one, and with various different plugs and terminations for different applications. See the whole range here.

 

 

Figure 1: Example of an RS branded Type K thermocouple in a 150mm probe

 

Resistance Temperature Detectors (RTDs)

Resistance temperature detectors (RTDs) are often specified instead of thermocouples for applications below about 500°C, because of better accuracy and stability. However, unlike the thermocouple they require a power source to operate. They are not suitable where a very fast response time or high sensitivity is required, and the probes can be quite large.

This type of sensor measures the change in resistance of a material, typically platinum, over a changing temperature.  Platinum is used because of its stable resistance-temperature relationship over a wide temperature range, meaning the measurements are accurate and repeatable. It’s also chemically inert, so it has a long lifetime.

The most accurate type of element is a coil or coils of thin wire inside a ceramic housing, which allows it to expand with temperature. There are also thin film elements, basically a thin platinum foil on top of a piece of ceramic, which are smaller, but not as stable as the wire wound type and often with a limited range.

By far the most common type used today is the PT100 – a platinum sensor with a resistance of 100Ω at 0°C, like this one, a wire-wound PT100 sensor that comes in single or dual sensor configurations.

See the whole range of platinum resistance sensors here.

 

Thermistors

A thermistor is basically a resistor whose resistance varies with temperature. Unlike RTDs, they are made from ceramic or polymer, achieving higher precision in a small package, albeit over a smaller temperature range than an RTD.

There are, broadly speaking, two types of thermistor. Those with a positive temperature coefficient (PTC thermistors or Posistors) increase their resistance with temperature. Most PTC thermistors are designed to have a sudden rise in resistance when the temperature hits a certain level, to be used as an indicator. Those with a negative temperature coefficient (NTC thermistors) decrease their resistance as temperature increases.

In addition to primary temperature sensing functions in toasters, coffee makers, refrigerators, etc., they can also monitor PCBs and other parts of electronics systems for ‘hot spots’.

Thermistors are also widely used as current limiting devices for circuit protection (instead of fuses) using the self-heating effect when a large current flows through the thermistor. As the current grows, the thermistor gets hotter, and its resistance increases, limiting the current. As an example, this Murata PTC chip thermistor can be used as an over current detector or a current limiting resistor. See the whole range of thermistors available from RS here.

 

Infrared

An infrared sensor measures the infrared radiation from an object and thereby determines its temperature; incident power is converted to an electrical signal, which can be measured. This means temperature can be measured from a distance as no physical contact is necessary. For this reason it’s typically used in handheld units which the user points at the hot object then presses a trigger to take a reading. IR thermometers can be used when the object is moving, or when it’s in a vacuum or another type of controlled atmosphere. Non-contact also means it can be used in environments that require cleanliness or hygiene, and they are often used in industry for very hot things, such as furnaces. They typically work less well at very cold temperatures.

As with most sensors, they are selected based on the required accuracy, temperature range and response time. They often also have an optimum distance between the sensor and measured object.

This infrared probe from Calex Electronics has a measurement range of 0 to +250°C and a response time of 250ms. The output is transmitted as a signal between 4 and 20mA. Different models are available for near or distant targets.  View the whole range of IR sensors available from RS here.

 

Integrated Silicon Sensors

Temperature sensors also come in silicon, in the form of bandgap temperature sensors, which can be integrated into modern ICs. A bandgap temperature sensor is based on the idea that the forward voltage of a silicon diode (in this case, the base-emitter junction of a BJT) is temperature-dependent.

They come in all the usual types of IC leadframe packaging, as well as sizes, so pick whatever is suitable for your application. You’ll also need to know whether you want an analogue or digital output, what the operating voltage is, whether you’re using a serial/I2C interface, etc.