Temperature

Temperature sensor

Types of temperature sensors:

• Thermostat (temperature switch)
• RTD
• Thermocouple

Temperature sensors measure the amount of heat energy or even cold generated by an object or system, allowing us to "feel" or detect a physical change in that temperature, resulting in an analogue or digital output.

There are many different types of temperature sensors available and they all have different characteristics depending on their application. A temperature sensor consists of two basic physical types:

Contact Temperature Sensors

These types of temperature sensors must be in physical contact with the object being detected and use conduction to monitor changes in temperature. They can be used to detect solids, liquids or gases over a wide temperature range.

Non-contact temperature sensors

These types of temperature sensors use convection and radiation to monitor changes in temperature. They can be used to detect fluids and gases that emit radiant energy when the heat rises and the cold settles in convection currents to the ground or to detect the radiant energy transmitted by an object in the form of infrared radiation (the sun).

The two basic types of contact or even non-contact temperature sensors can also be divided into the following three groups of sensors, electromechanical, resistance and electronic sensors. The most common ones are shown below.

The thermostat

The Thermostat is an electromechanical contact type temperature sensor or switch, which basically consists of two different metals such as nickel, copper, tungsten or aluminium etc. which are stacked together to form a Bimetal. The different linear expansion rates of the two different metals cause a mechanical bending movement when the bimetal is exposed to heat.

The bimetallic strip can itself be used as an electrical switch or as a mechanical switch control. This type of temperature switch is widely used to control hot water heating elements in boilers, ovens, hot water storage tanks and cooling systems.

The Thermistor

A thermistor is a special type of resistor that changes its resistance when exposed to temperature fluctuations.

Thermistors are generally made of ceramic materials such as nickel oxides, manganese or cobalt, coated in glass which can easily damage them. Their main advantage is their speed of reaction to any changes in temperature, accuracy and repeatability.

Most types of thermistor have a Negative Temperature Coefficient of resistance (NTC), which means that their resistance value goes down when the temperature rises, and of course there are some that have a Positive Temperature Coefficient (PTC), in the sense that their resistance value goes up when the temperature rises.

Resistive Temperature Detectors (RTD).

The Electrical Resistance Temperature Sensor or RTDs are precision temperature sensors made of high-purity conductive metals such as platinum, copper or nickel that are wound in a coil and whose electrical resistance changes as a function of temperature, similar to that of the thermistor. Thin-film RTDs are also available. These sensors have a thin layer of platinum paste that is applied to a white ceramic substrate.

Resistive temperature sensors have positive temperature coefficients (PTC), but unlike the thermistor, their output is extremely linear, resulting in very accurate temperature measurements.

The most common types of RTD's are made of platinum and are called Platinum Resistance Thermometers or PRT's, the most common being the Pt100 sensor, which has a standard resistance of 100Ω at 0C.

The thermocouple

The thermocouple is by far the most widely used type of all types of temperature sensors. Thermocouples are popular for their simplicity, ease of use and speed of response to temperature fluctuations, especially due to their small size. Thermocouples also have the widest temperature range of all temperature sensors from -200C to well above 2000C.

Thermocouples are thermoelectric sensors that basically consist of two nodes of two different metals, such as copper and constantan, that are welded or shrunk together. One node is kept at a constant temperature, the reference point (cold), while the other is kept at the measuring point (hot). When the two nodes are at different temperatures, a voltage is developed over the node which is used to measure the temperature.