Temperature Sensors; A Brief Introduction

From the humble thermometer to advanced electrical heat recorders, temperature sensors are used for all manner of purposes in a disparate number of industries. The more advanced forms of contact measurement device are labelled as thermocouples and resistance temperature detectors or RTDs. The measurement of temperature typically takes place in liquids or through contact with the target surface. Contact measurement is not always required however; there are sensors on the market that are able to make a measurement of heat without physical contact. Normally this is done with the use of either infra-red or air ionisation technology.

When selecting a sensor the large amount of considerations can make the decision making process difficult. Part of this process should be an assessment of the temperature range that must be measured, how accurate the device must be and how long it will take for any particular device to produce results. For those who consider accuracy to be of prime importance the most common choice is an RTD sensor. Cost may also come into the equation, if this is the case; thermocouples present a far more affordable and cost effective option. It is hoped that this following information will give those considering temperature sensors the knowledge required to decide between RTD, thermocouple and Infrared thermometer varieties.

The RTD uses the increased resistivity caused by heat to make an accurate measurement of temperature. The manufacture of RTDs uses materials such as platinum, nickel and copper for their chemical properties. Copper and nickel variants are most widely used in situations where low temperatures are measured; in contrast platinum, a far more expensive material has a greater range and hence are used for more far reaching applications. Despite their advanced uses the basics of RTD construction are relatively simple; normally the sensing element made from metal is connected to a non-conducting component made from either ceramic or glass. Resistance is measured across the two elements and from this the temperature can be deduced.

Thermocouples are fundamentally different to RTDs as they use two different metals, connected at one end that produces a voltage; this voltage is then converted into a heat reading. The meeting point of the two metals is termed the sensing junction; this component can be attached to wire extensions, subsequently increasing the physical range of the device. As almost any metals can be used in the manufacture of thermocouples, the construction costs are markedly less than those of RTDs. A downside is however that any interference in the circuit will lead to inaccurate results; in some applications this is simply not workable.

Infrared temperature sensors are becoming increasingly popular in many industries and scientific situations. They do not require contact with the target material in order to gain a reading. This remote measurement is particularly useful for some applications, such as in motor sport where the heat of tyres needs to be measured. The device focuses infrared rays through a lens and then measures the effects on the light to gain a reading. Other applications for this form of device are in the calibration of built-in thermostats, in fridges and ovens.

This article has been only an introduction into the different forms of temperature sensor on the market today. It is brief and literally scratches the surface of the types of sensors out there and how they work. The ultimate objective has been to give those without previous knowledge a good starting point for further research into the world of temperature measurement.

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