Contact And Non-Contact Varieties Of Temperature Sensors

By: Thomas Pretty

Generally temperature sensors fall into two distinct categories, defined as contact and non-contact. It must be remembered that sensors and particularly those that measure temperature inherently have problems with accuracy. What is important to recognise is that the discrepancy estimates should be accurate and many readings should be made in order for the final results to have decent levels of accuracy.

Part of this is to calibrate your temperature sensors whether they are contact or non-contact with a known temperature, by doing this you can minimise any discrepancies in the readings, making your measurement extremely accurate; this process is fundamentally important before any measurement can take place.

Contact temperature sensors essentially measure their own temperature at any given time. They do this by creating a thermal equilibrium between the sensor and the substance being measured. Thermal equilibrium simply means that the two bodies have no heat flowing between them and are hence the same temperature. There are however a variety of errors that can occur when using these types of sensors, for instance surfaces are especially hard to measure with contact sensors, in such instances a non-contact method is more avoidable.

Contact sensors are by far the norm for temperature measurement in science and industry. Types of contact sensor include thermocouples and thermometers. Thermocouples use an electrical current with specific resistance levels that through variations of this resistance can measure temperature; this is widely termed the Seebeck effect. Out of the temperature sensors utilised in science and industry thermocouples are some of the most widely used due to their ease of operation, relative inexpensiveness and their ability to measure across a wide spectrum.

Thermometers as a means of measuring temperature differ greatly, not only in terms of accuracy but in terms of operation. Of course there is the most widely used sensor in the world the liquid glass thermometer; the one that measured your fever when you were a child. As well as these however there are filled system thermometers used in ovens to regulate cooking temperatures as well as bimetallic thermometers that have two metals contained within them that measure at different rates. However there is no need to use an expensive thermometer, it is even possible to measure temperature with the use of materials that change state at certain temperatures, scientifically these are known as phase change devices.

Less widespread is the use of non-contact sensors, as a result the manufacture and use of these devices is less standardised than the contact type. The situation is changing however; in the medical profession the use of the infra red ear thermometer is becoming extensive as a more efficient and effective way of gaining patient temperatures. The lack of standardisation of other forms of non-contact sensors has meant that they are struggling to be utilised across a wide variety of industry sectors.

Fundamentally they work on the principles of Plank's law; that is the thermal emission of radiation, although the names given to this type of sensors are extremely varied, adding to the confusion that surrounds their usage. Names such as pyrometer, radiometer and thermal imager are most common but are in no way a comprehensive list for this type of temperature sensor.

Today temperature sensors are used in all manner of industries and professions. Naturally the medical profession uses both varieties of sensor extensively but those in the manufacturing of foodstuffs such as the fermenting process as well as the manufacture of goods also use thermal measurement regularly. The large number of different varieties stands testament to the importance of this piece of equipment in the world currently, fundamental is that they should be calibrated so that accurate measurements can be made.

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