Temperature Controllers and Infrared Thermometers

Temperature measurement and thermometers

Types of thermometers

Temperature measurement is used extensively in a wide range of fields, where it plays a very important role. Thermometers can be classified according to their operating principles into the categories shown below.

									Bimetal
						Mechanical			Thermal expansion
									Magnetic
			Contact						Thermocouple
									Resistance
						Electrical
Thermometers									Thermistor
									Semiconductor
									Radiation
			Non-contact
									Optical

Radiation

Different types of thermometers have different operating principles. Contact-type thermometers make use of the fact that the heat of an object transfers to other objects (thermal conduction), while radiation thermometers—one of the most common types of non-contact thermometers—use infrared radiation to measure temperature. In research, development, and production control in the natural sciences and a variety of industries, temperature data is used as important information. Non-contact thermometers in particular can dramatically expand temperature measurement possibilities, and are already used in a variety of fields.

Comparison of contact and non-contact thermometers

Type	Non-contact	Contact
Size of measurement target	Can also measure small objects	May affect the temperature of the target. Small objects are difficult to measure
Response time	Fast response time allows measurement of moving objects	Not suited to measuring fast-moving objects
Measurement situation	Can measure from a distance	Must touch the target
Points measured	Surface temperature	Can measure desired areas (e.g. can measure internal temperatures)
Temperature range	High temperature models possible	Up to around +2,000°C
Precision	Comparatively low	Easy to make very precise models
Service life	Long	Generally short

Infrared thermometers

Infrared radiation

Infrared radiation is a kind of light, just like light in the visible spectrum that humans can see. However, as it has a longer wavelength than visible light, it is not visible to the naked eye. (As shown in the diagram below, infrared light is in the range of 0.8 to 1,000 μm and has a higher wavelength than visible light.) While infrared light is not visible, when it hits an object it raises the temperature of the object.

Infrared thermometers

When you bring your hand close to your cheek it feels warm; this is because your skin detects the infrared radiation emitted from the palm of your hand. All objects emit infrared radiation like this, and the higher the temperature of the object, the stronger the infrared radiation emitted. Infrared thermometers use infrared radiation to measure temperature.

Measurement principle

Infrared light emitted by the target is condensed with a lens and then focused on a detection element called a thermopile. The thermopile produces an electrical signal proportionate to the strength of the infrared radiation. This signal is linearized and corrected for emissivity before being displayed.

Thermopile construction and operating principle

A thermopile is a detecting element that generates an electric voltage by absorbing infrared radiation emitted by an object. As in the figure below, a thermopile consists of multiple thermocouples arranged in series. The hot junction of the thermopile is heated by infrared light, and the difference that this creates between the hot and cold (reference) junctions produces an electric voltage. The temperature of the cold junction (which is equivalent to the internal temperature of the sensor) is measured with an integrated thermistor, and the temperature of the target is calculated from these two values.

 

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