Editorial Feature

Pyrometers - A Different Class of Thermometers

Oct 9 2013

Broadly speaking, pyrometers belong to a class of thermometers that are used for measuring only high temperatures. However, today they have been modified to also measure low temperatures.

Scientists use pyrometers to determine the heat and type of heat emitted from an object. The main difference between these instruments and other types of thermometers is that the incandescent levels from the heated objects are extremely hot for contact and only pyrometers can measure the heat without the need for making contact with the object. This is possible as pyrometers have built-in optical scanners.

As there are different types and levels of heat, there are various types of pyrometers to handle these variations in temperature. These instruments provide accuracy, economy, and speed. Their demand is increasing as non-contact heat measuring is a standard procedure in several industries today.

Pyrometers offer other advantages as well. They can measure temperatures of moving and inaccessible objects, and can also be used with highly aggressive materials such as acids in chemical processes, without any damage to the sensor.

Types of Pyrometers

Pyrometers can be broadly classified into two categories - optical pyrometers and infrared / radiation pyrometers.

Optical pyrometers – They are designed for measuring thermal radiation in the visible spectrum. They measure the temperature of extremely hot objects based on the color of the visible light they emit. It has the capability to perform visual comparison between a calibrated light source and the targeted surface. When the temperature of the filament and the target match, their thermal radiation intensity will also match causing the filament to become almost invisible as it merges into the targeted surface in the background. Once this happens, the current passing via the filament can be converted into a temperature reading.
Infrared / radiation pyrometers – These measure the temperature of an object from the radiation emitted by the object. They can handle thermal radiation in the infrared region usually 2 ~ 14 µm (80 ~ 550 µin). The radiation is directed at a thermocouple, which is capable of producing an electric current when it is partly heated. The thermocouple will generate a higher current according to the heat emitted. The current operates a dial that displays the temperature. These pyrometers are made from pyroelectric materials such as polyvinylidene fluoride (PVDF), triglisine sulfate (TGS), and lithium tantalate (LiTaO₃).

Working Principle

Radiation is emitted by every object with a temperature that goes beyond absolute zero (-273.15°C). This radiation is referred to as infrared as a majority of the radiation falls within the region above the visible red light in the electro-magnetic spectrum.

Radiated energy is useful in determining temperature of the body being measured. This heat energy is received by a detecting device and transformed into electric signals.