Basics of industrial temperature measurement 6

High temperature measurements sometimes are not possible using RTD or thermocouples, as is the case in iron and steel industries or other metal and alloy industries where excessive temperature requires devices for remote measurement of temperatures. Here PYROMETERS are utilized (Pyro=fire). These are categorized in two main types. Optical pyrometers and Radiation pyrometers.

Lets first see how are optical pyrometers work. The first and basic type is the disappearing filament type. It is a comparative measurement which measures the photometric brightness of any heated object against a standard regulated source. In this type of temperature measurement, a filament is placed in the line of sight of the object for which temperature is being measured. The telescopic tube has a monochromatic lens and a current control knob which controls the current of a heated element placed directly in the line of sight.. The intensity of the filament is controlled manually and at certain point, the filament seems to disappear completely when its radiation matches with the radiation coming from the object. The value of current going to the filament at that point is translated into the temperature of the object. This is based on the principle that light wavelength emitted by a hot body depends on its temperature. These work from normally 700°C to 4000°C

There are also automated commercial versions available which have an accuracy of upto 1% of the measuring range. These work on the same principle and require a wavelength of visible nature, hence are not suitable for lower temperatures.

The other type Radiation pyrometers are based on the heat radiations and measures the temperature based on the amount of heat radiated from the object. Since these are based on measuring temperature, the sensing element used are commonly thermocouples, clubbed together to form a THERMOPILE. This we have discussed earlier briefly in other article. The radiation is focused on a thermopile using a convex lens, which detects the amount of heat reaching and converts it into a temperature reading.

All these are directional measuring devices, and needs to be focused in the right direction for measurement of temperature and may sometimes be hindered by any obstruction in the line of sight.

Another device now very common is the IR laser thermometers which are used to measure a very wide range of temperature from negative temperatures to high positive temperatures as well. the device uses a series of mirrors and lenses to process the emitted energy from an object into the temperature reading. These also measure the emitted infra red energy of the objects and a laser is used to focus the required object or surface which is emitting, reflecting or transmitting the infra red energy. The more darker the object, the better the results, though it has quite good accuracy in general. Since almost all object emit, reflect or transmit this energy weather hot or cold, it is a very handy device to measure temperatures, from body temperature to cool surfaces and food temperature as well. It is a fast and reliable way of temperature measurement, but needs careful calibration of the device from time to time as per the manufacturer's recommendations.

Known as IR temperature detector or temperature gun, or Hand held temperature meter, it is now widely used in industry and other places as well to have a reference of measurement where some error is expected in other measuring devices. It is quite handy with ease of use is now a very popular device.

Henceforth we have seen the various methods of measurement of temperature in the industry and elsewhere. That almost concludes our topic of basics of industrial temperature measurement. If you like this article, feel free to share it. Also you can subscribe to our newsletter to get more from the field of instrumentation, automation and control as we unfold other topic of interest.

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