Radiation Pyrometer: A Non-Contact Temperature Sensor

Contents

A radiation pyrometer is a device that measures the temperature of a distant object by detecting the thermal radiation it emits. This type of temperature sensor does not need to touch the object or be in thermal contact with it, unlike other thermometers such as thermocouples and resistance temperature detectors (RTDs). Radiation pyrometers are mainly used for measuring high temperatures above 750°C, where physical contact with the hot object is not possible or desirable.

A radiation pyrometer is defined as a non-contact temperature sensor that infers the temperature of an object by detecting its thermal radiation emitted naturally. The thermal radiation or irradiance of an object depends on its temperature and emissivity, which is a measure of how well it radiates heat compared to a perfect black body. According to Stefan Boltzmann’s law, the total thermal radiation emitted by a body can be calculated by:

Where,

• Q is the thermal radiation in W/m\$^2\$
• ϵ is the emissivity of the body (0 < ϵ < 1)
• σ is the Stefan-Boltzmann constant in W/m\$2\$K\$4\$
• T is the absolute temperature in Kelvin

A radiation pyrometer consists of three major components:

• A lens or a mirror collects and focuses the thermal radiation from the object onto a receiving element.
• A receiving element that converts the thermal radiation into an electrical signal. This can be a resistance thermometer, a thermocouple, or a photodetector.
• A recording instrument that displays or records the temperature reading based on the electrical signal. This can be a millivoltmeter, a galvanometer, or a digital display.

There are mainly two types of radiation pyrometers: fixed focus type and variable focus type.

A fixed-focus type radiation pyrometer has a long tube with a narrow aperture at the front end and a concave mirror at the rear end.

A sensitive thermocouple is placed in front of the concave mirror at a suitable distance, such that the thermal radiation from the object is reflected by the mirror and focused on the hot junction of the thermocouple. The emf generated in the thermocouple is then measured by a millivoltmeter or a galvanometer, which can be directly calibrated with temperature. The advantage of this type of pyrometer is that it does not need to be adjusted for different distances between the object and the instrument, as the mirror always focuses the radiation on the thermocouple. However, this type of pyrometer has a limited range of measurement and may be affected by dust or dirt on the mirror or lens.

The thermal radiation from the object is first received by the mirror and then reflected onto a blackened thermojunction consisting of a small copper or silver disc to which the wires forming the junction are soldered. The visible image of the object can be seen on the disc through an eyepiece and a central hole in the main mirror. The position of the main mirror is adjusted until the focus coincides with the disc. The heating of the thermojunction due to the thermal image on the disc produces an emf that is measured by a millivoltmeter or a galvanometer. The advantage of this type of pyrometer is that it can measure temperatures over a wide range and can also measure invisible rays from radiation. However, this type of pyrometer requires careful adjustment and alignment for accurate readings.

• They can measure high temperatures above 600°C, where other sensors may melt or damage.
• They do not need physical contact with the object, which avoids contamination, corrosion, or interference.
• They have a fast speed of response and high output.
• They are less affected by corrosive atmospheres or electromagnetic fields.

• They have non-linear scales and possible errors due to emissivity variations, intervening gases or vapors, ambient temperature changes, or dirt on optical components.
• They require calibration and maintenance for accurate readings.
• They may be expensive and complex to operate.

Radiation pyrometers are widely used for industrial applications where high temperatures are involved or where physical contact with the object is not feasible or desirable.

Some examples are:

• Measuring the temperature of furnaces, boilers, kilns, ovens, etc.
• Measuring the temperature of molten metals, glass, ceramics, etc.
• Measuring the temperature of flames, plasmas, lasers, etc.
• Measuring the temperature of moving objects such as rollers, conveyors, wires, etc.
• Measuring the average temperature of large surfaces such as walls, roofs, pipes, etc.

Conclusion

A radiation pyrometer is a device that measures the temperature of a distant object by detecting the thermal radiation it emits. This type of temperature sensor does not need to touch the object or be in thermal contact with it, unlike other thermometers such as thermocouples and resistance temperature detectors (RTDs). Radiation pyrometers are mainly used for measuring high temperatures above 750°C, where physical contact with the hot object is not possible or desirable.

There are two types of radiation pyrometers: fixed focus type and variable focus type. The fixed focus type has a long tube with a concave mirror and a thermocouple at the rear end. The variable focus type has an adjustable concave mirror and a thermojunction with a small disc at the front end. Both types measure the emf generated by the heating of the receiving element due to the thermal radiation from the object.

Radiation pyrometers have some advantages and disadvantages compared to other types of temperature sensors. Some advantages are: they can measure high temperatures, they do not need physical contact with the object, they have a fast speed of response, and they are less affected by corrosive atmosphere or electromagnetic fields. Some disadvantages are: they have a non-linear scale and possible errors due to emissivity variations, intervening gases or vapors, ambient temperature changes, or dirt on optical components. They also require calibration and maintenance for accurate readings.

Radiation pyrometers are widely used for industrial applications where high temperatures are involved or where physical contact with the object is not feasible or desirable. Some examples are: measuring the temperature of furnaces, boilers, kilns, ovens, etc., measuring the temperature of molten metals, glass, ceramics, etc., measuring the temperature of flames, plasmas, lasers, etc., measuring the temperature of moving objects such as rollers, conveyors, wires, etc., measuring average temperature of large surfaces such as walls, roofs, pipes, etc.