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Infrared thermometer

What is an infrared thermometer?

An infrared thermometer is a thermometer that infers the temperature from a portion of thermal radiation sometimes called black body radiation emitted by the object being measured. They are sometimes called:

  • Laser thermometers (because a laser is used to help aim at the measurement area)
  • Non-contact thermometers
  • Heat guns

All of these terms refer to a device with the ability to measure temperature from a distance.

By knowing the amount of infrared energy emitted by the object and its emissivity, the temperature of the object can be determined with varying precision according to parameters set out later. Infrared thermometers are part of the family of "thermal radiation thermometers".

Sometimes, especially near room temperatures, readings can be prone to errors due to radiation reflection from a warmer body. Thus under certain conditions the person holding the instrument can influence the measurement.

The design basically consists of a lens to focus infrared thermal radiation on a detector, which converts radiant power into an electrical signal that can be displayed in units of temperature after being compensated for by ambient temperature. This allows the temperature to be measured remotely without contact with the object to be measured. A non-contact infrared thermometer is useful for measuring temperature in circumstances where thermocouples or other probe-type sensors cannot be used.

Examples of use 

Industrial and tertiary sector (measurement on objects)

In some typical circumstances, the object to be measured moves; where the object is surrounded by an electromagnetic field, as in induction heating; where the object is contained in a vacuum or other controlled atmosphere; or in applications where rapid response is required, the precise surface temperature is desired or the object temperature is above the recommended point of use for contact sensors, or contact with a sensor would damage the object or sensor, or would introduce a significant temperature gradient across the surface of the object.

Infrared thermometers can be used for a wide variety of temperature monitoring functions.

- Verification of mechanical or electrical equipment (verification of hot spots)

- Checking the heating temperature

- Verification of calibration and control furnace

- Checking hot spots in firefighting

- Monitoring of materials in heating or cooling processes

Medical area (measurement of people)

Measuring the temperature of patients in a hospital without touching them. During outbreaks of fever-causing illnesses, such as the SARS coronavirus and Ebola virus disease, infrared thermometers have been used to check the fever of arriving travelers without causing harmful transmissions among those tested.

There are many varieties of infrared temperature sensing devices, both for portable and portable use and as fixed installations.

How accurate is an infrared thermometer?

Infrared thermometers are characterized by two parameters the precision and angular cover. The simplest instruments can have a measurement error of ± 2 ° C or ± 4 ° F. GUILCOR infrared thermometers have an accuracy of ± 1 ° C and can be supplied with a calibration certificate.

The distance / spot ratio (D: S) is the ratio of the distance to the measurement surface and the diameter of the temperature measurement area. For example, if the D: S ratio is 12: 1, the diameter of the measurement area is one-twelfth of the distance to the object. A thermometer with a higher D / S ratio is able to detect a more specific and narrower area at a greater distance than one with a lower ratio. A 12: 1 rated device can detect a 1cm circle at a distance of 12cm, while a 10: 1 ratio device achieves the same circle from 1cm to 10cm.

The ideal target area should be at least twice the spot size at this distance.

An infrared thermometer cannot be placed too close to its target, or this proximity causes heat to build up in the thermometer housing and damage the sensor. The measurement error usually only decreases at too great a distance due to the effects of reflectivity and the inclusion of other heat sources in the sensor's field of view.

According to Stefan-Boltzmann's law, the radiant power is proportional to the square of the temperature, so when the measurement surface has both hot and cold areas, the indicated temperature may be higher than the actual average temperature and closer to the square of average power.

Most surfaces have a high emissivity (over 0,9 for most biological surfaces), and most IR thermometers rely on this simplifying assumption; however, reflective surfaces have a lower emissivity than non-reflective surfaces. Some sensors have an adjustable emissivity measurement, which can be set to measure the temperature of both reflective and non-reflective surfaces. A non-adjustable thermometer can be used to measure the temperature of a reflective surface by applying non-reflective paint or tape, with some loss of accuracy.

A sensor with an emissivity setting can also be used to calibrate the sensor for a given surface or to measure the emissivity of a surface. When the temperature of a surface is known precisely (for example by measuring with a contact thermometer), the emissivity setting of the sensor can be adjusted until the temperature measurement by the IR method matches the temperature. measured by the contact method; the emissivity setting will indicate the emissivity of the surface, which can be taken into account for subsequent measurements of similar surfaces.

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