Fermion's 100 Questions, Issue 7 | Infrared Temperature Measurement

Time: 2020.03.13Source: View: 1510

19. How does the infrared thermometer work? What does the object distance ratio mean? Will the temperature measured by infrared thermometer be different at the same temperature for different materials?

When the temperature of a natural object is higher than absolute zero, it will continuously radiate electromagnetic waves to the surroundings due to the existence of internal thermal motion, including infrared rays with a wavelength of 0.75 ~ 100μm. Infrared thermometers are made based on this principle and have many advantages such as easy use, fast response speed, high sensitivity, wide temperature measurement range, and the ability to achieve online non-contact continuous measurement.

Comparison of Common Temperature Measurement Methods

Blackbody Radiation

A black body is an idealized radiator that absorbs radiation energy of all wavelengths, has no energy reflection or transmission, and has a surface emissivity of 1. Other materials with a reflection coefficient less than 1 are called gray bodies. There is no real black body in nature, but in order to understand and obtain the law of infrared radiation distribution, a suitable model must be selected in theoretical research. Planck's black body radiation law, that is, the black body spectral radiance expressed in wavelength, is the starting point of all infrared radiation theories, as shown in the figure below.

As the temperature rises, the radiation energy of the object becomes stronger. This is the starting point of infrared radiation theory and the design basis of single-band infrared thermometer.

As the temperature rises, the radiation peak moves to the short-wave direction (to the left) and satisfies Wien's displacement theorem T *λm = 2897.8 μm*K. The wavelength λm at the peak is inversely proportional to the absolute temperature Τ. The dotted line is the peak line at λm. This formula tells us why high-temperature thermometers mostly work at short-wave and low-temperature thermometers mostly work at long-wave.

The rate of change of radiation energy with temperature is greater at short-wave than at long-wave, that is, the thermometer working at short-wave has a high relative signal-to-noise ratio (high sensitivity) and strong anti-interference. The thermometer should try to work at the peak wavelength, especially in the case of low-temperature small targets. This is particularly important.

Infrared thermometers use a point-by-point analysis method, that is, focusing the thermal radiation of a local area of an object on a single detector, and converting the radiation power into temperature through the known emissivity of the object. Due to the different objects to be detected, measurement ranges and usage occasions, the appearance design and internal structure of infrared thermometers are different, but the basic structure is generally similar, mainly including optical systems, photoelectric detectors, signal amplifiers and signal processing, display output and other components.

Test Error

Since infrared temperature measurement is non-contact, there will be various errors. There are many factors that affect the error. In addition to the factors of the instrument itself, they are mainly manifested in:

● Emissivity
Emissivity is a physical quantity that measures the radiation capacity of an object relative to a black body. In addition to being related to the material shape, surface roughness, and concavity of the object, it is also related to the direction of the test. If the object has a smooth surface, its directionality is more sensitive. The emissivity of different substances is different, and the amount of radiation energy received by the infrared thermometer from the object is proportional to its emissivity.

When the emissivity of an object is higher, the reflectivity is lower, the influence of background and reflection will be smaller, and the accuracy of the test will be higher; on the contrary, the higher the background temperature or the higher the reflectivity, the greater the impact on the test. In addition, the emissivity is also related to the test direction. The larger the test angle, the greater the test error. The test angle is preferably within 30°, and generally should not be greater than 45°. If it is necessary to test at a greater than 45°, the emissivity can be appropriately lowered for correction.

●Distance coefficient
The distance coefficient (K=S:D) is the ratio of the distance S from the thermometer to the target to the diameter D of the temperature measurement target. It has a great influence on the accuracy of infrared temperature measurement. The larger the K value, the higher the resolution. Therefore, if the thermometer must be installed far away from the target due to environmental conditions, and a small target needs to be measured, a thermometer with high optical resolution should be selected to reduce the measurement error. In actual use, many people ignore the optical resolution of the thermometer. Regardless of the size of the target point diameter D, turn on the laser beam and aim at the target to test. In fact, they ignore the S:D value requirements of the thermometer, so the measured temperature will have a certain error. ● Target size The measured object and the field of view of the thermometer determine the accuracy of the instrument's measurement. When using an infrared thermometer to measure temperature, generally only the average value of a certain area on the surface of the target can be measured. There are three general situations in the test: 1. When the target is larger than the test field of view, the thermometer will not be affected by the background outside the measurement area, and can display the real temperature of the measured object in the determined area of the optical target. The test effect is best at this time. 2. When the target is equal to the test field of view, the background temperature has been affected, but it is still relatively small, and the test effect is average. 3. When the target is smaller than the test field of view, the background radiation energy will enter the field of view of the thermometer to interfere with the temperature reading and cause errors. The instrument only displays the weighted average of the temperature of the measured object and the background. Therefore, it is recommended that the size of the target exceeds 50% of the field of view when measuring the temperature.

●Environmental factors
The environmental conditions of the measured object have a great influence on the measurement results. As the ambient temperature rises, the additional radiation effect will be greater, and the temperature measurement error will be greater.

In the process of infrared radiation transmission, due to the absorption of the atmosphere, the energy will always be attenuated to a certain extent. Atmospheric absorption refers to the process of converting part of the infrared radiation energy into other forms of energy or distributing it in another spectrum during the transmission process. The degree of atmospheric absorption changes with the change of air temperature. The farther the distance of the measured object is, the greater the influence of atmospheric transmission on temperature measurement. Therefore, when performing infrared temperature measurement outdoors, it should be carried out in an environment without rain, fog and relatively clear air. When performing infrared temperature measurement indoors, it should be carried out in an environment without water vapor, so that a more accurate value can be measured with the minimum error.

How to Correctly Choose an Infrared Thermometer

There are three aspects to consider when choosing an infrared thermometer:

● Performance indicators, such as temperature range, spot size, operating wavelength, measurement accuracy, response time, etc.

● Environment and working conditions, such as ambient temperature, window, display and output, protective accessories, etc.

● Other options, such as ease of use, maintenance and calibration performance, and price.

Infrared thermometers include three series: portable, online and scanning, and are equipped with various options and computer software. Each series has various models and specifications. To select an infrared thermometer from various models of thermometers with different specifications, you should pay attention to the following aspects:

1> First of all, you must clarify the measurement requirements and the problems to be solved, such as the temperature of the target to be measured, the target to be measuredSize, measurement distance, target material, target environment, response speed requirements, measurement accuracy requirements, and whether to use portable or online type, etc.;

2> Compare the measurement requirements and the problems to be solved with the existing various types of thermometers, and select the instrument model that can meet the above requirements;

3> Among the many models that can meet the requirements, select the best combination in terms of performance, function and price.

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Fermion's 100 Questions, Issue 7 | Infrared Temperature Measurement