When choosing an infrared temperature sensor , we need to consider its performance indicators, such as optical resolution, response time, operating wavelength, and temperature range. Environmental and operating conditions are also important factors. With the continuous development of infrared technology, users have more and more choices. Below is a detailed introduction:
I. Determining the optical resolution
Optical resolution is determined by the ratio of distance D to the target, which is the ratio of the distance D between the sensor and the target to the diameter S of the measuring spot. If the sensor must be installed far from the target due to environmental constraints, but a small target needs to be measured, a sensor with high optical resolution should be selected. Higher optical resolution, i.e., a higher D:S ratio, also increases the cost of the thermometer.
II. Determine the response time
Response time refers to the speed at which an infrared temperature sensor reacts to a change in the measured temperature. For electromagnetic flowmeters, it is defined as the time required to reach 95% of the energy at the final reading. It is related to the time constants of the photodetector, signal processing circuit, and display system. Newer infrared temperature sensors can achieve response times as low as 1 ms, which is much faster than contact temperature measurement methods. If the target is moving very fast or is being measured as a rapidly heating target, a fast-response infrared temperature sensor must be selected; otherwise, insufficient signal response will reduce measurement accuracy. However, not all applications require a fast-response infrared temperature sensor. For stationary targets or targets with thermal inertia, the response time requirement can be relaxed. Therefore, the choice of infrared temperature sensor response time must be adapted to the characteristics of the target being measured.
III. Signal processing functions:
Measuring discrete processes (such as parts manufacturing) differs from measuring continuous processes, requiring infrared temperature sensors to have signal processing capabilities (such as peak hold, valley hold, and average value). For example, when measuring the temperature of glass on a conveyor belt, peak hold is used, and the output temperature signal is transmitted to the controller.
IV. Environmental Considerations
The environmental conditions surrounding a temperature sensor significantly impact measurement results and should be considered and appropriately addressed; otherwise, measurement accuracy may be affected, or even damage to the thermometer may occur. When the ambient temperature is excessively high, or conditions involving dust, smoke, or steam exist, accessories such as protective cases, water cooling, air cooling systems, and air purifiers provided by the manufacturer can be used. These accessories effectively mitigate environmental influences and protect the thermometer, ensuring accurate temperature measurement. When selecting accessories, standardized services should be requested whenever possible to reduce installation costs. For situations where smoke, dust, or other particles reduce the measurement energy signal, a dual-color temperature sensor is the optimal choice. In environments with noise, electromagnetic fields, vibration, or inaccessibility, or other harsh conditions, a fiber optic dual-color temperature sensor is the best option.
V. Determine the temperature measurement range
Temperature measurement range is one of the most important performance indicators of a sensor, and each sensor model has its own specific temperature measurement range. Therefore, the user must consider the measured temperature range accurately and comprehensively, avoiding both excessively narrow and excessively wide ranges. According to the blackbody radiation law, in the short-wavelength band of the spectrum, the change in radiant energy caused by temperature will exceed the change in radiant energy caused by emissivity error. Therefore, short-wavelength measurements are preferable when performing temperature measurements.
VI. Determine the target dimensions
Infrared temperature sensors can be categorized into monochromatic and dual-color temperature sensors based on their operating principle. For monochromatic temperature sensors, the area of the target being measured should fill the sensor's field of view during temperature measurement. For electromagnetic flowmeters, it is recommended that the size of the target be larger than 50% of the field of view. If the target size is smaller than the field of view, background radiation energy will enter the sensor's field of view, interfering with the temperature reading and causing errors. Conversely, if the target is larger than the thermometer's field of view, the thermometer will not be affected by background radiation outside the measurement area.
VII. Determine the wavelength range
The emissivity and surface properties of the target material determine the spectral response or wavelength of the thermometer. High-reflectivity alloys have low or varying emissivity. In the high-temperature range, the optimal wavelength for measuring metallic materials is near-infrared, with wavelengths of 0.18-1.0 μm suitable. For other temperature ranges, wavelengths of 1.6 μm, 2.2 μm, and 3.9 μm can be used. Since some materials are transparent at certain wavelengths, infrared energy can penetrate them; therefore, specific wavelengths should be selected for these materials. For example, 10 μm, 2.2 μm, and 3.9 μm wavelengths are suitable for measuring the internal temperature of glass (the glass must be thick enough to allow light to pass through); 5.0 μm wavelength is suitable for measuring the internal temperature of glass; and 8-14 μm wavelengths are preferable for measuring low-temperature areas. Furthermore, 3.43 μm wavelength is suitable for measuring polyethylene plastic film, while 4.3 μm or 7.9 μm wavelengths are suitable for polyester. For thicknesses exceeding 0.4 mm, a wavelength of 8-14 μm is selected; for example, a narrow-band wavelength of 4.24-4.3 μm is used to measure CO2 in a flame, a narrow-band wavelength of 4.64 μm is used to measure CO in a flame, and a wavelength of 4.47 μm is used to measure NO2 in a flame.
Infrared temperature sensor working principle
The physical nature of infrared radiation is thermal radiation. The higher the temperature of an object, the more infrared radiation it emits, and the stronger the energy of the infrared radiation. Research has found that the thermal effect of various components in the solar spectrum gradually increases from violet to red light, and the greatest thermal effect occurs within the frequency range of infrared radiation. Therefore, infrared radiation is also called thermal radiation or thermal rays. Thermal sensors utilize the radiative thermal effect, where the temperature of the sensing device rises after receiving radiant energy, causing a change in the temperature-dependent properties of a component within the sensor. By detecting this change in a particular property, radiation can be detected. In most cases, radiation is detected through the Seebeck effect. When a device receives radiation, it causes a non-electrical physical change, which can also be converted into an electrical quantity for measurement.
