Specification of Determined Wavelength Range for Thermometers
The spectral‑response wavelength of a thermometer is determined by the emissivity and surface characteristics of the target material. Alloy materials with high reflectivity feature low or variable emissivity. For metal‑material measurement in the high‑temperature range, near‑infrared band is optimal, and a wavelength of 0.8‑1.0 μm can be selected. Wavelengths of 1.6 μm, 2.2 μm and 3.9 μm are available for other temperature ranges.
Some materials are transparent at certain wavelengths and allow infrared energy to pass through. Special wavelengths shall be selected for such materials. For instance, wavelengths of 1.0 μm, 2.2 μm and 3.9 μm are adopted to measure the internal temperature of glass (the measured glass shall be sufficiently thick; otherwise infrared energy will transmit through it). A wavelength of 5.0 μm is used for glass‑surface temperature measurement, and 8‑14 μm is suitable for low‑temperature‑range measurement.
For polyethylene plastic film, select 3.43 μm; for polyester materials, 4.3 μm or 7.9 μm; for materials thicker than 0.4 mm, adopt 8‑14 μm. For CO measurement in flames, use the narrow‑band wavelength of 4.64 μm; for NO₂ measurement in flames, use 4.47 μm.
Determination of Response Time
Response time characterizes the reaction speed of an infrared thermometer to changes in measured temperature. It is defined as the time required to reach 95 % of the energy corresponding to the final reading, and is related to the time constants of the photodetector, signal‑processing circuit and display system. New‑generation infrared thermometers achieve a response time as short as 1 ms, far faster than contact temperature‑measurement methods.
If the target moves at high speed or a rapidly‑heated target is measured, a fast‑response infrared thermometer shall be chosen. Otherwise, sufficient signal response cannot be obtained and measurement accuracy will deteriorate. Nevertheless, fast‑response infrared thermometers are not required for all applications. For stationary targets or targets with thermal inertia in their thermal processes, the requirement for thermometer response time can be relaxed.
Accordingly, the response‑time selection of an infrared thermometer shall match the conditions of the measured target. Response time is mainly determined by the target's moving speed and its temperature‑change rate. Requirements for response time may be loosened for stationary targets, targets with thermal inertia, or applications constrained by the speed of existing control equipment.
