Main Standards for Choosing an Infrared Thermometer
The application of a thermometer is mainly determined by its measuring range. Both the measuring voltage and the starting value of the measuring range must meet the requirements of the measurement task. The larger the selected measuring voltage, the lower the resolution and thus the poorer the accuracy. Especially at low starting temperatures, using an excessively large measuring voltage will reduce accuracy exponentially. Therefore, it is recommended to select the smallest possible measuring voltage.
The starting value of the measuring range determines the spectral sensitivity and thus the type of detector. Measurement errors caused by incorrect emissivity settings are significantly smaller for short-wave sensors than for long-wave sensors. For example, at 800°C, the measurement error of a thermal film sensor (8–14 μm) due to incorrect emissivity adjustment is about five times greater than that of a germanium photodiode sensor (1.1–1.6 μm). Germanium photodiode sensors typically permit a measuring range starting at approximately 250°C.
For instance, in the ceramics industry or in combustion processes at power plants, the measuring range is usually 0–1300°C. To avoid large errors, thermometers equipped with short-wave detectors should be used, even though their effective measuring range starts at 250°C up to 1300°C.
Another criterion for selecting a suitable thermometer is the distance-to-spot ratio (D:S ratio), which refers to the ratio between the measuring distance and the diameter of the measuring spot. If the target is small, the measuring distance is large, or a so-called "hot spot" exists on a large surface, a high distance-to-spot ratio is required. Conversely, for large measuring areas, a low distance-to-spot ratio is suitable because the sensor provides a stable output signal relative to the measuring spot.
It must also be determined whether the thermometer is equipped with a sighting device, as such a device can increase costs by 50%, making cost a key consideration. For large-area targets, a built-in sight is usually unnecessary. Instead, an external sight can be used for alignment during installation, offering a cost advantage since multiple measuring points can share a single sight.
For small targets or long measuring distances, continuous sighting capability is required. Using a viewfinder with a reticle allows the actual size of the measuring spot to be clearly seen. A more economical alternative is a laser sight, but it only supports spot-by-spot measurement.
In closed furnaces or similar applications, a window is required. It is therefore necessary to decide which functions the thermometer needs, such as averaging, special value storage, limit switching, or computer interfaces. Emissivity adjustment is essential to adapt the thermometer to the surface of the measured object. Other functions can be implemented inexpensively by connecting recorders, controllers, or programmable logic controllers.
In addition, the mechanical design is a decisive factor for certain applications. Under high ambient temperatures, a thermometer with only an optical lens unit connected via a fiber-optic cable to the electronics located away from the heat zone is advantageous, as cooling devices can be saved.
