Main Criteria for Selecting Thermometers
The application of a thermometer is primarily determined by its measuring range. Both the full-scale measurement value and the lower limit of the measuring area shall comply with actual measurement requirements. Selecting an excessively large measuring range results in lower resolution and, consequently, poorer measurement accuracy. Especially when the starting temperature is low, adopting a large measuring range will reduce accuracy exponentially. Therefore, it is recommended to select the minimum feasible measuring range.
The lower limit of the measuring area determines spectral sensitivity and further defines the detector model. When emissivity is set incorrectly, short‑wave sensors produce significantly smaller measurement errors than long‑wave sensors. For thermal sensors (8–14 μm) operating at 800 °C, measurement errors caused by incorrect emissivity adjustment are five times greater than those of germanium photodiode sensors (1.1–1.6 μm). Germanium photodiode sensors offer a measurable temperature range starting from approximately 250 °C.
For example, temperature measurement in ceramic production or power plant combustion processes generally covers 0–1300 °C. To avoid substantial errors, thermometers equipped with short‑wave detectors should be selected, even though their effective measuring range starts at 250–1300 °C.
Another key criterion for selecting a suitable thermometer is the distance-to-spot ratio. This refers to the ratio between measuring distance and spot diameter. For small targets, long measuring distances, or isolated hotspots on large surfaces, a high distance-to-spot ratio is required. Conversely, for large measuring areas, the sensor delivers a stable average output signal, making a low distance-to-spot ratio sufficient.
It is also necessary to confirm whether a sighting device is required, as an integrated sighting system can increase costs by approximately 50%, making cost control an important consideration. When measuring large-area targets, a built-in sight is usually unnecessary. An external sighting accessory may be used instead for calibration during installation, providing a cost advantage, since a single external sight can serve multiple measuring points.
For small targets or long-distance measurements, continuous aiming capability is essential. A transparent sight lens with scale markings allows direct observation of the actual measuring spot size. Low-cost options include laser pointers, which support only single-point measurement. For closed furnaces and similar enclosed environments, an observation window is required.
Users shall clarify required configurations and extended functions in advance, such as averaging functions, value memory, limit alarms, or data communication interfaces. Adjustable emissivity is essential to adapt to different target surface conditions. Additional functions can be implemented cost-effectively by connecting external recorders, controllers or programmable devices.
In addition, mechanical structure is also a decisive factor for certain operating conditions. In high ambient temperature environments, a split-type design is recommended: the optical lens assembly is installed on-site, while the electronic unit is remotely placed via fiber optic cables away from high-temperature zones. This design reduces the need for additional cooling equipment and lowers operating costs.
