The non-contact temperature measurement can be performed with 1-color pyrometers or 2-color pyrometers.
1-color pyrometers measure infrared radiation from objects in one spectral range, while 2-color pyrometers measure in two different spectral ranges simultaneously and determine the temperature from the ratio of the two radiation intensities. With 2-color pyrometers, it is not necessary to know the emissivity of the measurement material, it is canceled in the ratio formation. The following measuring conditions must be observed:
- The measured object must always be larger than the pyrometer’s spot size, since the mean value of the radiation is detected via the spot size. If the measured object is smaller than the spot size diameter, measuring errors occur because background temperatures are included in the measurement.
- As an aid in alignment, the device can be aligned to maximum temperature.
- The pyrometer’s field of view must be free from fumes, smoke or dust. These lead to a signal weakening and thus to a lower indication of the temperature.
- Measurements through windows are possible if they have constant transmittance properties in the spectral range of the pyrometer. The transmittance losses must be entered into the pyrometer.
- If the target is smaller than the spot size diameter, a warm background temperature effects the measurement. But an effect is to see from about 300°C and above the influence is also low.
- If the target is smaller than the spot size diameter, a lower signal strength occurs. Safe measurement is possible up to a signal strength of 10%. If this value is not reached, the measurement is switched off and the display no longer displays any measured value.
- Aligning the device to maximum temperature, such as with radiation pyrometers, does not work here, but it can aligned to maximum signal strength.
- If the emissivities of the target are different at the two pyrometer measurement wavelengths (for example, at bare metal surfaces), the emissivity slope of the target must be considered and entered into the device.
- If the emissivity changes due to the surface or depending on the temperature for the two wavelengths (also, for example, at an oil film on the lens), large deviations of the 2-color temperature may result. A 2-color pyrometer behaves several times more sensitive to wavelength-dependent signal changes than a 1-color pyrometer, the larger the difference and the lower the absolute value of the emissivity.
- Measurements through polluting viewing windows or through smoke or dust in the pyrometer’s field of view are possible because the radiation ratio remains constant in this case. Prerequisite: The signal attenuation is homogeneous on both wavelength ranges.
- For measurements through windows, the window material is to be observed. If the transmission properties are not uniform (for example for quartz glass), the corresponding emissivity slope must be entered.