TR Metrological Errors_201604_en
3Pages

Metrological Errors in Non-contact Temperature Measurement Applications by Albert Book Practitioners of metrology are often skeptical of non-contact temperature measurement methods, also known as pyrometry. The technical data provided by pyrometer manufacturers, however, document that these instruments do indeed provide very accurate and precise temperature readings. Aside from the importance of selecting an instrument best suited to the particular application, it is crucial to allow for material specific properties and ambient influences in order to obtain a reliable result. Measuring errors can be avoided if pyrometers are used in a skilled manner. This article exposes the most common sources of error and explains how to minimize or prevent them. Emissivity Pyrometers measure the thermal radiation which an object emits. The amount of infrared energy which is radiated will depend on the material properties and surface characteristics of the object. This ability to radiate thermal energy is referred to as “emissivity” (e). For precise temperature measurement, the pyrometer must be adjusted for the measurand’s specific emissivity. Selecting an incorrect emissivity setting can result in considerable errors. The table (Fig. 1 demonstrates the temperature deviations (AT) when a pyrometer is incorrectly adjusted for 80% emissivity rather than the true 90% emissivity. This error increases when measuring at longer wavelengths or at higher temperatures. Therefore one should always chose a pyrometer which operates at the shortest possible wavelength but is still feasible for the temperature range of the application. Fig. 1 Measurement errors depending on wavelength and temperature at a 10% deviation in emissivity (e. . . = 0.8 and e , = 0.9) Instrument real Especially in the case of metallic surfaces whose emissivities are either extremely variable or uncertain, measuring at shorter wavelengths will greatly minimize the potential for error. The emissivities of metals tend to increase at shorter wavelengths, and at the same time, the likelihood of error - in the event the emissivity was misadjusted - will tend to decrease.

TECHNICAL REPORTS Transmission loss Background and incident radiation In ideal conditions, the atmosphere between the pyrometer The displayed temperature reading will depend on the total and the target will be unobstructed. If gases or particles such as amount of infrared energy detected by the pyrometer’s sensor dust, vapour, smoke, or other media such as protective lenses (ΦΣ). As the equation below shows, the total incident radiation is or opaque materials are in the sensor’s sighting path, a portion the sum of the thermal energy emitted from the target plus extra- of the energy emitted...

TECHNICAL REPORTS Caused by physical conditions, the optical error increases with the wavelength. Therefore, an even greater effort is required to correct the optical error for long-wave measuring devices and thus devices for low measuring ranges. The negative impact is, that the displayed measured value of cheap pyrometers, which allow measurements from room temperature, is highly dependent on the selected measuring distance. The „Size of Source Effect” is negligible when the measured object is considerably larger than the target spot and the surface is almost at the same temperature...