Thermistor Calibration
The Guildline PRT is conductively coupled to the AERI Blackbody cavity . The PRT / Blackbody assembly is heavily insulated and placed inside a temperature chamber.
The AERI Blackbody is connected to the calibrated AERI Resistance Measurement Electronics in the same way as for normal use.
Thus, during calibration the AERI Blackbody thermistors are powered and sampled the same way as they are during normal use (using the same hardware and software).
This scheme eliminates thermistor self heating differences between calibration and normal use. In general, these self heating differences can be a problem if thermistor resistances are measured using a DVM.
Typically there are five temperatures used for AERI Blackbody thermistor calibration. A regression fit of these five (temperature, resistance) points is used to determine the three traditional Steinhart and Hart coefficients: A, B, and C. The fit relationship represents test results to with an error of less than ± 0.003 K.
The thermistors used in the AERI Blackbodies are YSI 46041 Super-Stable Precision Themistors. The thermal drift of these thermistors at 70°C after 100 months is specified to be less than 0.01°C.
The thermistors are mounted in holes drilled into the cavity wall using thermally conductive (non-creeping) paste, which provides a stress free thermal coupling.
Although the test set-up and procedure facilitate a very high degree of isothermality at each temperature calibration point, there is an uncertainty of ±0.02 allocated for the gradient between the Guildline probe and the thermistors that are imbeded in the cavity.