Normally, you want a 4:1 ratio between the accuracy of the calibration equipment and the accuracy of the equipment being calibrated. But acceptable calibration can be achieved even if this guideline cannot be met.
The 4:1 ratio comes from MIL-STD-45662A, paragraph 5.2, which specifies that the uncertainty of measurement standards should not exceed 25% of the acceptable tolerance for a characteristic being calibrated. However, MIL-STD-45662A allows deviation from this ratio provided the adequacy of the calibration is not degraded.
SBIR’s differential blackbody sources are specified with a total system uncertainty of .025°C for temperatures near 25°C and near zero DT. Calibration is performed using a thermometer with accuracy of .01°C, so a 2.5:1 ratio exists between the measurement standard and the characteristic being calibrated.
The limiting factor in calibration of differential blackbodies is the unavailability of a better thermometer. An SPRT and resistance bridge, such as the Rosemount 162CE and ASL F-17 used in SBIR’s primary standards lab, provide greater accuracy but cannot be used because of physical limitations on the probe: the SPRT diameter is too large and its immersion depth requirement is too great to allow use in a blackbody calibration. The SBIR Model 104 thermometer is the most accurate thermometer available with temperature probes of suitable form factor for blackbody calibration.
MIL-HDBK-52A, paragraph 5.2(c) permits ratios as close as 1:1 when state-of-the-art limitations preclude a higher accuracy ratio. The 2.5:1 ratio provided by the SBIR Model 104 thermometer is well above that minimum. In addition, the Calibration Data Sheets (QF214-009) that are recorded during recalibration of both the Model 104 thermometer and the differential blackbodies consistently show a level of error that is comfortably below the accuracy specification of the instruments. Also, cross-checking of blackbody calibration using a different Model 104 thermometer has shown correlation on the order of .01°C, demonstrating that the adequacy of the blackbody calibration has not been compromised.