A Radiometric Determination of the Stefan-Boltzmann Constant and Thermodynamic Temperatures between -40 degrees C and +100 degrees C
The total radiant exitance of a black body at the temperature of the triple point of water, Ttp (273.16 K), and at a series of other temperatures in the range from about 233 K (-40 degrees C) to 373 K (100 degrees C), has been measured by using a cryogenic radiometer. From the measurements at Ttp a value for the Stefan-Boltzmann constant σ has been calculated: σ = (5.669 67 ± 0.000 76) × 10-8 W m-2 K-4. This is the first radiometric determination of σ having an uncertainty comparable with that calculated directly from fundamental physical constants. This measured value differs from the calculated one by 13 parts in 105, which is less than the combined standard deviations of the measured and calculated values. From the measurements of exitance at the other temperatures, values of the corresponding thermodynamic temperature T have been calculated by using Stefan's fourth-power law. Since the temperature of the radiating black body was also measured by platinum resistance thermometers calibrated on IPTS-68, values of (T-T68) were obtained. These range from about -(5 ± 1.6) mK at 20 degrees C to -(28 ± 2.5) mK at 100 degrees C and +(5 ± 1.5) mK at -40 degrees C. The results confirm to within a few millikelvins the departure of T68 from T above 0 degrees C already discovered by gas thermometry and show that similar departures, but of opposite sign, exist down to the lowest temperature measured, -40 degrees C. The uncertainties associated with these new values of T and (T-T68) are similar to those of the best gas thermometry.
Philosophical Transactions of the Royal Society of London Series A
- Pub Date:
- November 1985