Abstract
We have performed measurements of a high emissivity fluid bath variable temperature blackbody source against our reference ammonia heat pipe blackbody from -45 °C to 25 °C. Although the two blackbodies are of very different designs, the spectral radiance results are consistent with calculations based on reference thermometer measurements and effective emissivity data.
Both blackbodies employ liquid coolant (ethanol) and two stage refrigeration: internal to the fluid bath LTBB and via an external recirculating bath for the AHPBB. In the case of the AHPBB, the coolant cools the ammonia heat pipe, which provides uniform temperature to the cavity, which it surrounds. The cavity of the LTBB is directly immersed in a bath, which also contains cooling coils for the refrigerant. Temperature uniformity is achieved by stirring the coolant. The radiating cavities are also of different designs. The AHPBB has a typical deep cylinder cavity with a shallow rear cone and is coated with a diffuse black paint, while the LTBB cavity is a trap design with a shorter cylinder section and a longer steeper cone, coated with a specular black paint. In each case, using coating data and cavity modeling, the effective emissivity was calculated to be ≥ 0.9999 from 3.5 µm to 14 µm.
We used the Infrared Spectral Emittance Facility (ISEF) to perform a comparison of the two blackbodies across the spectral and temperature ranges previously described. The radiance temperature and effective emissivity results were found to be within the expanded uncertainty of the measurement process, providing validation of the scale.
Validation of NIST's Low Temperature Infared Spectral Radiance Scale
We have performed measurements of a high emissivity fluid bath variable temperature blackbody source against our reference ammonia heat pipe blackbody from -45 °C to 25 °C. Although the two blackbodies are of very different designs, the spectral radiance results are consistent with calculations based on reference thermometer measurements and effective emissivity data.
Both blackbodies employ liquid coolant (ethanol) and two stage refrigeration: internal to the fluid bath LTBB and via an external recirculating bath for the AHPBB. In the case of the AHPBB, the coolant cools the ammonia heat pipe, which provides uniform temperature to the cavity, which it surrounds. The cavity of the LTBB is directly immersed in a bath, which also contains cooling coils for the refrigerant. Temperature uniformity is achieved by stirring the coolant. The radiating cavities are also of different designs. The AHPBB has a typical deep cylinder cavity with a shallow rear cone and is coated with a diffuse black paint, while the LTBB cavity is a trap design with a shorter cylinder section and a longer steeper cone, coated with a specular black paint. In each case, using coating data and cavity modeling, the effective emissivity was calculated to be ≥ 0.9999 from 3.5 µm to 14 µm.
We used the Infrared Spectral Emittance Facility (ISEF) to perform a comparison of the two blackbodies across the spectral and temperature ranges previously described. The radiance temperature and effective emissivity results were found to be within the expanded uncertainty of the measurement process, providing validation of the scale.