Certified primary and secondary blackbody sources
NPL can supply a range of fully certified, primary and secondary fixed-point blackbody sources for realising the ITS-90. Above the freezing point of silver the International Temperature Scale of 1990 (ITS-90) is realised and disseminated using radiation thermometry and the Planck law of radiation. The temperature T90 is defined as the ratio of the spectral radiance of a blackbody at T90 to that of a blackbody at the freezing-point of either Ag (961.78 °C), Au (1064.18 °C) or Cu (1084.62 °C).
NPL can supply primary standard fixed-point blackbody sources for Ag, Au and Cu to enable users to conveniently realise the ITS-90 in the laboratory. These sources, which are designed, built and tested by NPL, are certified by comparison with primary standard fixed-points, ensuring direct traceability to National Standards.
Fixed point blackbodies of this design are also available for Zn (419.527 °C) and Al (660.323 °C). In addition, NPL can supply secondary standard fixed-point blackbody sources for the freezing points of In (156.5985 °C), Sn (231.928 °C), Zn, Al and Ag.
Above the Cu freezing point, eutectic-point cells containing a mixture of metal and carbon can be used as reference fixed points.
NPL design – primary standards
The Ag, Au and Cu primary standard fixed-point blackbody sources consist of a graphite crucible containing an ingot of high purity (99.9999 %) metal. A high emissivity (> 0.99995) graphite blackbody cavity is immersed into, and is in thermal equilibrium with, this metal.
The standard assembly parameters are: nominally 42 mm outer diameter of the crucible, 110 mm crucible length, 3 mm aperture diameter and emissivity of >0.99995. Some adjustment is usually possible to meet customer requirements.
The crucible is contained in a 58 mm diameter outer graphite casing to protect the assembly from oxidation. A removable rhodium disc defines the blackbody aperture to facilitate alignment.
Blackbody sources of this design containing Zn or Al can also be supplied.
The Ag, Au and Cu fixed-point blackbody sources typically achieve melting and freezing plateaux of approximately 30 minutes duration, depending on the furnace conditions, with plateaux of over an hour possible for the lower temperature fixed-points. The uncertainty in the radiance temperature is assessed for each individual source, and is typically less than 0.1 °C.
All NPL fixed-point blackbody sources are supplied with a calibration certificate and operating/assembly instructions. Sources can be ordered separately, or complete with a three-zone electrically heated furnace and all the ancillary components necessary to establish the freezing point.
The secondary standard In, Sn, Zn, Al and Ag fixed-point blackbody sources are of a similar design to the primary standard sources but have a larger aperture (9 mm), and therefore a lower emissivity (~0.9999) and do not have a rhodium disc.