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 silver (961.78 °C), gold (1064.18 °C) or copper (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 zinc (419.527 °C) and aluminium (660.323 °C). In addition, NPL can supply secondary standard fixed-point blackbody sources for the freezing points of indium (156.5985 °C), tin (231.928 °C), zinc, aluminium and silver.

Above the Cu freezing point, eutectic-point cells containing a mixture of metal and carbon can be used as reference fixed points.

Technical information

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 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 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, and therefore a lower emissivity, and do not have a rhodium disc.

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 several hours 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.

Dimensions of the NPL blackbody assemblies are shown below. Some adjustment is usually possible to meet customer requirements.

 

Fixed point	Crucible outer diameter / mm	Crucible length / mm	Aperture diameter / mm	Emissivity
Zn, Al, Ag, Au, Cu (Primary standard)	42	110	3	>0.99995
In, Sn, Zn, Al, Ag (Secondary standard)	42	114	9	~0.9999

 

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 metal-carbon eutectic cells consist of a blackbody cavity within a graphite cruible containing the eutectic material. The aperture diameter of the blackbody is nominally 3 mm. The estimated effective emissivity of the cavity has been calculated to be 0.9996. The fixed-point tranistions are realsied by placing the cells within a suitable high temperature furnace. (Note that NPL does not routinely supply the furnace but can suggest suitable suppliers). The cells are fully evaluated using an NPL standard, precision radiation thermometer. The available cells, along with their fixed-point temperatures are given in the table below. Depending on available material purities the evaluation uncertainties can be as low as 0.05 % of the Celsius temperature.

 

Fixed point	Temperature/ °C
Co-C	1324
Pd-C	1492
Pt-C	1738
Ru-C	1953
Ir-C	2290
Re-C	2474
WC-C	2750

 

Other calibrated artefacts for temperature measurement

In addition to fixed-point blackbody sources, NPL can supply a range of other calibrated artefacts for temperature measurement. These include fixed points for platinum resistance thermometer and thermocouple calibrations, variable temperature blackbody sources, triple point cells, pyrometric lamps, and calibrated platinum-rhodium thermocouples. Information about these artefacts is available by clicking on the links.

Contact

Customer Services tel: +44 20 8943 6315
E-mail: temperature_enquiries@npl.co.uk