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Temperature and humidity measurements

Temperature fixed-point cells

Range: -189 °C to +1492 °C

NPL can supply a range of temperature fixed-point cells for calibrating SPRTs and thermocouples

We manufacture the cells in-house or source the cell from a dedicated manufacturer. In both cases, the cells are supplied with a UKAS (ISO 17025) accredited calibration. Many are additionally certified under the CIPM Mutual Recognition Agreement (MRA). The cell is compared against NPL national standards, either by direct inter-comparison or via calibrated reference thermometers.

Additionally, for proper implementation, NPL can provide the peripheral equipment required for operation of the cells, including the furnace, ancillary components and thermometers.

In-house manufacture (ITS-90 fixed points) 

Where ITS-90 fixed-point cells are manufactured in-house, the cells consist of a high-purity graphite crucible containing an ingot of at least 99.9999 % pure metal. The crucible is sealed in argon within a silica (quartz) envelope. The cells are supplied with an Inconel holder, thermometer guide tube and insulation as necessary for operation.

The approximate dimensions of the freezing point cells, in mm, are:

  • External length of crucible = 240 mm
  • Depth of graphite re-entrant well = 200 mm
  • Inner diameter of thermometer well = 8 mm
  • Outer diameter of Inconel holder = 52 mm
  • Length of Inconel holder = 460 mm

This service meets the requirements of the ITS-90 for local realisation, traceable to NPL’s national standards.

In-house manufacture (high temperature fixed points)

NPL is able to offer for sale the novel cobalt-carbon (Co-C, 1324 °C) and palladium-carbon (Pd-C, 1492 °C) eutectic fixed points for thermocouple calibration at high temperature with ultra-low uncertainties.

The high temperature fixed-point cells consist of a high purity graphite crucible containing an ingot of the eutectic alloy constructed from Co-C or Pd-C, each element having a purity of at least 99.999% (99.95% purity for palladium at present, due to limited availability).

The cells are approximately 120 mm in length with a diameter of 40 mm. The re-entrant well has a diameter of 7 mm.

Furthermore, NPL is developing miniature versions of these cells, which are available to purchase for in-situ validation of thermocouple readings. These must also be operated under an inert atmosphere. See the self-validating thermocouples page for more information.

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