Stacey, C; Salmon, D R; Simpkin, A (2009) NPL guarded hot-plate for measuring thermal conductivity of insulation from -175°C To 50°C. In: 30th International Thermal Conductivity Conference (ITCC 2009), 29 August - 2 September 2009, Pittsburgh, PA, USA.

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Abstract

NPL has developed a guarded hot-plate to determine the thermal conductivity of insulation materials at temperatures between 175 °C and 50 °C. Along with NPL's other guarded hot-plates, this apparatus forms part of a facility for addressing new European product standards for industrial insulation that require thermal conductivity measurements between 170 °C to 800 °C.
The NPL Low-Temperature Guarded Hot-Plate (LTGHP) is based on a double-sided configuration with a heater plate of lateral dimensions 305 mm x 305 mm and central metering area of 152.5 mm x 152.5 mm. The heater plate is constructed from a specially design inconel heating element in a polyimide film substrate and sandwiched between two aluminum plates instrumented with T-type thermocouples and a differential thermocouple between centre and lateral guard with sixty four junctions.
Heat extraction for both the cold plates and environmental chamber is provided by controlled pulses of both liquid nitrogen and heated compressed air injected into the system. The cold plates are made from copper their channeling for liquid nitrogen and air are based on a twin helix design. The cold plates are also instrumented with T-type thermocouples. The environmental chamber is insulated with a combination of micro-porous insulation panels and vacuum insulated panels.
The design of the LTGHP also employs a number of novel features that are required to meet the metrological demands of making accurate measurements at cryogenic temperatures. These features include a multi-cavity enclosure for maintaining the environment around the plate/specimen stack at the mean specimen temperature; an automated system to apply a constant compressive load; a temperature controlled isothermal junction box; an in-situ thickness measurement mechanism that links the displacement between plates at cryogenic temperatures with transducers at the laboratory temperature, and multi-segment calibration of these displacement transducers.
The guidelines and specifications given in ISO 8302:1991 and EN 12667:2001 were followed during the design of the LTGHP and the performance checks described in ISO 8302:1991 have been completed, including measurements relating to imbalance errors, edge heat losses and linearity. An assessment of the overall measurement uncertainty has been made following the approach outlined in the ISO Guide to Uncertainty in Measurement and the additional errors due to issues arising from the low temperatures have been considered, including equilibrium criteria, metering area correction and temperature uniformity.
Finally, measurements were carried out on European certified reference material IRMM 440 and compared with both the certified values and the additional low temperature values from DFT. The measured values were in agreement with the reference values to within their combined uncertainties down to temperatures of -150 °C

Item Type:	Conference or Workshop Item (UNSPECIFIED)
Keywords:	thermal conductivity, low temperature, guarded hot plate, insulation
Subjects:	Advanced Materials Advanced Materials > Thermal Performance
Last Modified:	02 Feb 2018 13:15
URI:	http://eprintspublications.npl.co.uk/id/eprint/4808

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