NPL Guarded Hot Plate for Measuring Thermal Conductivity of Insulation from -175°C to 50°C

摘要

NPL has developed a guarded hot plate for determining 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 [1] that require thermal conductivity measurements between -170°C and 800°C. The NPL Low Temperature Guarded Hot Plate (LTGHP) is based on a double-specimen configuration with a heater-plate that includes a specially designed heating element. The temperature control for both the cold-plates and the environmental chamber is provided by controlled pulses of both liquid nitrogen and heated compressed air injected into the system. The design of the LTGHP also employs a number of novel features that are used 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 laboratory temperature. The guidelines and specifications given in ISO 8302:1991 [2] and EN 12667:2001 [3] were followed during the design of the LTGHP and the performance checks described in ISO 8302:1999 [2] have been completed. An assessment of the overall measurement uncertainty has been made and the additional errors due to issues arising from the low temperatures have been considered, including metering area contraction and temperature uniformity. Finally, measurements were carried out on European certified reference material IRMM 440 [4] and compared with both the certified values and the additional uncertified low temperature values from DFT, University of Padova, Italy. The measured values were in agreement with the reference values to within their combined uncertainties down to temperatures of -150°C.