Determination of the internal pressure of vacuum insulation panels with the envelope lift-off technique - methods for analysing test data

摘要

For vacuum insulation panels the internal pressure is crucial for the thermal performance of the panel. Depending on the pore size of the core material the internal pressure is directly connected with the thermal conductivity. Compared to the determination of thermal conductivity, the measurement of the internal pressure can be performed very quickly, what makes it ideal for quality check purposes. Also for the proof of durability by applying an artificial ageing procedure and in the case of development of new high barrier film materials or production methods, the precise determination of internal pressure is very important. The reference method for internal pressure measurements on VIPs with mainly pyrogenic silica as core material is the so called envelope lift-off technique. For this method the atmospheric pressure around the VIP is lowered subsequently and the pressure as well as the deflection of the envelope is observed. As soon as the surrounding pressure is lower than the internal pressure of the VIP the envelope lift-off from the core material. To calculate the internal pressure of the VIP the measured curve of pressure as a function of the envelope displacement is evaluated. Up to now no detailed information of how to process the test data to achieve the best possible accuracy is available. To avoid a user influence, automated test data analyses based on adequate algorithms to identify the internal pressure are favorable. This paper describes algorithms for two methods to evaluate the observed data, which are referred to as tangent method and triangle method. The algorithms are tested on a set of internal pressure measurements on VIPs with pyrogenic silica core and flexible laminate envelopes. Both methods are suitable for automated analysis of test data. (C) 2018 Elsevier B.V. All rights reserved.