Evacuated glazing is a form of double glazing, utilizing the internal vacuum between the two glass sheets to eliminate heat transport by gas conduction. To achieve the target thermal conductance, the internal gas pressure must be $LSEQ 0.1 Pa. To estimate the useful service life of the glazing in terms of vacuum stability, it is necessary to establish the rate of degradation of the vacuum as a result of outgassing processes. We have measured the gas pressure of several samples of evacuated glazing which have been deliberately degraded by baking at high temperature. These measurements were made using two different methods of measuring the thermal conductance of the samples, on of which can be used while the glazing is at high temperature. The insulating properties of the glazing can be considerably degraded (gas pressure $GRT 10 Pa) by baking at temperatures close to those used in the evacuation procedure during the construction of the glazing. Such samples show an improvement in the vacuum with time if stored at temperatures significantly below the bakeout temperature. The experimental data are consistent with a qualitative model of the outgassing of evacuated glazing which involves diffusion of molecules within the volume of the glazing, and adsorption/desorption of gas molecules on the surfaces.
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