Evacuation and outgassing of vacuum glazing

The evacuation and outgassing of vacuum glazing is studied by modeling and experimental methods. A two-dimensional finite difference method of modeling the pumpdown process is validated by comparison with a one dimensional, cylindrically symmetric finite difference method, and with measurements on experimental samples. In the absence of outgassing, the pressure in the glazing decreases linearly with time in the viscous flow region; in the molecular flow region, the pressure decreases exponentially with time. The time taken to evacuate the sample from atmospheric pressure to the transition pressure is approximately equal to one time constant for the subsequent exponential pressure decrease. The time constant associated with the exponential pressure decrease in the molecular flow region can be written $τ=V/C,$ where V is the volume of the sample, and C is the series combination of the conductance associated with the radially inwards gas flow towards the pumpout port, and the conductance of the pumpout tube itself. For very small gaps, $τ$ varies inversely with gap; for large gaps, $τ$ is proportional to gap. The time constant is least for gaps in the range 0.1–0.5 mm, depending on the dimensions of the sample and the pumpout tube. The outgassing rate at elevated temperatures from the internal surfaces of the glazing is shown to decrease slowly, relative to the time constant for pressure reduction in a typical sample. Over a wide pressure range, the outgassing rate is unaffected by the pressure in the sample. The time required to evacuate a sample of vacuum glazing is therefore determined mainly by outgassing, rather than by pumping considerations.