As outgassing can greatly increase the length of time necessary to achieve a given vacuum, it is useful to know how being exposed to atmospheric pressure and air affects the pump down. To this end, we pumped down the dewar after leaving it open for 5 days (120 hours), 2 hours, and 30 minutes.
Qualitatively, the longer the dewar was exposed to air before pumping down, the longer it took achieve a given pressure. For all three runs, we started the timer once the dewar had reached 1 torr. The time to reach various pressures was recorded.
Plotting the achieved pressure as a function of time demonstates the result that longer open times result in more outgassing. This is manifested in the dewar pressure taking longer to fall. The straight lines are rough estimates at bounding curves for the minimum and maximum expected pump-down times (for a dewar pumped down immediately after being opened, and for a dewar pumped down having never been pumped down before). The two bounding curves converge at a point, which can be interpreted as the maximum achieveable vacuum for this system.
While it would've been nice to test additional intermediate times (for example, 1 hour), the data show a clear trend. Pumping down after 30 minutes open time proceeded very quickly, with the first pressure benchmark reached after only 8s. It is probable that shorter open times will result in decreased pump down times. This reinforces the notion that the best way to keep the system "clean" is to keep it under vacuum as much as possible, and to minimize the amount of time the system is exposed to ambient air.