High Vacuum Gauge Reading Stability

[weed78]

We have an old perlite insulated cryo tank in which we are trying to improve the level of vacuum insulation. We are down below 1 torr but are witnessing wild fluctuations in the reading of the vacuum gauge. It is a digital gauge made by Kurt Lesker. If we close the valve to the tank then the reading in the line comes down below 10 microns and is steady, but as soon as we open the valve we see the readings fluctuate as much as 100 microns. Does anyone know if this is expected with digital gauges, are we picking up some form of gassing off effect in the vacuum space, or is there another phenomenon taking place?

 

[eric2718]

Have you found a solution? It sounds like you have a leak into your vacuum space. I would imagine that out-gassing would be minimal in an old tank.