It tells you something the alarms will not, and it tells you considerably earlier.
Why Consumption Leads
The annulus vacuum is maintained by a getter, a chemically active material of finite capacity that absorbs gas from outgassing and from hydrogen and helium migrating through steel. As it saturates, annulus pressure creeps up and convection returns as a heat path. Nothing fails. No alarm sounds. The only thing that moves is how much nitrogen you buy.
Reading the Trend Honestly
A single high reading means nothing, because workload varies. What matters is a sustained rise at genuinely unchanged workload, measured against your own baseline. Without a proper fill log you have impressions rather than a trend.
What Confirms It
Frost or condensation on an outer shell that was previously dry, usually first near the base or a support point. Any cold spot on a surface that should be at room temperature. Also log any vessel that has been dropped or moved roughly and behaved differently since, because a knock can crack a weld without leaving a mark.
A degrading vacuum does not recover. Transfer the contents out. The mechanism is explained in vacuum insulated cryogenic storage tanks.
