A gauge used for monitoring the integrity of a vacuum drawn on a chamber or interstice.
In a bladder tank, a slight vacuum is drawn between the rigid outer tank and the flexible inner tank. The constancy of this vacuum can be observed through use of a gauge, attached to the tank. If the gauge reveals that the vacuum has dropped, this is an indication of a problem, either in the outer or inner tank.
Similarly, many double wall tanks and some double wall sumps are shipped from the manufacturer with a vacuum on the interstice to verify the integrity of the tank during shipping and installation.