Ensuring a Vacuum Jacket is Working Properly

To effectively measure pressure in the mTorr range, a vacuum dial gauge is simply not sensitive enough. All of the effects related to insulating occur while the needle is resting on the “0” of the dial (remember we need to know how much of the last 1/1000th of an atmosphere is left). For this reason, a specialized vacuum gauge that measures the thermal conductivity of the gas in the jacket is used to infer the pressure. Called a thermal conductivity, thermocouple, or TC gauge, this gauge is an effective and reliable way to ensure the insulating properties of the jacket. When you think about it, a thermal conductivity gauge uses the best technology for this application, because it directly measures how well the vacuum is insulating. To implement a simple scheme for continuously monitoring a vacuum jacket for stationary application (e.g. LNG, cryogenic bulk tank, or vacuum-jacketed pipe), the gauge tube and readout electronics can be permanently installed on the vacuum containment. For outdoor installations, the tube can be mounted on the tank with the electronic readout located remotely. In some situations, monitoring is only done periodically (dipstick check). This may be the case on a cryogenic tank truck or rail car where the tube is typically installed in a permanent location and checked with a hand-held, battery operated electronic unit.

While the ideal installation location of gauge tube(s) depends on the configuration of the system, some general guidelines should be considered. When periodic checks are made with a hand-held readout, the tube should be installed in an accessible location. Additionally, technical considerations would dictate that the gauge tube be located near anticipated leak points or moving seals like valve stems or rotary feed-throughs. In actively pumped vacuum jacket systems, installing a gauge tube near the pump will not necessarily indicate when leaks begin to adversely affect the performance of the jacketing. The pressure at the pump will always be the best (lowest) reading, while the information you need to know, is the worst pressure. For this reason, locations in remote branches or dead-end sections are ideal for ensuring that adequate insulating properties are maintained.