How can you ensure a vacuum jacket is working properly?

The technique of using a vacuum guard or barrier to thermally insulate a cryogenic or refrigerant tank is pretty obvious–-remove the air from around an object and you remove the conductive heat transfer. But in practice, this method is only effective if the less than 1/1000th of the air remains in the space! In other words, if the pressure is much above 1 Torr, the jacket is worthless. As the graph below shows, the insulating effectiveness improves as the pressure is reduced below 1 Torr and is essentially perfect at pressures below 10 mTorr. (Radiation and conduction through mechanical connections will then dominate the heat transfer.) So to know how well the jacket is doing its job, you need to know what its pressure is. If the pressure is above 1 Torr, there is very little benefit; if it’s in the mTorr range, then all is well.

Method

To effectively measure pressure in the mTorr range, a vacuum dial gauge is simply not sensitive enough. All of the effects that we’re interested in happen 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). However, there is a kind of vacuum gauge that performs very well in this range by actually measuring the thermal conductivity of the gas to infer the pressure. Called a thermal conductivity, thermocouple, or TC gauge, it 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. For some situations monitoring is only done periodically (dip-stick check). This may be the case on a cryogenic tank truck or rail car where the tube is generally installed permanently and checked with a hand-held, battery operated electronic unit.

While the ideal location of the gage tube(s) depends on the configuration of the system, some general guidelines can be considered. Of course, for the situation where periodic checks are made with a hand-held readout, the tube should be installed in an easily accessible location. The 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 is always the best (lowest), while what you need to know is the worst pressure. For this reason, locations in the more remote branches or dead-end sections are the best for ensuring that adequate insulating properties are maintained.