Penning Gauge - Theory of Operation
While Mr. Penning no doubt appreciates having this gauge named after him, one can get a better idea of how this gauge operates from its original name - the cold cathode ionization gauge. Limiting the discussion to the type we use here, the cathode is the outer cylinder of the gauge tube. It is cold; that is, it is at ambient temperature. The anode consists of a wire loop (tungsten, I believe) mounted in the center of the tube and mounted to an electrical feed-through. A potential of up to about 3KV is applied to the anode, which induces field emission of electrons from the cathode. The electrons ionize gases inside the vacuum chamber and the charged products are collected as a current. This small current is a function of the chamber pressure (yippee) and the species of gas present. There is a magnet mounted on the outside of the gauge tube. Its function is to increase the path length traversed by the electrons thus amplifying their effectiveness. And that's pretty much it for how they work.
There are some typical problems with these types of gauges that one should be aware of, however. First, they don't work if the vacuum is too good (damn the bad luck). No, seriously, they stop conducting when the pressure drops somewhere below 10-7 Torr. Sometimes one can initiate conduction by tapping the side of the gauge tube gently with a metalic object. (yes, really) The shock can aid in initiating field emission. Some systems (none of ours) use UV sources to aid in maintaining conduction. Secondly, they get dirty. Everything that they ionize is collected on either the anode or the cathode - yuck. After a while, they need to be cleaned. Of course, it is trivial to check the condition of the gauge tube when the system is open. This is always the best time. A dirty gauge will typicall read irratically. This can also be a symptom of outgassing, though. Lastly, they are not terribly accurate. This isn't so important in our lab. The key is to learn what is typical for your chamber and use that as a guide. Staff can assist with calibration of the control units. Contact Powell Barber or Greg Brown for assistance.