Note: this is the second half of a slightly revised version of an article series that originally appeared in The International Neon Association (INA) newsletter (a wonderful but unfortunately no longer active association devoted to the betterment of the neon sign industry. Many thanks to them.)
This month, let's look at various types of medium pressure electronic vacuum gauges, and how they work. This information is important to know, as various types of gauges are suitable for use in neon work, and others are not, depending on where and how they're used.
It should be readily apparent that some type of vacuum measuring device can assist one in producing more consistent and higher quality tubes. While I recognize that there are those out there who can produce a fine tube without any gauges whatsoever (and I applaud such skill), I feel that gauges give one the certainty that they are doing so, as well as indicating in advance when things are not well with the manifold (as happens from time to time...) On the other hand, having gauges on your manifold will not compensate for a lack of pumping experience - you still need to understand what the gauges are telling you.
The most common electronic gauges found on neon manifolds are the thermocouple gauge and the Pirani gauge. The basis of operation of both gauges is similar, and depend on the thermal conductivity of gasses at low pressure. Basically, as the pressure of a gas is reduced, so is its ability to dissipate heat. In both gauge tubes, a heated filament is surrounded by the manifold gas. The temperature of this filament is determined by the input power to the filament and the rate of heat loss from the filament to the surrounding gas.
In the thermocouple gauge, there is a thermocouple welded to the center of the filament. By measuring the voltage produced by the thermocouple, we know the temperature of the filament. If we keep the input power to the filament constant, then the filament temperature is determined by its heat loss to the surrounding gas - the more gas, the greater the heat loss. Therefore, we can calibrate what amounts to a filament temperature meter to directly indicate pressure.
In the Pirani gauge, there is no thermocouple. Instead, we exploit another characteristic of a heated filament - that its resistance varies with temperature. If we arrange the circuitry connected to the filament properly (generally in the form of a 'Wheatstone bridge', we can simultaneously heat the filament and measure its resistance. Again, this resistance meter is calibrated to indicate pressure directly.
There are a few caveats to thermal pressure measurement. [1] It only works if the composition of the gas is known, as thermal conductivity varies radically with the type of gas surrounding the filament. Generally, these gauges are calibrated for a nitrogen atmosphere (air is mostly nitrogen.) As such, one cannot use one of these gauges to measure the pressure of the neon or argon backfill gas (unless the gauge has special scales for doing so.) Their main value is in indicating how effectively the roughing pump is doing its job. [2] The range of pressure measurement is limited. Generally, thermocouple gauges are good down to about 1 micron, and a bit lower for the Pirani. An additional note: generally the accuracy of these gauges is limited, and can only be expected to be accurate within a factor of two1.
There are a few considerations to be made when choosing and installing one of these gauges. It must be recognized that the neon shop manifold can be a hostile place for a gauge tube to exist. The main hazard to be dealt with is flashback from the bombarder. A bombarder arc can fry both the gauge tube and/or any associated electronics. There are two methods of dealing with this problem. The first is to use a battery operated gauge. This generally eliminates any conductive path from the gauge back to ground, as there is no connection to ground via a power cord. However, there could be one through the manifold operator if he was to touch the gauge when the bombarder is active. As such, one should be extremely cautious, as bombarders generally take no prisoners.
The second method of protecting the gauge is to precede the gauge tube with a grounded electrode (we're speaking of a glass manifold here - if you use a metal manifold, you should already have it securely grounded.) Of course, you can/should use both methods, for extra security. My manifold has a grounding electrode preceding the gauge tube (and the stopcocks), and I ground the gauge tube shell as well. Of course, if your bombarder has a propensity for flashback, you may have to come up with alternate solutions, such as isolating the gauge tube from the manifold with a stopcock. Various manufacturers make gauges with protected circuitry for use in a neon manifold environment.
I have heard that some gauge tubes contain gold in their internal construction. These types should probably be avoided, as mercury combines with gold, and any mercury contamination within the manifold could 'poison' the gauge tube, destroying its accuracy.
For those of you interested in such things, there are articles on making your own gauge readout indicators (and for the truly adventurous, making your own Pirani tubes) in the publication The Bell Jar2. While I'm not advocating making your own equipment here, as good equipment is readily available on the surplus market at reasonable prices, it's interesting to experiment with this stuff to gain a further understanding of how this equipment works.
1Building Scientific Apparatus, by John H. Moore, published by University Press, 2009, ISBN 978-0-521-87858-6. This is a good general reference on constructing vacuum equipment.
2This publication devotes itself to
all types of experimental vacuum physics. For more information on the publication
The
Bell Jar, see http://www.belljar.net
for more information.