In my last post I talked about the 3 main types of Vacuum gauges available for use. They were:
- Force measuring (105 – 10-2 Pa)
- Heat transfer (10 – 10-2 Pa)
- Electrical charge transfer (ionization) (100 – 10-9 Pa)
I also talked about how I have a FRG-700 Inverted Magnetron Pirani Gauge currently connected to my Cryostat chamber. Well now I want to look at each of these types of vacuum gauges in a little more depth giving the general method of vacuum measurement for each. Understanding how each of these work at a basic level and their limitations can be very helpful when designing your own experiment. All of the information that I present below can be found either from a quick Google search/Wikipedia, or from the High-Vacuum Technology book, by Marsbed Hablanian, that I noted in my last post.
First, let’s look at the force measuring vacuum gauges. As noted above, these are generally useful in the 105 – 10-1 Pa region of pressures, or more specifically from atmospheric pressure ranges down to medium vacuum ranges that a single roughing pump can reach. The main two types I’ll cover in here are Bourdon gauges (105-103 Pa) and Diaphragm gauges (105-10-2 Pa). A Bourdon gauge works by using the elastic deformation of a curved, twisted, or helical shaped tube when a pressure difference is present between the gas inside and outside of the tube. This is a simple method that works very well at pressure above atmospheric quite well, and to a degree within a vacuum as well. The drawback is that as the vacuum is reduced more and more, the pressure differences change less and less as the jump between 104 -> 103 Pa is 1000 times higher than the jump between 100 -> 10-1 so the force changes are smaller and smaller till eventually the tube does not move anymore as the pressure is still being decreased.
The next type of force measuring device is a diaphragm gauge; this system extrapolates the pressure from measurements of the deflection of a small membrane because it deflects in approximate proportion to the applied pressure difference across the membrane. The exact pressure ranges that these are useful for are dependent on the size and thickness of the membrane; smaller and thinner membranes can measure smaller pressure differences, but fail at larger pressure differences. For this reason, often a separate vacuum chamber is necessary to keep the pressure difference low enough for these to be used. Larger and thicker membranes are not as accurate because of their size, but they are able to resist the forces from much larger pressure differentials.
I’ll be moving on to heat transfer vacuum gauges now. The two types that I will be covering are the thermocouple gauge and Pirani gauge. I believe one of the best ways of describing these types of vacuum gauges can be read directly from ‘High-Vacuum Technology’: These gauges measure pressure in the range for which the mean free path is comparable to or greater than the dimensions across the flow of heat occurs – in other words, in the free molecular regime. This means that the pressure needs to be low enough for these types of gauges to work properly, namely in the 102-10-1 Pa range. The basis for their measurement comes from the fact that gas loses its ability to conduct heat as pressure is lowered because there are significantly less particles to transfer the heat through convection. From what I understand about these gauges a constant source of heat is produced by voltage run through a resistor. As the pressure of the gas is reduced, its ability to conduct away the heat is lessened, so the resistor will start to heat up. By using the temperature reading of the thermocouple, the pressure of the surrounding air is able to be extrapolated.
A Pirani gauge (the type in our Varian vacuum gauge as noted previously) is the other type of heat transfer vacuum gauge type I will cover. These gauges are accurate in pressure ranges from 102-10-3 or 10-4 Pa depending on the type of equipment used. This type of gauge used a Wheatstone bridge with a thermistor exposed both to the vacuum chamber and a compensating element that is sealed off in a glass enclosure at a pressure below 10-1 Pa. If the voltage is kept constant across the circuit then as the pressure changes in the test chamber, the resistance across the bridge is unbalanced, and depending on the magnitude of unbalance, the pressure is able to be extrapolated from this.
The last type of vacuum gauges that I will cover is ionization gauges. These are for high and ultrahigh vacuum range (10-1-10-8 Pa), which require a second vacuum pump, a turbomolecular pump, for a test chamber to reach. These types of gauges are split into cold and hot cathode versions. The cold cathode variant is useful for the 100-10-7 Pa range, well within the lower range of what I expect to get my test chamber down to (~10-5 Pa). While there are other types of cold cathode vacuum gauges, ours is of the Inverted magnetron variant, also called a Redhead gauge. The basis of operation is that electrons are produced by a high voltage electrical discharge through the surrounding gas molecules creating ion-electron pairs that start to fly around. The number of these pairs is proportional to the gaseous molecular density times the voltage that produces the electrons. It’s all a very complicated process that I’m happy is something I can just plug into my computer and have it give me what the vacuum pressure is! A hot cathode is very similar to the cold cathode mode of operation; however it uses a hot filament to introduce the electrons into the system instead of a straight electrical discharge.
Well that about wraps up everything I have to say about vacuum gauges. Its most likely way more information than anyone cares about, but I’ve found that getting the information into my mind, and then onto paper really helps me get it compartmentalized in there and I actually understand better how everything fits together better. If you want more information on pressure measurement in general, I would suggest looking at the Pressure Measurement page on Wikipedia. It gives a quick run through all the types I mentioned above plus many more! Also if you want more information on High-Vacuum technology then take a look at the “High-Vacuum Technology” book that I keep referencing in these posts.