Donate to Science & Enterprise

S&E on Mastodon

S&E on LinkedIn

S&E on Flipboard

Please share Science & Enterprise

What is an Ion Gauge?

– Contributed content –

Ion gauge

(Skitterphoto, Pexels)

6 Oct. 2020. Ion gauge is a must-have component for every modern high and ultra-high vacuum system. An ionization gauge is a low-pressure measuring device for anything in the region of 10-3 to 10-10 Torr. However, if you take a look at the Internet, you will notice that there is a wide assortment of UHV gauges to choose from. This can make it difficult when creating your vacuum, as you may not know what option to select. That is why we have put this post together, providing you with everything you need to know about ion gauges.

A glimpse at one of the most popular UHV gauges

There is only one place to start when it comes to the ion gauge, and this is with the Bayard-Alpert type ionization gauge. R. T. Bayard and D. Alpert invented the Bayard Alpert gauge in 1950 in order to overcome the limitations of the triode gauge in vacuum pressure measurement. The triode gauge presented problems because it did not have the ability to indicate pressures beneath 10-8 Torr due to photoelectrons being emitted when electrons struck the ion collection, as they would generate low-energy x-rays.

Consequently, they developed the vacuum gauge which took their name, and which operates by ionizing the gas particles in the gauge volume. After this, a tiny ion collector wire is used to collect those ions. You can have this manufactured by a specialist wire manufacturer. Then, pressure is determined by measuring the resulting current to the ion collector, as this deciphers the number of molecules that are present. The Bayard-Alpert gauge is a type of ionization gauge known as ‘hot-filament’, which is the most widely used form of the low-pressure measuring device. A hot-filament gauge is simply one that uses a cathode, i.e. a heated filament, to emit electrons toward an anode, which is a grid. All in all, the main reason why people opt for this style of gauge over the other options that are available is that it can reach and measure much lower pressures. But what about the cold ionization gauges that are available?

What about the cold cathode ion vacuum gauge?

Now that you have an understanding regarding the hot ion gauge filament, it is important to look at the other side of the spectrum, i.e. cold ionization gauges. There are many different types of cold cathode gauges that are utilised for vacuum measurements, including the inverted magnetron, the magnetron and the Penning. These gauges operate by utilising crossed magnetic and electric fields to trap electrons, with the typical operating range being between 10-2 and 10-9 Torr. If you find a company claiming to sell a cold cathode gauge that will measure pressures below this, you should be very cautious.

One thing that is worth noting is that the pressure in the chamber and the ion-induced current are not linearly related. Instead, the relationship is exponential, and the presence of spurious discontinuities in pressure vs. current characteristics complicates matters. A cold ion vacuum gauge are not recommended for high vacuum transfer standards because they are not as accurate as hot-filament gauges. Another downside important to note is that cold cathode gauges can be difficult to start. The moment high voltage is applied, the discharge in a cold ionisation gauge does not automatically start. In fact, the striking time can vary from gauge to gauge.

What about an ion gauge controller?

The right controller improves efficiency and may even reduce pumping times as controls can be automated. However, this is only possible if the right control unit is purchased for your system. Knowing what to expect from your ion gauge control center is the first stage in ensuring you get the quality that you need from the device. Firstly, the unit you choose should be easy to use, provide a guided set up and offer the option of password protection. This reduces the risk of mistakes and heightens your security, reducing the risk of deliberate or accidental tampering with the settings. It also needs to have a clear and easy to read display and a choice between automatic and manual controls.

In terms of its technical abilities the ion gauge control unit needs to have a wide and continuous measurement range that at least covers pressures from normal air pressure to the very low pressure required for UHV and extremely high vacuums. It is a benefit to you if the controller is able to run two gauges sequentially. This means that you are able to add a Pirani gauge to your system alongside your ion gauge. The reason that this is a benefit is because the Pirani gauge is more effective at measuring pressures during the early stages as the system passes through the low or rough, vacuum stage. A system that automatically switches from the Pirani gauge to the ion one as the former reaches the end of its effective working range is a further benefit available on some models.

Ion gauge filament replacement tips

Ion gauge filament replacement is needed when the filament comes to the end of its effective working lifespan. The exact lifespan depends on a number of factors including the material that the filament is comprised of and the residual gases present in the vacuum system. Ion gauge filaments are generally made from either Tungsten or Iridium. Where is used it can be coated in either Thoria or Yttria. gauge’s Tungsten filament, you need long nosed pliers, side cutters, an Allen key (often supplied with the replacement filament), isopropyl alcohol and clean room gloves. Before starting the process, the pliers and cutters should be cleaned with the alcohol. The alcohol can also be swabbed on the setting screws to improve lubrication. The feed-through wire should be supported with the long-nose pliers while the Allen key is used to undo the filament support screw.

Then loosen the remaining support screws, this will enable you to remove the old ion filament assembly. Next remove the new assembly’s restraining sleeve and fit the center support into the barrel connector. Once in place tighten the screws. Continue by holding each ion filament termination with the pliers and extending them down to the outer barrel connecters. These should them be fixed in place with the set screws. The method for vacuum gauge Iridium filaments is similar, but you should always refer to your manufacturer for specific instructions.

*     *     *

Comments are closed.