tained in the system. If the discharge changes color, you have found the leak. If leaks are located far apart on a vacuum system, it is possible to find a leak while looking for others. However, leaks that are close together may appear, or act, like one. Therefore, after finding and repairing one leak(s), you should reexamine the area for other leaks.

When using this technique, turn off all lights and shut all windows (and doors) that allow light into the room. The darker your workplace, the more subtle the variations in the discharge may be observed. If your room can be brought to total darkness, you may want to place a small table lamp near where you are working. Then, you will be less likely to bump into equipment as you make your way from the room's light switch to the testing area after turning the room lights off.

If you spray a probe gas lighter than air (such as helium) on a vacuum system, start at the top of the system. Start at the bottom with gases heavier than air. Otherwise, the drifting of the respective gases may provide false or inconclusive leak identification. It may be necessary to close windows and doors and might even be necessary to set up baffles to minimize drifting gases to prevent false or inconclusive leak identification.

Once a leak is verified and localized, pinpointing the leak can be done by taking an acetone-soaked cotton swab and rubbing it all around the section in question. Once the acetone is wiped over the leak, there will be an immediate color change of the discharge to white or turquoise.

The shape and angle of a leak can have a significant effect on the time before a vapor is removed from a system and the colors of a discharge turn back to normal. If the glass is very thin and the leak is perpendicular to the glass, the time for recovery can be very short (<10 seconds). On the other hand, if the glass is thick and/or the leak is on the diagonal through the glass wall, a leak can hold the acetone for a period of time, even many minutes. Thus, the length of time necessary for the colors to go back to normal provides a clue as to the nature of the leak.

7.6.8 Soap Bubbles

As stated before, the Tesla coil cannot be used on metal systems. If a leak on a metal system is large enough to prevent you from using a mass spectrometer (or you do not own a mass spectrometer), you may be able to use positive pressure to locate leaks in a vacuum system.* Place the vacuum system under pressure with dry air, nitrogen, or helium up to about 60 psig. Then squirt a soapy solution on areas in question while looking for the formation of bubbles. This technique is the same that is used on all pressure systems and is even used by plumbers when installing gas pipe.

Some important rules are:

Positive pressure cannot be used on glass systems because the pressure can cause glass stopcock plugs to blow out or joints to separate. The pressure may be sufficient for damaged to the system to occur.

1.The gas used to pressurize a system should be clean and dry. Nitrogen and argon are very good because they are nonreactive gases. Do not use pure oxygen, especially if there are any greasy areas in the system because an explosion may result.

2.Any components that may be damaged by high pressure (i.e., ion gauges) should not be exposed to any pressure, or limit the pressure to 1 to 2 psig.

3.Watch out for fittings not intended to be used in pressure conditions.

4.Use a very bubbly solution (dish soap in water is good). Professionally made solutions such as Snoop™ or kid's bubble-blowing solutions work as well.* Window-cleaning solutions are not recommended because they typically have nonfoaming additives.

5.Apply the solution slowly and carefully so as not to create bubbles.

6.Do not immediately assume that there is no leak if no bubbles readily form; they may take a few seconds to an hour to form.

7.Use a waterproof marking pen or crayon to mark each leak as you proceed.

7.6.9Pirani or Thermocouple Gauges

As mentioned in Sec. 7.5.14, many gauges read an inferred pressure, not real pressure. Some vacuum gauges use the thermal conductivity of gases present in the system to infer the pressure of the system. These gauges are based on the concept that "less" gas will conduct "less" heat. Because different gases have different thermal conductivities, the user needs to make allowances if the gas in a system has a different thermal conductivity than the particular gas a gauge has been calibrated to use.

The standard reading for a thermionic gauge is based on air or nitrogen. If a probe gas is used whose thermal conductivity is higher (or lower) than air, its presence would be detected by the Pirani or thermocouple gauge as a change of indicated pressure. As would be expected, light gases with low viscosity can find smaller leaks. Hydrogen and helium are such gases, and they show a leak by an apparent increase in pressure because they have higher thermal conductivities than air. Other gases, such as argon, show an apparent decrease in pressure because of their lower thermal conductivities. The liquids in Table 7.14 can also be used with Pirani and/or thermocouple gauges because they can provide similar apparent changes in pressure.

Regardless of which direction the gauge varies, the important thing is that the gauge varies because the gauge is detecting a change from what was in the system before the probe gas (or liquid) was introduced. Sometimes there may be a slight

*Soap solutions must be rinsed off of stainless steel because soap can corrode the metal.

drop in pressure caused by the liquid filling up a long, thin "tunnel" type of hole. This drop is then followed by a rise (or depression)* in pressure as the vapors of the liquid affect the thermocouple. Do not expect equal swings of low followed by high pressure, because the causes of pressure swings are not related.

Another leak detection system that uses thermal conductivity is a gas leak detector that samples the air around the system being checked by a simple pump (often a diaphragm pump). The system under examination is filled with any type of gas (except combustible gases). Then, changes in the gas composition surrounding the piece being tested are "sniffed" by a probe. Any changes detected by a thermocouple within the detector are an indication of a leak, and the user is notified by a swing of a dial's needle (one way or another) and/or an audio alarm. The sensitivity of these devices varies depending on the probe gas used. Helium provides the best sensitivity. They are capable of detecting leaks as low as 10~5 atm/ sec, or, in other words, they can be used with simple vacuum systems.

One particular advantage with gas leak detectors is the constant distance from the leak to the thermal conductivity sensing device. This constant distance makes the time lag consistent wherever you are "sniffing" with the probe. When relying on installed thermocouples (on the vacuum system), a leak located right near the thermocouple will provide an immediate and full-strength response, whereas a leak some distance from the thermocouple will have a more subtle effect and will require some time before this effect is observed.

As is standard when using probe gases on the inside for detection outside, the greater the pressure exerted within the system being tested, the greater the sensitivity that can be expected from the detection device. With glass systems, it is not recommended to use a pressure greater than can be developed by attaching a balloon filled with the probe gas to the system. It is possible to flow helium through the system before beginning the testing, but it then becomes necessary to wait until all the excess ambient helium in the room drifts off. Otherwise, there is too much helium "noise" in the room to allow for any sensitive readings. The wait could be minutes to hours depending on the size of the room, ventilation, and sloppiness of administering the helium to the system.

Do not breathe too close to the sniffing probe or its reference chamber because the CO2 from your breath could cause a deflection of the needle. If you are going to use a helium test gas after doing a bubble test, you must wait until the system is completely dry. The water will cause a negative deflection and the helium will cause a positive deflection, providing a combined weaker deflection. If, coincidentally, the water and helium are perfectly balanced, there will be no deflection.

Once a leak region is indicated, move the detector away from the system being tested and allow the needle to come to a normal position. Then you can go back in and zero in on the specific location.

*If the thermal conductivity of the probe gas (or liquid) is higher than nitrogen (the base standard), the gauge will indicate a drop in pressure. If the thermal conductivity is less than nitrogen, the gauge will indicate an increase in pressure.

7.6.10 Helium Leak Detection

Imagine a room full of people. Now imagine you want to find the one(s) named Bill. If there was no way to easily identify the Bills, the process would become significantly laborious. However, if you called out and asked all those named Bill to please raise their hands, the task of identification becomes easy. The helium leak detector has the ability to isolate and count the helium atoms from a vacuum system full of many other atoms and molecules. In reality, the helium leak detector is nothing more than a customized mass spectrometer. The mass spectrometer was developed in the 1920s, but became a leak detection tool in the 1940s when the need for ultra-vacuum systems became critical on the Manhattan project.*

The operation of a mass spectrometer is fairly straightforward. It places an electrical charge on all the gases passing through its system. This electrical charge allows a flowing path of charged gas molecules to be bent by a magnetic field. The angle of the bend depends on the mass of the molecule* and can be calculated with high precision. Because the particles have an electrical charge, they can be collected on a grid and counted. If a collection grid is placed at a specific point of the possible radius path, it is possible to collect only one type of molecule. A helium leak detector is a mass spectrometer that is permanently tuned to collect and record only the helium ion.

Without the magnet, the grid would collect all charged particles and the total gas pressure could be recorded. With the magnet separating the charged particles by their molecular weights, only helium is detected. This quality allows the helium leak detector a sensitivity of leak detection that is independent of total gas pressure. The leak detector will indicate that it is sensing helium by swinging a needle on a meter, sounding an electronic signal, and/or illuminating some type of display. For quantitative work, a digital or analog readout is required.

When helium leak detectors first became available, they were large and cumbersome and required highly trained technicians. They were also very expensive. Now, like most electronic equipment, they are smaller (relatively), easier to use (for some operations, they can be automated), less expensive, more reliable, more sensitive, and faster. As is typical of most things, helium leak detectors cannot have all the aforementioned attributes at once. However, it is possible to pick and choose which options are more important to you and select a detector that has features that are best suited to your needs. The usual tradeoff is sensitivity for speed.

Although there are other leak detectors and leak detection techniques that rely on detection of a change in gas, none have enjoyed the success of the mass spectrometer tuned to helium. There are other types of tuned mass spectrometers that specifically look for oxygen or halogen, but they are not as common. Their opera-

*An interesting article on the development of the helium leak detector can be found in the article "History of Helium Leak Detection" by Albeit Nerken in the Journal of Vacuum Science, and Technology A, Volume 9, #3, pp. 2036-2038 (May/June 1991).

fThe more the magnetic field can bend the path, the smaller the radius and therefore the smaller the molecule.

tor during leak analysis. Helium, like other noble gases, is completely nonflammable.

Helium is found in extremely small quantities (approximately five parts per million) in the atmosphere. Any discovery of concentrations greater than normal are easily discernible. Imagine the challenge of someone looking for water leaking out of a vessel held underwater.

Helium is relatively inexpensive.* Neon (mass 20), which could pass many of the qualifying tests for leak detection, fails here because of its greater costs.

Finally, it should be pointed out that helium leaks better than air because it is a much smaller molecule than air. The kinetic theory of gases predicts that helium will flow 2.7 times faster than air through an ideal leak at molecular flow condi-

Of

tions. Because of this property, helium can indicate a leakage rate that is greater than would be encountered in practice. For instance, because helium can permeate through O-rings, a helium leak detector could indicate a leak (albeit very small) at an O-ring joint despite the fact that oxygen or nitrogen are not likely to leak past the same O-ring.

*This level of leak detection is not inexpensive: A new mass spectrometer helium leak detector with the accompanying equipment can easily cost between $20,000 and $30,000. Purchasing used equipment can significantly reduce the initial costs, but one should not enter the level of helium leak detection because it seems like a good idea. On the other hand, if you need a helium leak detector, you cannot afford not to have one.