1.When possible, usea gas over a liquid.

2.Do not spray or squirt a liquid on. Usea cotton swab (or Kimwipe) to wipe it across parts of the system. Inaddition to safety, this method provides more control in finding leaks.

3.Solvents can damage parts of your system such as O-rings, causing a greater leak than theone you were originally trying to find. In addition, if the solvent drips on a water hose andcreates a hole, you then have a water leak and a mess. If it drips on electric wires, you mayhave a short.

4.All of the chemicals listed in Table 7.14 are dangerous to breathe or

have in contact with skin from short to extended periods of time. Some of the chemicals listed have severe OSHA restrictions foruse without a fume hood, andsome aretoxic chemicals andshould not beused at all (such ascarbon tetrachloride and xylene).

5.Do not have any open flames orsparks while working onleak detection. Unplug iongauges and other electrical equipment.

6.Besure to have plenty of ventilation andlimit yourself to thefirst three liquids in Table 7.14. These liquids are(relatively) the safest, andthey are the best from the list. Still, you should use a buddy system, and check with OSHA and/or local/state regulations to verify if there are

any legal restrictions onthe uses of any of these chemicals in your area.

7.When possible, select a low-vapor-pressure liquid over a high-vapor- pressure liquid toprovide faster cleanup time. Inaddition, nonpolar solvents aremore easily removed from glassware than polar solvents (for example, methanol vs. acetone).

8.When spraying probe gases on a vacuum system, be sure to start at the top with gases less dense than air and start at the bottom with gases denser than air.

Other aspects ofTurnbull's four factors will beconsidered further in Sec. 7.6.9.

7.6.7 The Tesla Coil

If you have a glass system andcanachieve a vacuum between approximately 10 to approximately 10~3 torr, then you can use a Tesla coil (sometimes called a "sparker") to look for moderate-size leaks. Because this range is the vacuum range of a mechanical vacuum pump, theTesla coil provides anexcellent tool for examining such systems.

The Tesla coil will ionize thegas molecules remaining in a vacuum systemand cause them to glow. Above pressures of about 10torr the gas molecules quench a discharge. That is, they aresoclose together they lose their extra energy bybumping into other molecules rather than giving off light. Below 10"3 torr, the mole-

Heavy-duty Tesla Coil

Fig. 7.57 The lightand heavy-duty Tesla coil.

cules are too far apart and the mean free paths are too long to maintain a discharge.

As the tip of a Tesla coil is slowly waved within 1-3 cm of an evacuated glass vacuum system, the gases remaining will discharge with a glow characteristic of the gases within the system. If you bring theTesla coil near a leak, a large white spark will jump from thetip of the coil to thespecific leak spot. This event is very dramatic anddemonstrates the location of a leak in a very effective manner. The spark is actually seeking ground, and the leak provides a path to the discharge inside the vacuum system. In turn, this discharge (which is an electrical conductor) provides a path to themechanical pump for completion of the ground.

Metal components confound Tesla coil use: Their ground is easier to obtain than the ground found by passing through a glass leak andthepoorer conducting discharge within.

There area fewlimitations to theuseof a Tesla coil:

1.It cannot be used near metal clamps or glass-to-metal seals on a glass system. Themetal provides a ground for theelectric discharge, bypassing theionization of the gasinside thesystem.

2.A large quantity of very small holes mayprevent youfrom obtaining a decent vacuum. However, none of the holes may belarge enough for the Tesla coil to indicate a leak.

3.The self-contained Tesla coils often found in many labs are recommended by themanufacturers notto be used for longer than 10minutes of continuous operation. If your needs require long continuous useona consistent basis, heavy-duty Tesla coils are available that can be used continuously. These coils areeasily identified because they have small boxes (approximately 5 in. x 5 in. x 8 in) connected to their hand-held sections. These hand-held sections arethesame asthose found on standard light-duty Tesla coils (see Fig. 7.57).

4.TheTesla coil will not find a leak within a demountable (seals such as stopcocks orjoints) that is caused by poor application of grease or old stopcock grease that hassheared.

Table 7.15 Appearance of Discharges

in a Gas Discharge Tube at Low Pressures76

() indicates a different observer's opinion of the color.

1.Turn off some (or all) of the lights in a room so that any discharge from the coil can more easily be seen (it may also be necessary to close doors and/or cover windows). In addition, try not to face an open window.

2.A metal wire (such as copper or nichrome) wrapped around the tip of the Tesla coil (see Fig. 7.57) and bent in a rightangle can extend your reach and/or reach behind glassware.

The Tesla coil can also be used to make the vacuum line a discharge tube to light up probe gases or liquids. Gases or vapors in a vacuum will light up in specific colors from a discharge caused by a Tesla coil. A list of the colors that can be achieved with pure gases in a discharge tube can be seen in Table 7.15.

charge tubes, which have very little to do with the discharge from the Tesla coil because of the following conditions:

1. The applied voltage and pressure can vary greatly.

450 Vacuum Systems

Table 7.16 Appearance of Discharges in Gases and Vapors at Low Pressures When Excited by a Tesla Coila

a From Journal of Chemical Ed.ucation, 68, pp. 526-528, (1991 Reproduced with permission.

*Void: a lack of discharge due to a loss of vacuum.

2.The Tesla coil discharge, in leak testing, is a through-glass discharge, whereas a discharge tube is a metal-electrode discharge.

3.The distance between electrodes in a discharge tube is fixed, as opposed to the distance between the Tesla coil and ground (this distance can vary constantly from as little as several centimeters to many meters during leak detection).

4.The colors indicated in the tables are for pure gases. In a working vacuum system there will always be some air and moisture mixed with the probe gases. There will also be a trace amount of hydrocarbon vapors (from the mechanical pump) as well as other gases and vapors that incidentally may be in the system.

Work by Coyne and Cobb provided a more effective list of discharge colors because the colors were observed in the act of actual leak detection. They are presented in Table 7.16. Their work demonstrated that spraying gases had limited success in leak indication, but moderate success when the item being tested was enclosed within the gas (by way of a bag). Vapors from applied liquids provided better indicators and more easily demonstrated the specific location of a leak.

If you have items with glass-to-metal seals or metal supports you wish to leakcheck with a Tesla coil, you can encase them in a bag. Initiate a discharge with the Tesla coil while filling the bag with a test gas, such as oxygen or helium. This procedure can help to verify whether or not there is a leak. However, it will not help locate the leak. To specifically locate the leak, you may wipe a probe liquid, such as acetone on a cotton swab, over the suspected areas while a discharge is main-