7.3.15 Diffusion Pump Limitations

378 Vacuum Systems

vapor pressure oils may backstream into the system, while the rest of the oil remains in the pump. Some of the low vapor pressure fractions may turn into a sludge. In a poorly designed or operated system, a considerable amount of higher vapor pressure oil may leave the diffusion pump and drift into the system or be drawn out by the mechanical pump. This loss of the oil from the pump can be a significant problem. The effects of this loss might not seem all that important, but if the loss is great enough to begin exposing heating elements, then other, more significant damage can occur. By including baffling and/or traps, this loss is greatly minimized. Unfortunately, the more effective the baffling, the more the pump slows down. Some baffling can decrease pumping speed by up to 50% (which is why some of the stated pump speeds can be irrelevant).

Back migration can also be caused by improper cooling around the orifice of the pump. With this problem, pump oil that had condensed begins to re-evaporate. This problem is more significant with ultrahigh-vacuum systems.

Evolution of gas (outgassed) from pump parts or attachments during use will decrease pumping potential. Any outgassing beyond general water vapor or high vapor pressure oil components is generally of greater concern to ultrahigh-vac- uum systems. However, you need to be conscious of the types of materials you are placing on high-vacuum systems. For example, some stopcock greases are adequate for student quality labs, but will fail tremendously in high-vacuum systems. In addition, connections in high-vacuum systems need to be solid and permanent. Flexible elastomer tubing and epoxies cannot sufficiently seal against leaks, nor can their high outgassing rates be contained by typical pumping systems.

Factors 1 and 3 from the previous list are of greater concern to systems that are trying to obtain ultrahigh-vacuum conditions. Factors 2 and 4 can affect attempts of highand ultrahigh-vacuum demands. The gases dissolved within pump oil from contamination during condensation and later released when pump oil is reheated is the easiest of these four problems to prevent and the hardest to eliminate once established. To prevent contaminants from entering a pump or to prevent the pump from contaminating the system, the placement of effective traps between the system and the pumps cannot be ignored.

7.3.16 Diffusion Pump Oils

The six major categories of diffusion pump fluids are explained in Table 7.9 (based on information from Laurenson ). Diffusion pump oils, like mechanical pump oils, need to be protected to maintain their properties. Sudden exposure to the atmosphere can destroy hot hydrocarbon pump oils (or even cause them to flash or explode) and damage others oils. Silicone oils, on the other hand, can easily survive contact with oxygen while hot, but their vapors can damage electronic equipment. Occasionally, undesirable vapors can speed the disintegration process of diffusion pump oils.

When operating in peak condition, a perfect diffusion pump oil should: 1. Be thermally stable

2.Be chemically inert

3.Exhibit a low vapor pressure over a wide temperature range

4.Trap vapors and particulate matter from the vacuum system

5.Be compatible (and not interfere) with, and protective of, the environment

6.Be nontoxic

This list differs from mechanical pump oil requirements primarily in the exclusion of lubricity demands. Because there are no moving parts in a diffusion pump, there are no lubricating requirements whatsoever (within some metal diffusion pumps there may be some rust prevention requirements under certain environments).

Of the six diffusion pump oil types, none has all of the above properties. In general, lighter oils pump faster than heavier oils, but heavier oils can achieve lower ultimate pressure. When your work demands varying pump oil requirements, it sometimes is easier to have separate vacuum systems than to simply change pump oils because the pump oils are seldom compatible, and mixing may impair potential peak performance.

To change a diffusion pump fluid requires the complete removal and cleanup of all previous fluid before adding a new, different oil. Before adding a new oil, be sure the type of oil (or mercury) that you wish to use will work in the pump that you have. Some oils require specific tolerances or pump designs for optimum performance. One example (in the extreme) is the use of mercury in an oil pump or vise versa. Mercury may work in some types of pumps, but the performance will not be very good. Oil, on the other hand, will not work in any mercury diffusion pump design.

The decision to use mercuryor oil-based diffusion pump fluid may be academic to those living in areas where mercury use is banned, but for others, such decisions are beyond idle curiosity. Mercury does not break down on contact with air when hot (although it may oxidize somewhat), it will not react with most compounds, and gases do not dissolve within mercury to the same degree as they do with oils. On the other hand, mercury can affect and/or damage electrical components in a vacuum system such as thermocouples (see page 422). In addition, mercury is a health hazard and once mercury has spilled on the floor, the only way to truly remove mercury (it is said) is to burn the building down.

Silicon oils do not break down on contact with air while hot, but on the other hand they can polymerize and develop an insulating film on electronics. Thus, their use is not acceptable for instruments such as mass spectrometers (including He leak detectors). Some specific properties of a spectrum of diffusion pump fluids are shown in Table 7.9.

*Mercury amalgamates with a few metals such as gold and aluminum, so attention must be paid to metal part selection for use on vacuum systems.

the entire system. Not only does the vacuum potential go down as dirt piles up, but so does pumping speed.

The most common problem with hydrocarbon diffusion pump oil is its fractionation into multivapor pressure components. As pump oil breaks down, it develops both lower and higher vapor-pressure characteristics. Oils with high vapor pressures can potentially drift into the system, although they are more likely to be effectively removed from the system by being trapped in the alembics of the central vertical tube, in the cold trap between the system and the diffusion pump, or in the cold trap between the diffusion pump and the mechanical pump. If not trapped, they are free to travel into the vacuum line itself or into the mechanical pump. Diffusion pump oils that collect in a mechanical pump are not likely to have any significant performance effects (as opposed to the degrading effects of mechanical pump oil collected in diffusion pumps).

In twoand three-stage glass pumps, a special distillation pot is added onto the end to separate fractionated oil (this doesn't apply to the Wheeler pump design). The three (or four) pots on twoand three-stage glass diffusion pumps are all wired in series. The first, and largest, stage of the pump has the longest resistance heating wire and therefore the greatest amount of heat. The last (and smallest) distillation pot has the shortest wire and therefore provides the least amount of heat. As in the single-stage pump, low-boiling oils (those with a high vapor pressure) are trapped in the alembics above the first stage. Each successive stage allows oils with higher boiling points to pass through connecting tubes toward the distillation pot. The lower heat of the distillation pot is not hot enough to significantly heat the oil, leaving heavy tarry oils remaining.

Before cleaning any diffusion pump, it is important to remove and/or unplug any electrical leads. Water should be turned off and removed if necessary. If tubing needs to be replaced, it is always best to cut off rubber or plastic tubing with a razor blade and replace it, rather than to try and pull the tubing off a hose connection.

To clean a metal diffusion pump, it must be removed from the rest of the system. Pour the used oil (or mercury) into a proper receptacle. Do not throw the mercury away because it is a toxic waste (a heavy metal). Fortunately, mercury may be reclaimed and reused. As far as diffusion pump oils, check with the health and safety and/or environmental officer in your institution and/or the waste disposal management of your city. Be sure to mention any hazardous materials that may have been absorbed by the pump oil during its operation to the proper authorities.

Because it is not recommended to repeatedly remove a glass diffusion pump that has been fused onto a vacuum system, an alternative approach is available to remove the oil. On the bottom of all glass diffusion pumps should be a lone glass tube that seems to do nothing. Its purpose is to drain the oil from the diffusion pump without requiring pump separation from the system. First unplug the diffusion pump from its electrical source, then, with a glass knife, scratch and snap off the drainage tube at about half its length. Assuming that the pump was properly

sidered by the EPA to be toxic. For seriously contaminated oils, and/or oils that are very tarry, something that is more heavy-duty such as decahydronaphthalene or trichloroethelene may be needed, but the latter is also considered to be a toxic waste. Each of these solutions should be followed by acetone and ethanol rinses.33 Another diffusion pump oil, polyphenylether, can be dissolved in either trichloroethelene or 1,1,1-trichloroethane. Again, both of these are considered by the EPA to be toxic materials.

Be aware that the old oil from a pump (and any solvent used to clean out the old oil) more than likely contains any toxic materials that may have come from the vacuum system. For example, if the system had a McLeod gauge, it is likely that the old oil is contaminated with mercury. The amount of contamination concentration determines how the oil or solvent can be disposed of. Unfortunately, because of the possibility that specific EPA-established concentration levels will change before you read this book, no disposal procedures are provided. Therefore, contact the EPA, or local regulatory agencies, to verify the various toxicity levels and the proper disposal procedures for materials of those levels.

If you are using a fluorinated oil such as fiuoropolyether, do not fire polish the end of the drainage tube and do not seal any glass onto the area until you have thoroughly cleaned the area. Any remaining grease that is heated higher than 280°C will turn into toxic fumes and could be lethal. You may use a fluorinated solvent such as trichlorotrifluoroethane or perfluorooctane to clean the system. The biggest problem when using fluorinated materials (either as stopcock or joint grease or as pump oil) is the environmental hazard of the solvents needed for cleaning.

Instead of using the above solvents, most of which are (or will be) banned, one can use an industrial detergent, such as BH-38, to remove the fluorinated oils. Simply by soaking the contaminated parts in the detergent can break up the oil for removal. There is no question that this will remove the vast majority of fluorinated solvent, but there is some question as to whether a film of the oil will remain. Thus, severe heating (above 280°C) is still recommended against.

Several recommended procedures for working with solvents begins with using as little solvent as possible. In addition, many solvents can be reused. For example, a solvent used for a final rinse can later be reused for an earlier rinse on the next cleaning operation. After a solvent has been reused, it can be distilled, or roto-evaporated, so you end up with a smaller amount of contaminated liquid. Because disposal costs are based on volume, any decreased amount of waste can provide significant cost savings.

After the cleaning has been completed, it may be necessary to remove the end of the drainage tube to remove any burnt deposit. Then, add an extension of the draining tube and close it off at about 1 to 1 V2 in. (see Fig. 7.26). Now the pump can be refilled. The reason for cutting the drain tube in half (as mentioned) is to provide distance from the diffusion pump onto which you can fuse an extension. If an extension were to be sealed directly onto the pump, extensive and formal annealing of the pump would be required.