Diffusion pumps cleaning

Nozzles at the diffusion pump are clogged Clean the diffusion pump. 

A special cleaning procedure was developed which virtually eliminated the characteristic decomposition of dilute metal-ammonia solutions. The optical cell was evacuated with a diffusion pump for several hours, about 50 ml. of anhydrous ammonia condensed onto the walls of the apparatus, and this wash ammonia was forced into a waste flask containing sodium and cooled with dry-ice. The optical cell was rinsed a total of four times. Approximately 500 ml. of anhydrous ammonia was condensed into the cell, about 5 mg. of potassium was introduced into the cell with the winch assembly (Figure 3a), and the solution was allowed to remain in the cell for at least 48 hours. The metal-ammonia solution was then forced from the cell into the waste flask the cell was rinsed four times with anhydrous ammonia and evacuated before introducing ND8. 

The catch to the vacuum method is that you must have a controlled boil without which the material and/or solvent are liable to be sprayed all over your vacuum system. Although a solvent can easily be pumped out of a vacuum system, it can cause serious problems if it remains in contact with stopcock grease, O-rings, or mechanical and/or diffusion pump oils. Any particulate material deposited within a vacuum line can only be removed from the vacuum line by disassembly and cleaning. With a glass vacuum system, such a cleaning may be difficult or impossible. 

For years, glass diffusion pumps provided (relatively) easy attachment to a glass vacuum system (but rather difficult removal), are mostly free from attack by corrosive substances, provide easy observation of the materials inside the pump, and can be cleaned (with some difficulty). 

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. 

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. 

The cleaning of a mercury diffusion pump is somewhat simpler because mercury does not break down as most pump oils do. However, mercury gets dirty, and a dirty mercury pump still needs to be cleaned. After you have drained the mercury out of a glass diffusion pump, refill the pump with approximately a 6 molar nitric acid solution and let it sit until the mercury has been removed. Do not pour this liquid down the sink Check with local waste management and/or your health and safety officer. The pump should be flushed with distilled water and then rinsed with methanol for drying. 

Second only to the mechanical gauge as the easiest device to measure and read a vacuum (and decidedly easiest in construction) is the liquid manometer (see Fig. 7.37). A well-made mercury manometer, kept very clean, can measure vacuums of up to 10 3 torr. This sensitivity can be increased by up to 15 times if a liquid with less density, such as diffusion pump oil, is used. However, diffusion pump oil is far more difficult to keep clean and can require either (a) a very tall (and thereby impractical) column or (b) a manometer of very limited range. In addition, because of the strong surface tension between diffusion pump oil and glass, long waiting periods between readings are required as the oil settles into place. 

Within a helium leak detector, there are valves that isolate the mass spectrometer and gauge sections, the diffusion pump, and the trap from the rest of the system. These valves are used to prevent contamination or damage to these sections when cleaning, adjusting, or venting is required during use. Find and use them. 

Surfaces with oil or grease contamination should be cleaned by an organic solvent such as methanol or acetone in an ultrasonic cleaner. It is also important to avoid touching the cleaned specimen with bare hands because fingerprints contain volatile hydrocarbon compounds. Sometimes a dark mark occurs on a well-cleaned specimen. This might result from hydrocarbon contamination from a dirty vacuum system. Hydrocarbons escaping from the diffusion pump oil into the SEM chamber can also contaminate a specimen under electron beam. 

The vacuum pumps discussed in Chapter 6 are capable of reducing the pressure to about 0.1 torr when they are new, are well trapped, and have clean oil in them. This is not a sufficiently low pressure to permit a high efficiency molecular distillation. A previous example calculation indicated that 10" torr or less is necessary. The most common method for reducing pressures from 10" to 10" torr (13.3 - 0.013 Pa) is the diffusion pump. Multiple stage diffusion pumps are capable of reducing pressures to 1 O torr. A diagram of a down jet diffusion pump is shown in Figure 7-11, p. 79. 

Vacuum evaporators have been in use for several decades for the thermal evaporation of materials, such as metals, onto a specimen to provide a conductive layer and dissipate charge during electron iiucroscopy. Holland [312] is an early authority on the topic of vacuum deposition. Typically, a 12 inch diameter bell jar is fitted onto a vacuum system which includes a rotary pump and diffusion pump. Electrodes are fitted onto the baseplate of the evaporator and connected to a transformer. These electrodes are used for attachment of the metals and the carbon rods for evaporation. An important accessory is a liquid nitrogen trap which is fitted above the diffusion pump to trap oil vapors and keep the vacuum clean, which is essential for carbon coating. 

Under normal circumstances, there is no need for the operator to be concerned about routine maintenance of the mass analyzer. With modem turbomolecular pumping systems, it is highly unlikely there will be any pump- or sample-related contamination problems associated with the quadrupole, magnetic sector, or TOP mass analyzer. This certainly was not the case with some of the early instruments that used oil-based diffusion pumps, because many researchers found that the quadrupole and prefllters were contaminated by oil vapors from the pumps. Today, it is fairly common for turbomolecular-based mass analyzers to require no maintenance of the quadrupole rods over the lifetime of the instrument, other than an inspection carried out by a service engineer on an annual basis. However, in extreme cases, particularly with older instruments, removal and cleaning of the quadrupole assembly might be required to get acceptable peak resolution and abundance sensitivity performance. 

Figure 6a. Flow apparatus, inlet system, and mass spectrometer used by Clyne and Watson for kinetic studies of FO radical reactions. Block diagram of mass spectro-metric system. Pumping lines as follows A, differential chamber (between pinholes X and Y) B, mass spectrometer ion source C, flow tube D, flow tube diffusion pump (used only for overnight clean-up of flow tube) X and Y, sampling pinholes. System shown is for study of FO radical reactions.

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