A newly designed glass diffusion pump by Michael Wheeler*27 has eliminated most of the problems of the classic glass diffusion pump design while considerably improving the pumping capacity. The pump (see Fig. 7.23) was a direct copy of a metal three-stage pump, and was intended to assist teaching the operation of the metal pump. Remarkably, the pump performed better than the metal pump it was copied from. Aside from speed (the operational pumping speed of this pump was determined to be an impressive 188 liters/s), the pump provides easy attachment and removal from a vacuum system, easy to remove internal components for cleaning,1 and external heating via a heating mantle. This latter point is important because it eliminates the through-glass electrodes of the classic design that are easily susceptible to breakage. Consistency of pump manufacturing is enhanced by the pump's design plans that Wheeler provides to anyone who wants them. Quality of manufacturing is still dependent upon the quality of the glassblower.

7.3.13 Attaching a Diffusion Pump to a Vacuum System

As previously mentioned, a diffusion pump works in tandem with a mechanical pump. The simplified alignment of such a system should be considered: vacuum line, diffusion pump, mechanical pump. However, in reality, the setup is a bit more complex.

1.A trap (i.e., a liquid nitrogen trap) must be placed at the inlet (system) side of a vacuum line to trap any vapors coming from the line as well as any lighter fractions of the diffusion pump fluid (see Sec. 7.4 on traps for the proper alignment and use of traps on a vacuum system).

2.The tubing between the vacuum system and diffusion pump must have as few restrictions (in size and shape) as possible. Glass or rotary stopcocks with bores from 12 to 15 mm minimum (on the system side of the diffusion pump) will help to ensure minimum hold-up. In addition, tubing in these areas should be as straight (unkinked, with few bends) as possible.

3.Because the pressure on the fore side of the diffusion pump is considerably higher than the pressure on the system side, large-bore stopcocks are not necessary and a stopcock with a bore of 8 to 10 mm is sufficient.

4.Oils used in mechanical pumps have significantly higher vapor pressures than the oils used within diffusion pumps. Therefore, it is important to prevent backstreaming of mechanical pump oils into the diffusion pump. This barrier can be done either with liquid nitrogen cold traps, molecular sieves, water-cooled thimbles, or chevron baffles.

University of Arizona, Department of Chemistry.

+ Because glass is less susceptible to chemical attack than metals, it may be desirable to use glass components whenever possible.

5.Avacuum system needs to achieve a sufficiently lowpressure before the diffusion pump canbegin operation. A mechanical pump that works in

tandem with a diffusion pump is called a fore pump. Fore pumps must be capable of achieving =10"2-10~4 torr and have sufficient speed to effectively work with a diffusion pump.

Most mechanical pumps with sufficient speed and ability are twostage pumps. However, it is not good to run a two-stage mechanical pump for an extended period of time without pulling on a load. Therefore, a fairly large system that would require running a mechanical pump for over 5-10 minutes before a =l-torr vacuum was achieved should include a smaller roughing pump. Once the vacuum has been "roughed" down, the fore pump can be started to bring the vacuum line to a low enough pressure to start the diffusion pump.

Fortunately, vacuum systems in most laboratories are small enough that the fore pump can double as the roughing pump with no problems.* For the few times that one needs to go from atmospheric to vacuum conditions, all that is necessary is to close off

To system

To mechanical

vacuum pump

Fig. 7.23 Wheeler diffusion pump design.

* Operations that require you to cycle your vacuum line back and forth from atmospheric to low pressure can put significant strain on the fore pump and its oil, so a roughing pump should again be considered.

the line to the diffusion pump and open the line to the roughing pump. Then, once the pressure is within the range at which the diffusion pump can operate, close the valve to the roughing pump and open the line to the diffusion pump.

A glass vacuum system requires a glassblower to assemble and fuse the various sections together (if you have not had any experience working with glass, do not start to learn on a vacuum system). This technique provides the best approach to assembly because it provides a solid, leak-tight seal between all parts. The main problem with this permanent form of connection is that if you ever need to dismantle the items for any reason, you again need the services of a glassblower. In addition, you should be forewarned that glass can be worked only so many times by a glassblower, after which it begins to devitrify easily (phase separation) and lose its strength and chemical durability. The attachment and disassembly of any glass apparatus should be only used in case of strong need, not as regular lab practice.

The second best method of attachment is with O-ring seals. These seals can provide an easy method of attachment and disassembly. Regrettably, O-rings can leak if the alignment between the two members is not perfect. In addition, some O-ring elastomers may decompose if left in contact with diffusion pump fluids for an extended time.

The third form of attachment is with some type of ground joint. It would be wonderful if this attachment could be made with a full-length standard taper joint (see Sec. 3.1.1), but alignment of several joints that allow for easy assembly and disassembly of several parts is seldom possible. Ball-and-socket joints would seem like an alternative, but they are not really acceptable for high-vacuum work and should not be considered. The use of ball-and-socket joints is not really impossible, but because their reliability is always in doubt, the use of these joints for vacuum line assembly should never be your first choice.

7.3.14 How to Use a Diffusion Pump

In the stylized drawing of Fig. 7.24, you see the layout of a vacuum line, traps, roughing pump, fore pump, diffusion pump, and related stopcocks (because this drawing is stylized, it's not intended to look exactly like your vacuum system). First find the parts on your line that are represented by the parts in Fig. 7.24. If not every item shown is on your line, determine which parts your line and this drawing have in common, and proceed accordingly. The following general guidelines should be followed when using a vacuum system with a diffusion pump (see Table 7.8 for diagnostic checks and corrections of diffusion pump problems):

1.The system must be in a vacuum state before the diffusion pump can work.

2.Let the mechanical pumps evacuate the traps before they are placed in liquid nitrogen to prevent freezing oxygen within the traps.

Roughing pump

(if needed)

Stopcock 2

Stopcock 5

Stopcock 6

The Vacuum Line

C X

Stopcock 7

Stopcock 4

Fig. 7.24 The arrangement of stopcocks and traps for a vacuum line with a diffusion pump.

3.The diffusion pump should be bypassed in the early stages of the system's pump-down, the cooling water for the diffusion pump must be turned on early, and the diffusion pump must be in a vacuum before the diffusion pump heaters are turned on. If your air-cooled diffusion pump has a fan, turn it on now. Some diffusion pumps are wired so that turning on the heater also turns on the cooling fans. If you have this setup, verify that the fan is working when the heater is turned on. Be sure that the diffusion pump is open to a pump before turning on the heater. Otherwise the heated pump oil's expansion could cause the diffusion pump to explode.

If you are starting a glass diffusion pump for the first time and need to adjust the heat (from a voltage regulator), first place the heat at a very low (10%-20% of full) setting and let it stabilize (this procedure will also help to outgas the oil). Then, over the course of time, increase the setting some 10 units at a time, letting the temperature stabilize at each setting. When you see the "wet marks" of vapor condensing on the horns inside the pump, the temperature is set. If the oil is furiously pumping away, the voltage regulator is on too high and needs to be reduced. Once the temperature is established, mark your voltage regulator for future reference. If the pump is water-cooled, connect a relay to shut off power to the pump in the event of a water pressure loss.

Metal diffusion pumps require no adjustment because they are fac- tory-set to provide the right temperature. The important thing is to make sure they are filled with the proper amount of oil. After attaching to a vacuum system, simply plug the outlet of the metal diffusion pump into an appropriate (110-120 V or 220-240 V) outlet. If the pump is water-cooled, connect a relay to shut off power to the pump in the event of a water pressure loss.

4.When shutting down the system, isolate the diffusion pump and the system from the mechanical pump(s). Vent the mechanical pump(s) after shutting them off.

Aside from the generalized description above, the following procedures should be used for operating a large vacuum system with a diffusion pump. If you are beginning a pump-down operation, proceed as follows:

1.Check to make sure all stopcocks are closed.

2.Turn on the roughing and fore pumps.

3.Open Stopcock 1, 4, and 7.

4.After pressure in the vacuum line is 10"1 torr or better, close Stopcock 1 and open Stopcock 2.

5.Place Dewars with liquid nitrogen under cold traps.

6.Be sure cooling water is running in (or that the fan is blowing) the diffusion pump and turn the diffusion pump's heater on.

7.After pressure in the vacuum line is 10'2 to 10~3 torr or better, close Stopcock 2 and open Stopcock 3.

Notice that once the mechanical pumping was transferred over to the forepump, the roughing pump was not turned off. Let the roughing pump run, because there may be ancillary parts of a vacuum system (such as the drawing of mercury back into a McLeod gauge) that are operated from the vacuum of the roughing pump. In addition, the roughing pump typically receives the bulk of the water vapor and other condensable vapors. Letting the pump run (against a load) helps to remove the condensed vapors from the pump oil. If the pump has a ballast, leave it open. Otherwise, be sure that nothing else on the vacuum line requires the use of the roughing pump before it is turned off.

If the line you are working with is small and does not require a roughing pump, proceed as follows:

1.Check to make sure that all stopcocks are closed.

2.Turn on the fore pump.

3.Open Stopcock 2, 4, and 7.

4.Place Dewars with liquid nitrogen under cold traps.

5.Be sure the cooling water is running in the diffusion pump and turn the diffusion pump's heater on.

6.After pressure in the vacuum line is 10"1 torr or better, close Stopcock 2 and open Stopcock 3.

Table 7.8 Diffusion Pump Fault Diagnosis and Correction"

" From N.S. Harris, "Practical Aspects of Constructing, Operating and Maintaining Rotary Vane

and Diffusion-Pumped Systems," Vacuum,\ol. 31,Pergamon Press, Elmsford, NY, 1981 (ref. 28\) reproduced with permission.

To turn off a vacuum line with a diffusion pump, do the following:

1.Close Stopcocks 2, 3, 4, and 7. Although there are none displayed in Fig. 7.24, any stopcocks that go to vacuum gauges or other peripheral equipment should also be closed.

2.Turn off the heater to the diffusion pump, do not turn off the cooling water or fan until the pump has been off for at least an hour, or is cool to the touch.

If the line you are working with has a roughing pump, and you have a reason to shut of this pump,* the shut down procedure is as follows:

1.After shutting off the roughing and fore pumps, open Stopcocks 5 and 6 to vent the pumps (this action prevents the mechanical pump oil from accidentally being sucked into the vacuum line).

2.If there is material in the cold trap near the vacuum line that needs to be

removed, open Stopcock 1 to vent the trap. Then remove the trap so that it may defrost in a fume hood. It is unlikely that the cold trap near the fore pump will need to be removed for cleaning.

If the line you are working with does not have a roughing pump, the shut-down procedure is as follows:

1.After shutting off the fore pump, open stopcock #5 to vent the pump (this prevents the mechanical pump oil from accidentally being sucked into the vacuum line).

2.If there is material in the cold trap near the vacuum line that needs to be removed, open Stopcock 2 to vent the trap. Then remove the trap so that it may defrost in a fume hood. It is unlikely that the cold trap near the fore pump will need to be removed for cleaning.

Until you need the vacuum that can be achieved with a diffusion pump, the water content on the walls of a vacuum system is mostly irrelevant. However, once you need to achieve 10~5 torr and beyond, the presence of water adsorbed into the vacuum system's surface will slow down, and even prevent, achieving greater vacuums. Because of this idiosyncrasy, when first trying to bring a vacuum system down to 10"6 torr, you may need to pump on the system from a half day to an entire day before you have removed enough water to obtain 10'5 torr. Even with auxiliary pumps, water vapor is difficult to remove. The most effective way to remove it is to heat the entire vacuum system and drive the water off the system walls. This approach cannot effectively be done to glass vacuum systems because of damage to stopcocks, valves, and glass connections, although localized heating (with a heat gun) can be directed on glass with limited success. (Do not let a hot air gun sit on any given area of the vacuum line because it can potentially cause the line to suck in at that point.)

*There is nothing wrong with letting mechanical pumps run continuously. If no work is going to be performed for an extended period of time, it is acceptable to shut the pump off.