EXERCISE 2: THE VACUUM SYSTEM Because vacuum system techniques are important to all of experimental physics, every undergraduate student should have had some experience in them. This exercise ensures that you will! In this exercise, you will pump the bell jar (chamber) down to 10-6 Torr and then bring it back up to atmospheric pressure again, recording the chamber pressure and the backing pressure. You will also do some pumping speed calculations to understand what is involved in the design of a vacuum system. Later, you may wish to use this system for the "Evaporation of a Silver Film" experiment. The Vacuum System and How To Use It The pumping station you will use consists of a bell jar pumped by a very small diffusion pump (or pumping speed 50 liters/s) backed by a rotary mechanical pump. There are four pressure gauges, each useful in a different pressure range, measured on different parts of the system, and there are five valves for directing the gas flow. For descriptions of the workings of the system parts, see the reference 1
Note the various pressure units used. The SI unit of pressure is the Pa = N/m 2. The bar is also used: 1 bar = 10 5 Pa, as is the Torr: 1 Torr = 1 mm Hg = 1.333 mbar. Thus atmospheric pressure is approximately 100 kpa = 1 bar 760 Torr. The mechanical (roughing or backing) pump rotates an eccentrically mounted vane inside a cylinder to compress and expel a volume of air in each revolution. It is used either to rough out the bell jar or to back the diffusion pump. It becomes ineffective at low pressure 10-2 Torr. The diffusion pump is effective down to pressures 10 7 Torr, but must have a backing pressure of 10-2 Torr in order to operate. Its high pressure limitation derives from the necessity for the gas being pumped to be at molecular flow pressures. The pump works by boiling oil with a heater and back-jetting the oil vapor with a system of jets. The oil vapor collides with the molecules of the air being pumped and pushes them forward. The upper walls of the pump are water cooled to re-condense the oil, while the air finds its way to the exit. Diffusion pumps can be gigantic in size but yours is among the smallest we could find, having a 38 mm (~1 inch) or 63 mm (~ 2 inch) opening. The cold trap is cooled by liquid nitrogen (boiling point = 77K) to both condense any oil that might back-stream from the diffusion pump into the bell jar and to increase the pumping speed for condensable vapors. (Vapors that condense on the baffle of the cold trap are, in essence, pumped from the vacuum chamber.) The valves: Referring to the figure: the Mechanical Pump Bleed-up Valve, when opened, permits the mechanical pump s (low) vacuum line to be vented to air. Similarly, the Chamber Bleed-up Valve, when opened, permits the bell jar to be vented to air. The Chamber Roughing Valve is normally kept closed when the diffusion pump is being used to pump the bell jar. However, because diffusion pumps cannot be used to pump pressures higher than 10-1 Torr, the chamber roughing valve may be opened to allow the mechanical pump to rough-out the bell jar. The Diffusion Pump Backing Valve, which connects the diffusion pump to its backing pump, will normally be open whenever the diffusion pump is operating. However, it must be closed whenever the mechanical pump is being used to roughout the bell jar in order that the diffusion pump not be subjected to higher pressures. The Chamber Diffusion Pump Valve is opened whenever the bell jar is to be pumped by the diffusion pump. 2
Cautions: - The first law of vacuum systems is: Do not leave the mechanical pump under vacuum when not running. For many types of mechanical pumps, oil may be sucked back from the pump body. On final shut-down, (a) close all valves to the system, (b) turn off the pump, ( c ) open the mechanical pump bleeder valve to bring the mechanical pump up to atmospheric pressure. - The second law of vacuum systems is: Keep the backing pump and cooling water on while the diffusion pump is hot. Otherwise the oil will back-stream into the bell jar and/or be oxidized. Never let air into the diffusion pump while it is hot, otherwise the oil will burn! Gauges: The thermal conductivity of a gas is proportional to the pressure. The Pirani Gauge has a hot wire in the vacuum space. Its electrical resistance, measured in the control unit, depends on the wire temperature and therefore the pressure. It gives useful readings from 10-2 to 10-4 Torr. In the present apparatus there are two Pirani gauges. One indicates the diffusion pump backing pressure and the other indicates the pressure in the chamber. The Penning Gauge monitors the current in the gas from a cold emission cathode to an anode. The electrodes are located in the field of a permanent magnet to give long trajectories. It overlaps the Pirani gauge and goes to lower pressures - down to 10-7 Torr. Before you startup the pumps examine the system and locate all the parts. There are four pressure gauges - you should figure out what part of the system each gauge samples. Check the positions of all the valves (which are open, which are closed). Try to ascertain the state of vacuum in the various parts of the system. To start the system you must: Be certain that the mechanical pump bleed-up valve is closed Be certain that the valve joining the bell jar to the diffusion pump (Chamber Diffusion Pump Valve) is closed Start the mechanical pump and check that it pumps down to about 20 10-3 Torr reasonably quickly Open the valve between the diffusion pump and the mechanical pump (Diffusion Pump Backing Valve) and pump the diffusion pump down to better than 20 10-3 Torr Turn on the diffusion pump water 3
Turn on the diffusion pump heater. After about 15 minutes, the diffusion pump should be operating. Cool the cold trap with liquid nitrogen. To pump out the chamber you must: Make sure that the plastic implosion shield surrounds the bell jar Be certain that the chamber bleed-up valve is closed Rough out the chamber by opening the valve to the mechanical pump (Chamber Roughing Valve) (Be careful to temporarily close the diffusion pump backing valve to protect the diffusion pump. This valve closing admittedly is a transient violation of the 2 nd Law, but it protects the diffusion pump from the major surge of air that would certainly violate part 2 of the 2 nd law.) When the pressure is about 10-1 Torr close off the roughing valve to the mechanical pump (Chamber Roughing Valve) (and, of course, re-open the diffusion pump backing valve.) You may now open the valve between the bell jar and the diffusion pump (Chamber Diffusion Pump Valve) Note the rapid decrease in pressure as the diffusion pump grabs the bell jar. It will finally go to 10-5 Torr or better. To access the bell jar you must let it up to air. (Why can t you just pull it off?) Valve off the diffusion pump from the chamber and then vent the chamber to air. To shut down the system: Close the valves to the bell jar (Chamber Diffusion Pump Valve) and turn off the diffusion pump heater. After the diffusion pump has cooled and the cold trap has warmed up, close all valves and turn off the diffusion pump water Turn off the mechanical pump and vent it to air with mechanical pump bleed-up valve. Leave the apparatus with both the diffusion pump and the bell jar each under vacuum. This helps keep clean the high vacuum parts of the apparatus. Experimental Part 1. Pump the bell jar to better than 10-5 Torr and then return it to atmospheric pressure. (How good a vacuum can you achieve?) 4
2. Insert the metal plate with the small aperture into the neck of the diffusion pump (above the valve) and pump the bell jar again. 3. Close the valve between the bell jar and the diffusion pump and, using the bleedup valve, carefully raise the pressure in the bell jar to approximately 0.3 Torr. (If you raise the pressure too high, rough-out the bell jar to the desired vacuum.) Now open the diffusion pump valve and observe the bell jar pressure as a function of time. Continue pumping for 30 minutes. Plot a graph of pressure versus time on a semilogarithmic scale (see 7. below). 4. Familiarize yourself with the terms molecular flow, viscous flow, throughput, in reference to gas flow; also pumping speed of a pump and conductance of an aperture. (Check for references) 5. Calculate the mean free path in air at the ultimate vacuum that you achieve. 6. Calculate (from its dimensions) the conductance (in liters per second) of the pumping aperture that you have inserted. 7. If the bell jar of volume V is pumped with a pumping speed S (in the molecular flow regime), you can show that the pressure would vary as S t V 0 p = p e + p b From a straight-line region in the graph you plotted in 3, estimate the conductance of the aperture. 8. Explain why the pressure settles to an ultimate value of p b and not zero. Reference: Vacuum System Detail Book, (available in room 229) RMS(2010), td-1996, gmg-1984, jbv-1993,1994 5