Standard Operating Procedure
Turbomolecular Pumping System ("Edwards" System)
Located in V12, this vacuum system is used for the processing of gas laser tubes as well as mass spectrometry. The system, as seen in figure 1 and diagrammed in figure 2 below, consists of:
Summary of Valves in the system:
This valve (see figure 3) isolates the turbopump and RGA head from the rest of the system to keep it under high vacuum (to stay clean) when not in use. It is an absolute sealing valve which does not leak.
Ths valve (see figure 3) is a high-conductance type allowing quick evacuation of the manifold(and anything attached to it). Do not open this valve unless the manifold is already at low pressures as the inrush of gas from a high-pressure manifold will overwhelm the turbopump (This is called 'dumping the pump' and is hard on a running turbopump).
A needle valve (see figure 3) allowing very small, controlled, amounts of gas to pass from the manifold to the high-vacuum parts of the system (i.e. the turbopump and RGA head). It is used (a) to lower manifold pressure in an controlled manner by leaking gas into the pump and (b) to alow a slow leak of gas from the manifold into the RGA head for composition analysis allowing the RGA head to stay at a pressure below 10-4 torr (the upper limit for the RGA) while the manifold is at a higher pressure. Even when turned off it has a small leakage rate into the high vacuum part of the system. When the manifold is pressurized to >1 torr only 30 to 60 degrees of counter-clockwise movement (one-sixth of a turn) is required to open the valve enough to sample manifold gas and keep the RGA head below the upper pressure limit of 10-4 torr.
These needle valves (see figure 4) allow a small amount of pure gas from each supply bottle into the manifold. Gas cylinders have average pressures of 3000 psi (!) which is reduced by the regulators to about 1 psi. The flow of this low-pressure gas into the manifold is then regulated by each needle valve.
This valve (see figure 4) allows a small amount of air into the manifold. It is used for adjusting manifold pressure (if air is used) as well as slowly venting the system to atmosphere when removing components. Alternative uses for the vant include allowing connection to a gauge to be calibrated such as an ion gauge.
Download a System Drawing updated to include the new gauges.
Summary of Gauges in the system: Operation of the system for processing a gas discharge or laser tube: Connect the laser tube or gas discharge tube (if not already connected) to the Swagelok connector on the isolation valve for the system. Most tubes will have their own isolation valves allowing them to be disconnected from the vacuum system once refilled. If single-shot processing is desired, a pinch-off tube (as used on commercial HeNe tubes) may be employed.
Twin capacitance manometer gauges (MKS type 870 and 622 with a PDR-2C auto-switching readout) are extremely accurate and produces readings which are independent of the gas mix involved (which is not true of other type of gauges such as the TC gauge). Pressures indicated are those in the manifold (and gas discharge tube if connected). The lower box is an instrumentation amplifier for the 870 gauge: the blue LED indicates both power supplies are operating and the gauge may be zeroed using the front control. It is necessary to turn the gauges on well before usage allowing them to stabilize and to zero the 870 gauge before each use (pull the manifold below 10-2 torr, switch to gauge 2, and zero the 870 gauge).
The 870 gauge head has a range of 1000 torr. At pressures below 9.0 torr the readout will automatically switch to the lower-range gauge, a type 622 with a range of 10 torr. The second gauge (a 622) will read accurately from 9.000 torr down to 0.001 torr.
This gauge (a Varian 843) reads pressures on the high-vacuum side of the system where the RGA ionizer is located and is used strictly as a go/no-go interlock to prevent filament burnout - It is used to indicate when a suitable pressure exists to activate the RGA and is inerlocked accordingly. This gauge will only read accurately between 1 torr and 10-3 torr. When the system is operating properly, it will often be 'buried' in the low end of the scale.
This instrument, which details the components of a gas mixture, can also be used as a high to ultra-high vacuum gauge in the range 10-4 torr to 10-12 torr range. In analog scan mode the absolute pressure reading may be activated from the Graph menu. It is also extremely accurate and is compensated for gas mixture. To use the RGA, power must be applied via the interlock controls on the small 19-inch rack under the PC - this switch applies power to the RGA, interlock controls (including the 843 gauge), and the PC as well.
The main valve is kept closed at all times when the system is not in use to keep the RGA under vacuum. This ensures that water vapour and oxyen are not adsorbed by system components leading to longer pump-down times and possibly the requirement for a bake-out. Leave this valve closed unless actually using the system.
Configure the system so that all valves are closed. Open only the isolation valve to connect the manifold to the gas discharge tube being pumped. Turn ON the TC and Baratron gauge controllers. Turn ON the room vent system which will remove vacuum pump oil vapour (the red switch near the main electrical panel at the rear of the room). Start the turbo pump via the EXC120 controller. This controller also operates the cooling fan for the turbo pump as well as the forepump. Once the forepump has started and a pressure drop is indicated on the TC gauge (which monitors pressures at the turbo pump intake), open the main valve slowly to evacuate the manifold between the evacuate and the main valves. The main valve will now remain open until the system is shutdown.
Open the sampling needle valve to pump the manifold down to about 1 torr. Be sure to open the valve only enough so that the pressure at the intake of the pump never rises above 10 torr (as per pp. 59 of the text). Once the manifold has reached a low enough pressure (as read on the Baratron gauge on the manifold), close the sampling valve and open the evacuate valve (which has a much higher conductance) in order to fully evacuate the tube and manifold. At this point pressure may be monitored via the TC gauge at the intake of the turbo pump. Pump the tube until this gauge 'bottoms-out' at which point the system is below 10-4 torr. Close the evacuate valve to isolate the tube and manifold from the pump. Pressure in the tube may now be monitored by the Baratron gauge on the manifold and may be controlled with several valves. The vent valve is used to allow small amounts of air into the manifold to increase pressure. When filling a gas discharge tube one may use the gas supply needle valves to admit small amounts of pure gas into the system, or the vent valve is air is used. To lower the pressure in the system the sampling valve can be opened to allow small amounts of gas in the manifold to be pumped out.
To begin, pump the tube down as far as the vacuum system will allow. With the pressure below 10-2 torr, it is necessary to zero the 870 gauge by switching the Baratron gauge controller to gauge 2 (it is normally in the AUTO position), adjust the zero control on the amplifier so that the readout reads zero, then restore the switch to the AUTO position again.
Ensure both the evacuate and sampling valves are closed to isolate the tube and manifold. Open the gas supply needle valves as required to fill the tube. Standard procedure is to overpressure the tube then pump out excess gas using the sampling valve which has a much finer control than the rest. Regardless, keep the TOTAL pressure well below 600 torr. Leak some back into the vacuum pump using the sampling valve as required to lower the pressure.
Operation of the system for processing a gas discharge or laser tube:
Connect the laser tube or gas discharge tube (if not already connected) to the Swagelok connector on the isolation valve for the system. Most tubes will have their own isolation valves allowing them to be disconnected from the vacuum system once refilled. If single-shot processing is desired, a pinch-off tube (as used on commercial HeNe tubes) may be employed.
Operation of the system for mass spectrometry:
The system is used much like above except the isolation valve is kept closed. The idea behind pressure controls when using the RGA is to keep the RGA head at a pressure no higher than 10-4 torr. If the pressure rises above this level the unit will give false readings (the 'total pressure' as read on the PC connected to the RGA will show negative readings). As well the filament coud burn-out (at $360.00 per thoria filament !). The TC gauge is interlocked to the RGA however so an increase in pressure will result in power being cut from the RGA unit to protect it. An indicator light in front of the RGA shows when power is applied. Usually about 30 degrees of turn in the clockwise direction is enough on the sampling valve to give adequate readings (30 degrees is one-twelfth of one turn !) - Go easy on the poor thing! Several LED indicators on the rear of the RGA unit may help in diagnosing any problems as well as ensuring the filament is OFF before allowing pressures higher than 10-4 torr at the RGA head.
The RGA program on the PC should be used in ANALOG Mode (selected from the MODE menu). As well, be sure to toggle TOTAL PRESSURE on from the GRAPH menu. Connect to the RGA head using the RS232 SETUP option from the UTILITIES menu (press the CONNECT button). The scan may then be started usng the GO icon. If the system is known or planned to be overpressured (e.g. when pumping the manifold to lower pressure) simply TURN OFF THE FILAMENT using the FILAMENT ICON on the toolbar (pictured to the left).
Figure 3 shows an example of the type of output to expect from the RGA when sampling air. Total pressure is visible in the box in the upper-left of the display (currently 8.1E-5 torr). The system was run with the sampling needle valve partially open. The large peak at 28 amu and the smaller one at 14 amu are from residual nitrogen in the system (from air). Also seen is a large amount of oxygen with peaks at 32 and 16 amu. The peak at 18 amu originates from water vapour in the system.
Figure 4 shows an example of the type of output to expect from the RGA when the system is sealed and pumped. Total pressure is now much lower at 2.7E-6 torr. Residuals from air such as nitrogen and oxygen are now in much smaller quantities than before with the scan being dominated by the large peak at 18 amu (and smaller peaks at 17 and 16) originating from water vapour in the system - the main component of the residual gases in the system. This also illustrates how an RGA is used to detect leaks as opposed to outgassing or other sources of contamination. Leaks usually show components of air (i.e. nitrogen and oxygen) while outgassing shows different components.
When a gas is required, open the bottle valve and then close it immediately. This will trap a small amount of gas inside the regulator itself (the high pressure gauge will show the pressure in the supply bottle). To prevent contamination of the ultra-high purity gases, Do NOT leave the bottle valve open. Set the regulator for an output pressure of 1 psi as indicated on the low pressure gauge. The needle valve maynow be used to admit gas into the manifold. Open this valve SLOWLY as the manifold has a small volume to fill.
Close all gas bottle main valves and the isolation valve. Close the main valve and press the START/STOP button on the EXC controller. The turbo pump will slowly wind-down and the mechanical pump will shutdown automatically at the correct point. No other controls need be adjusted. Once the mechanical pump has stopped, turn OFF the room vent via the switch near the electrical panel.