Standard Operating Procedure
Turbomolecular Pumping System #2
Located in V12, this vacuum system is used primarily for experiments in the vacuum and thin-film classes. The system is flexible and features a basic vacuum system and gauges to which experiments may be attached via one of two NW-40 high-vacuum fittings. The system, as seen in figures 1 through 3 below, consists of:
Summary of Valves in the system:
This valve (see figure 2) isolates the turbopump from the gauge manifold section (with the convectron gauge, ion gauge, and mass spectrometer) to keep it under high vacuum (to stay clean) when not in use. It is an absolute sealing valve which does not leak.
This valve (see figure 2) connects the gauge section to the experiment manifold (shown in these photos with the gauge experiment attached).
Summary of Gauges in the system:
A type of thermocouple gauge, this instrument (a Granville-Philipe type 275) reads manifold pressures up to 800 torr and down to 0.001 torr. This gauge is used to indicate the pump inlet pressure (often zero in operation) and is useful when starting the system to indicate the pressure where the turbo pump may be turned on. WARNING: This gauge is sensitive to the type of gas being measured. For gases other than nitrogen, a conversion chart is required: see the link at the bottom of the page to obtain the PDF for this gauge (see section 6 of the manual). While this gauge is accurate with a single gas (e.g., nitrogen or argon) the gauge is difficult to use in a mixed-gas environment. The primary usage is in system start-up.
Operation of the system
Connect the experiment (if in use) to one of the NW-40 fittings (i.e. through the MANIFOLD or AUX valves). Usually the fitting off the MANIFOLD valve is used (as shown in the photos on this page).
Turn the main power ON (to power the system gauges - the ones on the left) and turn the mechanical pump ON using the switch on the rear of that pump. The system will begin rough-pumping. Open the MAIN valve and the Convectron will show the current pump inlet pressure. The MANIFOLD valve can also be opened now allowing evacuation of the experiment as well.
Continue monitoring the turbo-pump inlet pressure via the Convectron gauge until a pressure of 1 torr is achieved at which point the turbo-pump may be started using the START switch on the CFV-100 controller. The controller will not allow the pump to start until this point (i.e. if it fails to start, the inlet pressure is not low enough and it will be required to wait longer until the system pressure falls). Pump operation may be verified by the yellow 'underspeed' indicator. The pump will accelerate to 27,000 rpm and Convectron gauge will show a decreasing pressure, eventually reading zero.
At this point, operation of the system is largely determined by the experiment. The Ion gauge may be used by selecting turning the hot cathode gauge controller on, selecting the PRESSURE display function via the front buttons, and switching the filament ON via the toggle switch. If the pressure is too high, the filament may not light, otherwise the filament will be visually observed to lluminate and the gauge will display current pressure. Be sure to limit filament usage and duty cycle to 30 seconds ON, one minute OFF as required.
The mass spectrometer may also be used but to ensure pressures are low enough (i.e. total pressure is below 1 * 10-4 torr), use the ion gauge to verify the pressure FIRST before turning the RGA/Mass Spectrometer on.
Operation of the system - Gauge Experiment
The most common use of this system is for the roughing gauge experiment for vacuum and thin-film laboratories. This experiment consists of four gauges of different technologies on a manifold along with two valves allowing introduction of gases as per figure 4 below:
In order to reduce pressure in the experiment manifold, the MANIFOLD valve may be opened. To introduce gases into the system, needle valves are used along with absolute-sealing toggle valves. Open the toggle valve to admit gas then use the needle valve to regulate the flow of gases into the manifold. Once pressure is at a desired level, the toggle valves should be closed since needle valve are not absolute selaing and so pressure will continue to rise continually.
When the experiment is complete, be sure to turn off gas supplies at the source (i.e. the supply bottle) and be sure the toggle valves are closed.
More details may be found in the experiment itself (on the course home pages)
Turn off all gauges, especially the ion gauge if currently on). Close both the MANIFOLD and MAIN valves and turn the turbo pump OFF. Wait until the pump has stopped spinning (about five minutes) then turn the mechanical pump OFF. Shut off the main power.
Updated 2012/01/11 by M. Csele