Equipment/High Vacuum Station
The HVS is an in-progress equipment project to provide LondonHackspace members with access to a multi-purpose high vacuum system.
- Resin degassing (backing pump ONLY - separate degassing chamber)
- Creating x-ray sources, CRTs & other electron-beam experiments
- Vacuum glasswork (EG; thermionic valve making/repair)
- Surface coatings (optical coatings, photocathodes, metalic finishes, titanium oxide coatings)
- Electron microscopy
To enable all the uses people are interested in, the system needs to have vacuum access at a number of stages in the pumping system.
The backing pump is a rotary pump that should be capable of reaching near 0mbar. On a double valve setup one pump should be able to act as backing pump to the diffusion pump or to directly pump a separate degassing chamber. This should allow the degassing to be used without the worry of contaminating the ultraclean conditions of the High-Vacuum chamber or diffusion pump.
There is a pledge for a suitable pump, as none offered so far have sufficient ultimate pressure and/or pumping speed. http://wiki.london.hackspace.org.uk/view/Pledges/Vacuum_Pump
The initial chamber is a home-made one made by Sci from an old fire extinguisher and water fittings.
Degassing resins allows small air bubbles to expand under low pressure and burst, removing the gas from the liquid. In plain resins this leads to stronger smoother castings, and with filler powders it creates a stronger bond between filler and resin.
As resins can bubble quite a lot in degassing, the vacuum line on the chamber should have a catch-pot fitted for safety. This is an empty vacuum-sealed container with two connections at the top. Any spilt resin or spray that makes its way down the initial vacuum line will be caught in this "dog-leg" and prevented from contaminating or damaging the rotary pump.
If you want to watch marshmallows expand, this is the portion of the system you'll be using.
The diffusion pump donated by James Collings is an Edwards E04. It's ultimate vacuum capacity is 5x10^-7 mbar (0.0000005mbar) with a pumping speed of air of ISO 600l/s^-1 (hydrogen: ISO 800l/s^-1). It has a maximum starting pressure of 300 mbar and requires a backing pump capable of keeping the backing pressure at about 1mbar (assuming DC704EU pump fluid). Recommended backing pumps are the Edwards RV8 or RV12. The RV8 has a displacement of 9.7m^3h^-1 and an ultimate pressure of 2x10^-3. The RV12 is advised for fast roughing, has a displacement of 14.2m^3/h^-1 and an ultimate pressure of 2x10^-3 mbar.
The diff pump operates by boiling a special silicone oil to vapour and forcing it out through special baffles onto a water-cooled jacket. There the oil re-condenses and trickles down along with any air molecules it grabbed on the way. It sets up a pressure differential across the pump, where at its exist it should be at a high enough pressure the backing pump will be able to pump it away.
Current prices for the compatible fluids (as of October 6th, 2011, direct from Edwards Vacuum) are;
- DC702 fluid - £117, 500ml (critical backing pressure 1.3 mbar, ultimate pressure 5x10^-6 mbar)
- DC704EU fluid - £134.10, 500ml (critical backing pressure 1.0 mbar, ultimate pressure 5x10^-7 mbar)
- DC705 fluid - £211.50, 500ml (critical backing pressure 0.9 mbar, ultimate pressure 5x10^-7 mbar)
- Santovac 5 fluid - £852.30, 500ml (critical backing pressure 0.8 mbar, ultimate pressure 5x10^-7 mbar)
- Edwards L9 fluid - Unknown price, possibly discontinued (critical backing pressure 0.9 mbar, ultimate pressure 5x10^-7 mbar)
On this I would recommend DC704EU fluid. For £17 extra you can achieve a magnitude better vacuum than the absolute cheapest. The spec sheet also describes it as a good durable and general-purpose fluid.
The cold baffle is a small plate in the pump stack with water-cooled plates. These cold plates minimise any pump fluid vapour that might otherwise deflect upward and contaminate the vacuum processes above.
Above the diffusion pump are a series of manifolds for various purposes, including the cold baffle mentioned as part of the diffusion pump above.
To fulfil all the uses proposed, placing an extension piece with a branch next with the main plate valve above it will allow the full power of the diffusion pump to be accessed by a side-valve without having to pump down the main chamber. This high-vacuum access valve could then be used for attaching vacuum tubes, glass-forming manifolds, electron microscopes, fusors or any other number of parts. A foreline vacuum pressure sensor will be necessary here.
The valve currently need a slightly wider packing piece to be used in the stack at this location. The current one is only 3/4" thick, and in this position fouls the valve control gearbox.
The vacuum chamber is a recessed base, likely for a glass bell-jar. However once fitted with a new O-ring gasket it will likely be fine for initial use with a simple flat cover. 25mm polycarbonate will probably do.
The chamber has three ports for vacuum feedthrus, though one is modified and may limit use. There are a number of threaded mounting points and, most notably, a large angled port at the circumference. This large port will require a special blanking plate making, but after that can probably be used for any custom or special-use interfaces required.
Both diffusion pump and cold baffle require a self-contained cooling loop. Possibly even separate radiators if one effects the critical temperature of the other.
The parts from James contained four Pirani Gauge heads and two Penning Gauge heads. These will need gauge controllers/readouts finding or making for them.
Some surface-coating processes work best if a glow-discharge plasma is used to remove surface oxidisation before coating. Sci has a 5.5Kv mains transformer that should be capable of producing this in vacuum.
Surface coatings also require a tungsten crucible or wire to be heated to vaporise metals. A high-current variable PSU or Variac would be used here.
To keep all the component parts secure and in-line, as well as keeping dangerous parts from being exposed, building a steel framed trolley to house it all seems like a sound idea.
Servicing and care
A wire mesh over the pumping hole of the vacuum chamber should help a lot to prevent any debris reaching the pump. After any use of the chamber for deposition purposes, it should be cleaned with fibre-less cloth and isopropyl alcohol.
Some additional parts will be needed.
- Air conditioning radiator matrix or similar.
- Water pump (for cooling loop).
- 1" thick aluminium or stainless plate, at least 7 1/4" diameter.
- I have a block of aluminium 250mm sq x 75 mm. a bit wasteful but could be used. artag.
- Clean box section or angle-iron (a push-fit connector frame would be fine too).
- 1.5-2" thick, ~4" wide sturdy plastic (delrin?) to turn an large threaded fitting for the chamber.
- I have a 100m dia, 45mm thick lump of nylon and a fair bit of 50mm delrin if it's any use. artag.
- 20mm+ thick polycarbonate sheet (will probably order cut to size).
- Small metal or plastic pressure vessel with removable end (to act as
- debris catch-pot on degasser side).
- Pirani and Penning gauge controllers.
All components are currently being stored in the Out Of Order workshop toilet on Jontys suggestion. This should keep them away from being damaged or contaminated by workshop dust. All manifolds seem in good order and have been in use until recently. Diffusion pump heater and thermal cut-out both test as good with a meter. Wiring should be replaced however. Pump should be drained & cleaned as a precaution. Fluid should be measured and filtered/inspected. A temperature sensor could be installed on the drain port.