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Help with Speedivac F203 Diffusion Pump

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LAN_403

Ash Small

Sat Dec 11 2010, 11:37PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

RichNormand, have you bothered to read this article?

testtest

Sun Dec 12 2010, 12:12AM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

Ash Small wrote ...

RichNormand, have you bothered to read this article?

I would suggest to take this to a new thread considering the original subject. My point was "electronic" TV and how it was brought to mass utilisation.

Ash Small

Sun Dec 12 2010, 08:44AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

So was mine.

IntraWinding

Mon Dec 13 2010, 11:37AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Proud Mary wrote ...

So where's the heating element, Alan? In that round box structure at the bottom?

The disc at the bottom is the heating element (ceramic on the reverse) and fits into the round box at the bottom of the pump. The small round circles on the disc are its electrical terminals and to the left are a pair of ceramic bead insulated leads/terminal blocks that fit to these.

plazmatron wrote ...

Sorry I missed this thread people!

Thanks for dropping by and giving such useful advice.

plazmatron wrote ...

WRT the Asbestos in the shell, I would use ordinary rock wool as found in loft insulation for that.
Normally anything below the watercooled section should be very hot to the touch during use. The insulated shell prevents you from incinerating stray digits, and helps maintain a steep temperature gradient between the condenser and the boiler.

Would you completely fill the gap between the pump tube and the outer aluminium cover tube with rock wool to keep in as much heat in as possible?
I can see 50ml of DC-704 might work out very cheap if I can find 50ml (thanks Ash Small).
I remember reading about the effectiveness of copper wool for preventing back streaming to the backing pump. I've got a good aluminium oxide trap for that too so no worries there. I am concerned about back streaming into the high vacuum chamber though becauseas I've got nothing to stop that.

Ash Small wrote ...

IntraWinding wrote ...

Yes, thanks, I'm sure it's a forline baffle now. No water cooling for it though. I think mine is a rather old design. .

Having seen the photo, I'm sure your forline baffle is watercooled. The cooling pipe coils around the foreline baffle, then around the pump body, if I'm not mistaken.
Your pump looks very similar to my two. What is the internal diameter of the body?

The photo's deceptive. The pipe which appears to connect to the baffle actually makes no contact with it.
Pump throat diameter is 2 1/2 inches.

Pump detail 1

Pump detail 2

Pump detail 3

plazmatron

Tue Dec 14 2010, 08:42PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Yes, I would fill th gap with rockwool, no need to pack it tight though.

Backstreaming from the diff pump into your chamber should be insignificant for most experiments, unless you are vacuum coating, or require an ultraclean environment.

That is indeed a baffle on the foreline arm, and is far enough away from the boiler, that active cooling is not really required.

Les

IntraWinding

Tue Dec 14 2010, 09:36PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Vacuum coating is one of the things I'd really like to try.

Ash Small

Tue Dec 14 2010, 10:01PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

That's what I'm working on as well, Intra.

IntraWinding

Wed Dec 15 2010, 01:46AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

So how will you control back streaming into the high vacuum chamber?

plazmatron

Wed Dec 15 2010, 10:29AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Backstreaming can be significantly reduced by installing a baffle in the pump throat.

They dont have to be of complicated design, a disk bolted to the top of the jet assembly, with a gap between it and the pump body will significantly reduce backstreaming, at the cost of lowering pumping speed.

Les

Ash Small

Fri Dec 17 2010, 06:24PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The main factor here is the lowest pressure you'll be pumping down to. As long as the vapour pressure of the oil is above this you should be ok.

Plasmatron's suggestion, or something like it, is probably the single most effective measure to take. Although refrigerated baffles or coldtraps are an improvement. Assuming you don't have liquid nitrogen, you can cannibalise an old fridge/freezer and get down to around -50C. There are some details here:

Peltier modules can also be used in combination with refrigeration, as long as the peltier module itself is not exposed to vacuum due to outgassing.

Another tip is to 'soft start' chamber pumpdown . By-pass the isolation valve with small bore tubing, open the valve in the small bore tubing and wait until pressure gets down to 1 micron before opening the isolation valve.

One thing that is often overlooked is backstreaming from the foreline pump. Again, use a foreline pump oil with a low vapour pressure. Use foreline traps. Some have already been described here, using copper wire wool. Activated alumina traps are best. Foreline traps should be by-passed during initial pumpdown as large amounts of water vapour can poison them, reducing the backing vacuum to the stall point of the diff pump.

The diff pump should be switched on around 100 microns to prevent backstreaming from the foreline pump at lower pressures.

A needle valve or similar providing a small leak into the foreline can also help to prevent backstreaming from the foreline pump by 'flushing' the oil out. (this is different to the purge valve that is used to flush contaminants out of the foreline pump oil)

Utilising a 'second chamber' in the foreline to provide a vacuum resevoir can be beneficial, as this allows the foreline pump to be switched off (after valving off) when pressure reaches around 20 microns and is switched back on when pressure rises to around 100 microns. (I'm not certain about these figures and this obviously requires a guage on the resevoir)

Back migration is a separate problem and involves the oil on the walls around the first stage of the diff pump 'migrating' up the walls towards the chamber. This can be minimised by ensuring the cooling water always enters the diff pump at the top and exits at the bottom.

Hope this helps. Maybe others can add to it or correct any points that may be incorrect.

(Ultimate vacuum required will depend on the type of vacuum coating you are planning to undertake. )

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