vacuum question...

The "worse case" is assuming that there was some liquid water in the system before you started drawing vacuum.

At vacuum, the amount of water left is just the vapor density X the volumn.

Note that in a "dirty" system, there is crap that only releases the water very slowly.

The quick and dirty test for this is to close off the pump and watch to see if the pressure slowly starts to increase. At the limit it will increase to the vapor pressure of H2O at the temperature of the system. But some stuff holds water beter than does "other stuff."

I mean, if I have a storage cylinder and I pull a vacuum, will it "dry out" material? Like a welding rod?

Bullony. The ONLY factor is the pressure (or lack thereof) that any water present is exposed to. ANY medium should "draw out fully" pretty quickly.

It is all based on the porosity of a medium within the chamber. The welds, the walls, everything. Including the medium under discussion.

A circuit board will release all of its trapped water in a slightly lower than full vacuum at a slightly elevated temp of 60°C or so.

That is a porous substrate yet appears fairly "sealed" when examined.

Epoxies and such have had a vacuum drawn on them before they reached their pot life figure, and should also be free of any water.

And, in a vacuum, there will be no "vapor pressure of water at a given temp"

In making HV power supplies, even "coffee breath" blown onto a ceramic capacitor before the potting cycle can cause a failure mode of the potting and the unit.

So reduced pressure, elevated temp "de-wetting" of the assemblies was performed, and a less than full vacuum chamber will do the job fine.

You should see how long it takes to get transformer varnish to stop foaming in a vacuum., so that it can be used to impregnate transformers.

Of course... that wasn't water, though I am sure some of it was (from the transformer). Also, the chamber itself should only be "dirty" for one cycle. After it gets baked and vac'd dry, it will not be nearly as "dirty" on ANY subsequent use, short of dowsing the chamber with water again in the future, "re-vitalizing" the "dirty" parts of the chamber.

It still will all go away after the first cycle again. It does not simply re-infuse as the pressure is brought back up.

Yes. Most certainly. Add a couple of silica packs to suck it up as it gets pulled out of the "rod" (sample under process).

This is actually the proper and preferred method of removing all of the water from ANY device or item that is not harmed by the reduced pressures.

This is why our assemblies with EL caps that were not sealed mil variety could only be subjected to the vacuum for specific periods. We also generally kept EL and their vent hole out of the potting, though soft RTV potting would not stop a cap from venting.

What do you mean "sample under process"?

DUT (Device Under Test) UUT (Unit Under Test) UIP (Unit In Process)

There are all kinds of monikers for "what is in the chamber".

Like the Windows recorder setting "What you hear", that captures what is currently in the Windows sound system pipe(s).

OR WYSIWYG.

ANYway. Whatever it is that you are trying to desiccate. Sheesh.

The relevant components of the interior of a vacuum chamber during use are:

1) The chamber interior surfaces and any moisture or other volatile compound capable of 'gassification'.

2) Any exposed sealing media at non-welded access areas or "hatches" that may present slight gasification action.

3) The moisture in the air in the chamber at closure time (a nice blast of nitrogen or other 'dry' displacement gas, used as a 'purge' will help reduce that moisture)

4) The item you are placing into the chamber, which you wish to subject to reduced atmospheric pressures, and presumably remove some or all of the water it contains from it.

They all act in parallel to add up to the resultant 'remaining vapor cloud' if any, that would be in the chamber after some 'settling period' at said given low, near vacuum pressure. Regardless, the moisture will no longer be in "the part" in the chamber. I would place a silica bag in front of the pressure relief vale ingress port so that returning air can 'splash over it' and provide a slight desiccation of the incoming rush.

We could place it in a cylinder and FORCE the return air through it, perhaps fully desiccating it.

Don't know how well these bags work in chambers.

My idea was to create a welding rod contailer that I could pull a vacuum on to dry/keep dry my welding rods. I wonder if this is a marketable idea?

I don't think that is a strong enough vacuum to really 'dry' things. That corresponds to a dew point of about 90-100 degF.

When 'drying' A/C units and such, we used to pull a much stronger vacuum, over 29 hg (at sea-level of course).

If you can get a stronger vacuum, then this is a good way to dry things out. Or if you can heat the material up a bit while drawing the vacuum you mentioned, that would help.

daestrom

This process is used to dry out large transfortmers. They are sucked down to a few mm of water and held for a few days then vaccuum filled with clean oil. The water all boils off. Waater boils at 100C at atmospheric pressure and boils way below freezing in a vacuum. You don't see any lakes in space.

No to to silica packs. If you want to try to dry something out by just putting into a sealed contain, the silica will reduce the moisture and "dry out" the other stuff.

But when you are drawing a "serious" vacuum, the silica will as likely or not give up water.

All you want in the vacuum system is stuff you want to dry out.

Of course if you want to dry out your silica packs ...

Really?

If there is any free water, the best vacuum you can pull will be the vapor pressure of the water still remaining. Of course, why typically happens is that the water freezes.

The same thing happens in space.

Actually, not true.

If their is something in the system that absorbs water, each time you vent the system that "something" will pick up some moisture again.

Let's say you have a piece of fruit in the system. If you pump long enough you will "freeze dry" the fruit. But when you vent the system is will pick up some moisture from the air.

It definitely will be easier the second (and later times) but unless you like changing the oil on a vacuum pump you get rid of as much water absorbing stuff in your system as you can.