vacuum

[ddanhra69]

Can anyone help me with questions about moisture and vacuum? I was thinking that if one applied a vacuum to anything it removed humidity or moisture from an object. Since my experience is with automotive A/C (very little I might add), I thought I knew that when a 30 in. vacuum was pulled on the system it reduced the boiling point of water to ambient temp. and would suck it out (moisture). I've got some engines (aerotech) being stored under 20 in. or so with silica gel with it and the silica gel is changing colors as if there's moisture in there. ?????????

[CPMcGraw]

Can anyone help me with questions about moisture and vacuum? I was thinking that if one applied a vacuum to anything it removed humidity or moisture from an object. Since my experience is with automotive A/C (very little I might add), I thought I knew that when a 30 in. vacuum was pulled on the system it reduced the boiling point of water to ambient temp. and would suck it out (moisture). I've got some engines (aerotech) being stored under 20 in. or so with silica gel with it and the silica gel is changing colors as if there's moisture in there. ?????????

Not that I'm an expert in vacuums (just hot air...) my first (simple-minded) thought is "How good are the seals on your storage container?" Could the silica be changing color due to a leak in the box?

[ddanhra69]

Not that I'm an expert in vacuums (just hot air...) my first (simple-minded) thought is "How good are the seals on your storage container?" Could the silica be changing color due to a leak in the box?
No, I really don't think so. I'm using Mason Jars. I still can't remove the lids without a lot of effort, but i'll check anyway.

[CPMcGraw]

No, I really don't think so. I'm using Mason Jars. I still can't remove the lids without a lot of effort, but i'll check anyway.

I used to keep Bettas in Mason jars...

Changing color was a bad thing for them, too, especially if they turned white with cloudy eyes... :eek:

I think you may still have a leaky seal. A Mason jar is great for canning, as you create the seal under water and with added heat. The lid gets pulled securely as the mass and the jar cools, which creates the vacuum. If the lid was not heated during the sealing process, it may not have made the "perfect" seal...

If it were a jar of pickled carrots, they'd probably not be fit to eat now... :(

[ddanhra69]

Ok thanks. Now on to the vacuum thing. What does happen when a vacuum is applied? Does it take all the moisture out or what do you think?

[mojo1986]

Your seals are not leaking air (and moisture) back into the jars. If they were, you would no longer have a vacuum. When you pull a vacuum on a system, you increase the tendency for anything volatile to get into the gaseous phase. If there is enough of it, you will gradually lose vacuum again as the volatile substance gradually 'gets airborne', exerting its own gaseous pressure. This is why you keep pulling a vacuum on a system, to remove those things that volatilize over time. I suspect that if you had any significant amount of moisture in the motors when you pulled the vacuum and then sealed them, the dessicant that you had in the jars absorbed it, causing the color change. So it seems to me that as long as you still have a vacuum in the jars you also have dry motors (as long as your dessicant isn't saturated, in which case you could still have some moisture in the atmosphere above it..............can you tell by the degree of color?).

Joe

[kapton]

Mojo's got the answer for you. Any moisture remaining when you stop pulling the vacuum will end up in the reduced pressure 'atmosphere'. Any water within the jar will evaporate until either 1) there is no liquid water left, or 2) until the water vapor reaches the saturated vapor pressure of water at the given temperature. The saturated vapor pressure of water at 68*F is 17.54 mmHg/.34 PSI/9.4" water.

[ddanhra69]

The silica gel has blue colored pellets that turn pink if wet I guess. They are only slightly pink. I've seen wet or moist propellant grains and these guys are difinitely still dry. What is "saturated vapor pressure of water" mean? That's when it's in liquid form or when it turns to water from vapor? Mostly i'll pull at least 15-25 in.Hg to store them. I've never tried 30 in.Hg. I'm just trying everything I can to keep these composites dry after opening their packages to get one engine out. Pack of 3 you know.

[kapton]

Saturated vapor pressure is the equilibrium pressure of the water vapor over water. In other words, it is the partial pressure of water vapor at 100% relative humidity.

If you take a container and pull a complete vacuum on it, then add a bit of liquid water to it, the water will evaporate until the vapor pressure equals the saturated vapor pressure at the specific temperature.

It sounds like you are just fine. Pumping the container down removed most of the moisture - the silica gel scavenged the remainder.