Does humidity change vacuum

[smiao]

I have a process that involves picking up a 3/16"x30"x60" piece of glass with a small vacuum system. Recently we have experienced a situation where the glass falls from the device. We currently have very high humidity and I am curious if vacuum increases or decreases as the humidity increases.

 

[CorBlimeyLimey]

http://www.vacuumlab.com/Articles/VacLab27.pdf

 

[Twoballcane]

How do you think the humidity would affect negative pressure? It more sounds like your nozzle material is not making a complete connection/seal with the glass.

Tobalcane
"If you avoid failure, you also avoid success."

 

[MintJulep]

How are you creating vacuum?

 

[smiao]

I am using a small electric motor connected to a vacuum pump.

 

[CRG]

smiao,
Did you measure the system pressure during the lifting cycle? If it is high, then there a few things to check.

Are there any pockets in the system where water could condense? If so, the demand may be significantly greater for the vacuum pump on days of high humidity where water would need to be flashed prior to obtaining the design vacuum.

Are there any leaks in the system that only show up when you move the lifting tool/head?

 

[Ron]

Clean the rubber pad...it's probably dirty. Moisture will enhance the sealing but to my knowledge, has no substantial effect on the vacuum.

 

[zeusfaber]

Is there a depression associated with the increased humidity?

Since a vacuum lifter actually relies on atmospheric pressure to do its lifting, you would expect the available force to vary with barometric pressure. The difference between a high pressure and a low pressure day is usually less than 10%, but if your margin between lifting and dropping is a bit narrow, reduced pressure isn't going to help.

Direct answer to your theoretical question is that the lift should reduce with rising humidity (water vapour is about half the density of dry air, so the more there is in the mix, the lower the barometric pressure you should expect).

A.

 

[zdas04]

Back to your title "does humidity change vacuum". No. Changes in humidity can change vacuum, but if you pull a vacuum the remaining gas will have a humidity that is reflected in the vacuum gauge. On the other hand if the remaining air is at high humidity then a temperature change can cause some of the humidity to condense which will significantly increase the vacuum (fill a vessel with steam, isolate it, let the temperature bleed of to ambient, pressure will drop to vacuum with condensation).

David

 

[Ron]

David...are you nuts?! You can't throw common sense into this mess!!
Ron

 

[MintJulep]

But in all likelihood your problem is something much more mundane - like a leak.

 

[rmw]

I have to disagree with David on this one. Humidity (the presence of water vapor) does affect how much vacuum you can pull. Where no water vapor is present, vacuum can be pulled to very very low torr values (if you have the equipment capable of doing that). However, where moisture is present, the vacuum attainable is limited by the vapor pressure of the moisture at the temperature of that moisture - until (and if) of course the moisture is all 'boiled' away by the vacuum.

I could always get to a deeper vacuum more quickly on A/C systems by taking a propane torch and applying some gentle heat to key parts of the system while pulling vacuum - areas where oil accumulated.

rmw

 

[zdas04]

Water vapor is just a gas. If I pull all of the gas that is in a container down to a very low remaining mass, the mixture of remaining gases will be the same provided there was nothing in the vessel except gas to start with. If I started with 100% RH and that number represented 2,000 lbm/MMCF then at the end, the remaining gas will have 2,000 lbm/MMCF just like I started with (although the RH humidity will now be closer to 10% than 100% because at very low pressures the gas can hold more water vapor). If I had 19% oxygen at the beginning I'll have 19% oxygen at the end.

In other words, there is nothing magical about water vapor. It is just a gas. The vacuum equipment will suck out whatever gases it encounters be they nitrogen, oxygen, or water vapor.

I've seen very deep vacuum pulled inside vessels with a coherent gas/water interface. In that case the volume percent of water vapor increased by at least an order of magnitude because the liquid kept evaporating to maintain 100% RH. By the time we reached 29"Hg (we were at sea level or just below), there was very little in the vessel besides water vapor and not much of that.

David.

 

[moon161]

How much vacuum do you use?

 

[FACS]

It doesn't matter.

If you are dropping glass, then your vacuum is inadequate.

If it was working before, but isn't now, then something changed, and relative humidity or not, it needs to be fixed to cover the range of requirements.

So... what was the vacuum gauge reading when you installed the system, and what is it now?

What was the air flow reading AT THE CUPS when you installed the system, and what is it now?

How old is the vacuum pump? They are only good for so long before they need to be replaced or rebuilt.

Did you change the filter?

Are you using a filter?

Did you check the lines for leaks and kinks?

Are you using stiff tubing of the right diameter?

Do you have a lot of fittings in the mess?

Are the lines dirty?

How old are the vacuum cups? They don't last forever.

I deal with vacuum systems every day, it's never been the humidity. It's one of the above issues. The humidity inside the vacuum pump is the same air as outside.


Charlie

http://www.facsco.com

 

[gerhardl]

...coming in late, and will give further the advice from qualified instrumentation mechanics i know:

if a device do not work, take it carefully apart, clean it, set it equally careful together again, without touching or screwing on any control or regulating part. In more than 90 percent of all cases, it will work properly.

For vacuum: even tiny, microscopic cracs will give leaks, rubber or elastic devices can have 'invisible' breaks in unseemingly places, for instance metal/rubber bridges or on glass contact surfaces or at lips.

All elastic cups to be exchanged.

 

[Cockroach]

Interesting discussion.

I would think that humidity DOES change with the creation of a vacuum. Is not the definition of humidity the percentage of water vapor per unit volume of medium?

A vacuum IS NOT negative pressure, but correctly any pressure below atmospheric. The negativity aspect comes from the fact that the surrounding atmosphere weights more heavily on the membrane of the vessel, i.e. the outside air is crushing the vessel since we no longer have a balanced pressure on either side of the wall of the containment. I would like to point out that in space, there are an average of only three molecules per litre, absolute vacuum means zero molecules. This lends itself well to the physics of the chemistry, pressure is the reactive force of molecular impact at the wall of the pressure vessel. So vacuum IS NOT negative pressure in the true sense of the mathematics.

As all as we are doing is pulling medium from a confined, enclosed space so would I not be changing the "unit volume" measure in the ratio of water vapor per volume of medium? The denominator decreasing means that the humidity is rising.

To visualize this, I start sucking out air from a containment and sooner or later begin to see "fog". That would be the dewpoint, would it not? In other words I would be creating "rain" inside the containment vessel as the pressure begins to drop since I hit 100% humidity.

I believe this is consistent with thermodynamic computations in the two phase region of the CV curves.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Cockroach]

Okay, the above mental exercise assumes that as the vacuum is being created, the volume of water remains constant inside the container. But as I draw out air, would I not be also drawing out water vapor in an unbais way?

So then humidity would remain constant, since the percentage of water vapor to volume of air are removed at the same rate. The ratio of water to air remains constant so then humidity would not change.

So I just confused myself. Not a clue if it rises (argument 1) or remains constant (argument 2). I'm very happy this is not an exam question, I would probably take the pipe on this one!

But have fun with it!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[zdas04]

Think in terms of mass. If the air has a water content of 0.1% at atmospheric pressure it still has 0.1% by mass at 30 inHg of vacuum.

It gets confusing when you start talking about relative humidity. As pressure goes down (or temperature goes up), the amount of water vapor that the air can hold (i.e., the mass of water that constitutes 100% RH) goes up dramatically. So, if the 0.1% mass at atmospheric pressure corresponds to 100% RH, then at 30 inHG that same mass percent is now 25% RH since the air can hold so much more water vapor at low pressure.

The "fog" discussion above doesn't seem quite right. I could see pulling a deep vacuum on a vessel with a coherent air/water interface and letting it equilibrate to 100% RH (probably have to circulate the air to do it). Then when you start raising pressure (say by inserting a piston to reduce volume). At some point, you will exceed 100% RH and the condensing water can fog. Going the other way, the air will be consistently more able to hold water vapor and shouldn't fog.

David

 

[smiao]

Thanks to everyone for their help with this issue. Apparently I asked a reasonably good question.

As a fairly simple minded person I believed the problem to be mechanical in nature so after a good cleaning of the air filter, hoses and suction cups the issues seems to be corrected (we can not pull the glass off the device).

I posted this question more from a theoretical perspective, I do believe that after watching the gauges on the machine that elevated levels of humidity coupled with elevated levels of temperature do have an impact (seems to increase vacuum) but in this case not enough to be relevant.