Vacuum leak detector question

If you start with a relative humidity in your housing at 100% at sea level, and pump the housing down to negative 5 inches Hg, the relative humidity in the housing will be about 85%. Continue pumpung down to negative 10 inches and the relative humidity will be about 70% at constant temperature. Interceptors above statement that compressing the air having a drying effect is completely wrong. Relative humidity = partial pressure of water vapor / saturated vapor pressure of water.

There are plenty of pressure dew point diagrams on the net. And it seems you just invented a dehumidifier based on hand pump. Am sorry to say that if on the previous topic i was in principle agreement but not convinced on the benefits what you are saying here is incorrect and it takes very little to demonstrate.

Stuart it rains in Denver before it rains in New York at the same temperature. That is why you have snow on the mountains and not on a plain. As in a higher pressure environment you have a higher dew point it feels drier at the same temperature and lower pressure it feels wetter. It is in your same Wikipedia article

Rain is not mentioned in the article. Neither is snow, or santa claus for that matter.

You'll find snow on the mountains and not on the plain because the mountains are higher, and therefore colder.

As for the rest, no further explanations from me I'm afraid. It's really not that difficult a concept to grasp and I can't think of a way of explaining it in any simpler way than I already have. So I'm afraid your tenacity for being right has beaten me. Live in your world, where the Laws of Physics have changed if you wish, or sit down with a cup of tea and read, digest, think about the previous posts, and then be a big man, put your pride to one side and admit that you're wrong.

Rain is not mentioned in the article. Neither is snow, or santa claus for that matter.

 

You'll find snow on the mountains and not on the plain because the mountains are higher, and therefore colder.

 

 

As for the rest, no further explanations from me I'm afraid. It's really not that difficult a concept to grasp and I can't think of a way of explaining it in any simpler way than I already have. So I'm afraid your tenacity for being right has beaten me. Live in your world, where the Laws of Physics have changed if you wish, or sit down with a cup of tea and read, digest, think about the previous posts, and then be a big man, put your pride to one side and admit that you're wrong.

This is a good reading

http://www.oxywise.com/en/news/What-is-the-dew-point-and-how-do-you-calculate-it

And not you can't change the humidity just with an hand pump. Take a weather station if you have one and close it into your housing and measure the humidity before and after you apply vacuum

And the point is exactly that one once you set up your recompression chamber it is dry in side because you are pressurizing things

Take a weather station if you have one and close it into your housing and measure the humidity before and after

Perhaps you should do exactly that. Do come back with your findings once you've completed your experiment.

That is exactly what I have done to test a leak detection system. I invite you to do the same. Meanwhile have a read at your own links.

A dehumidifier works by drawing air across cooling elements. Saturated vapor pressure is directly proportional to temperature. Since Relative humidity = partial pressuer of water / saturated vapor pressure of water, the relative humidity jumps up and condensation forms in the unit. A dehumidifier could also work by compressing the air, causing condensation to form in the unit. BTW, I monitor the relative humidity in a hyperbaric chamber on every treatment. I do this multiple times on a daily basis.. Interceptor is simply misinformed OR he has been very successfully trolling us. I think the latter is more likely.

A dehumidifier has nothing to do with vacuum. It moves air around and removes moisture inside. The water precipitated within it and the air circulates back in the room. There is no material change to the vacuum of the room being dehumidified as not environment is air tight

Please have a read of the following;

http://www.vaisala.com/Vaisala%20Documents/Application%20notes/Dew-point-compressed-air-Application-note-B210991EN-B-LOW-v1.pdf

The paragraph of interest is on page 2, section 3 (right hand column) where it states:

 

Increasing the pressure of a gas increases the dew point temperature of the gas. Consider an example of air at atmospheric pressure of 1013.3 mbar with a dew point temperature of -10 °C (14 °F). From the table above, the partial pressure of water vapor (designated by the symbol “e”) is 2.8 mbar. If this air is compressed and the total pressure is doubled to 2026.6 mbar, then according to Dalton’s law, the partial pressure of water vapor, e, is also doubled to the value of 5.6 mbar. The dew point temperature corresponding to 5.6 mbar is approximately -1 °C (30 °F), so it is clear that increasing the pressure of the air has also increased the dew point temperature of the air.

 

Conversely, expanding a compressed gas to atmospheric pressure decreases the partial pressures of all of the component gases, including water vapor, and therefore decreases the dew point temperature of the gas.

So, pumping air out of the housing will reduce the pressure inside, effectively the same as expanding the gas. Dew point temperature will reduce.

Yes that's exactly correct and looking at your Wikipedia paragraph 2 At temperatures below the dew point, water will leave the air. As you are moving the dew point down you are making more likely to have a wet environment. Phew it did take a long time!!!

The following document is also of use:

http://www.supertron.com.sg/download/appnotes/13dpconv.doc

In case you'd rather not open a document from an untrusted source, it states:

 

Dew point temperature is always directly related to the partial pressure of water vapor in the gas. In other words, if you know the dew point, the partial pressure of water for that dew point temperature can be found in published tables. See General Eastern’s Humidity Handbook.

 

The ability to predict the dew point change from the total pressure change is based on physical laws which govern the behavior of gases. The Ideal Gas Law and Dalton's Law are both in operation here. The Ideal Gas Law gives the relationship of pressure, temperature, and volume in PV = nRT. Dalton's Law states that total pressure is equal to the sum of all the partial pressures of the component gases.

The important part to pick up on is that dew point is dependant on the partial pressure of water vapor in the gas. Increase overall pressure, you increase the partial pressure, decrease pressure (e.g suck some air out of the housing) you decrease the partial pressures (Dalton's Law).

I watch the relative humidity INCREASE in the chamber with every treatment. I have supervised 2 treatments already today. I will certainly post is the converse happens for any remaining treatments today, and I will also let you know if the sun decides to set in the east today.

I watch the relative humidity INCREASE in the chamber with every treatment. I have supervised 2 treatments already today. I will certainly post is the converse happens for any remaining treatments today, and I will also let you know if the sun decides to set in the east today.

I've just seen Santa fly over towing the sun behind his sleigh, so it could well happen this evening

Another good illustrative example is if you put a cup of water in a vacuum chamber, and pump it all the way down, it will start to boil at room temperature. This is the condensate (water in cup) going to a gasseous state. Once the vacuum is removed, condensation forms on the walls of the chamber.

Yes that's exactly correct and looking at your Wikipedia paragraph 2 At temperatures below the dew point, water will leave the air. As you are moving the dew point down you are making more likely to have a wet environment. Phew it did take a long time!!!

I'm not entirely sure what you're saying here, sincerely hope you are finally agreeing with what we've been trying to explain to you and the reams of information that has been shared

I'm not entirely sure what you're saying here, sincerely hope you are finally agreeing with what we've been trying to explain to you and the reams of information that has been shared

Doubtful, Interceptor, you have just agreed that dew point temperature will decrease as you create a vacuum, this means that the realtive humidity decreases BY DEFINITION. So you CAN derease the relative humidity with a hand pump. Q.E.D.

Wow, didn't think my comment would have caused this much debate. I would have thought you could have logically solved this by just looking at the extremes. At one end you have a case full of air that is fully saturated with water, which will absolutely lead to condensation. At the other end, you have a case under a complete vacuum, there would be no water whatsoever, so there would be no chance of condensation. Of course, these are static examples, and maybe something weird happens going from point a to point b. But I think we intuitively know that at lower pressures, its easier to get water to be its gaseous state than its liquid. Climbers will know that water boils at a lower temperature at higher altitudes (ie lower pressure). Or NASA urine dumps, where it vaporizes if its not too cold. And if i remember anything from my thermo class 20 years ago, lowering the ambient pressure in the closed system that is a housing would mean the vapor pressure of water (which is unchanged) is now higher than the ambient pressure in the housing, and this differential would drive the water into its gaseous state or keep any water vapor from condensing. I think a better chart is the phase diagram, where you can see the effects of lower pressure on gaseous water.

http://en.wikipedia.org/wiki/Ice#Phases

There was a comment that at a complete vacuum, your housing would implode. I would think that if the housing could handle 10msw, it could handle a complete vacuum.

Wow, didn't think my comment would have caused this much debate.

I'm not sure that debate is the right word for it. More increasingly frustrated attempts to try and get someone to understand and accept that 2 + 2 = 4, not 0.

"I would have thought you could have logically solved this by just looking at extremes" Indeed Watboy, but there is the rub. Sometimes logic falls upon deaf ears.

"Knowledge and truth may be within us without judgment, and judgment without them; but the confession of ignorance is one of the finest and surest testimonies of judgment that I know" Michel de Montaigne,

You could spare the insults as those are inappropriate and rude

If this was a closed system being compressed and decompressed I would not have any issue in agreeing that pressure increase changes humidity levels.

However this is the other way around you are decreasing pressure by pumping matter out you are not applying pressure on the same amount of air.

I have done again the measurement with the weather station and it does not report any changes. maybe this device is not accurate but it would seem that either the vacuum pump is not sucking the moisture out or that it is sucking everything out moisture and dry air and the relative masses stay the same so that the reading does not changes at all.

Or it could also be that 200 mbar delta pressure mean absolutely nothing to the device and the changes need to be much more conspicuous to produce any effect.

Air and water are weird substances and modelling them with perfect gas laws is not always effective