Filling and Humidity

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So you are saying the article is wrong.

The article states, by example, that the amount of water in the air reaching the filter stack is relative (0.7%) to the water in air at the intake of the compressor. You are saying this is not true.
youare mixixng references. it will always be 100% RH hitting the filters. if the environment is30% RH you may or may not get much if anything form the first separator. once the RH hits 100 it goes no higher. You saying that the filter is .7% may be true so long as you reference the gas to 1 ata and not the 150 bar it is under in reality. for sake of argument all RH's are reference to what you are going to breath not what is in the tank. your tank may have 80% RH in it at 200 bar. as bar drops so does the RH because as the Bar drops the gas is capable of holding more.

Until some one educates me difrferently there is but 2 reasonss to have the BPR or PMV as you called it is this.

1. to prpovide a back presure such that the force on the final piston equals teh force of hte other cylendars. that keeps it runn8ing vibration smooth instead of like a tire on your car that threw a weight.

2. It provides a back pressure on the final moisture separator so that it constantly is under say 140 bar when the tank is only under 50 bar. the higher the back pressure the more moisture is removed. ( relate to squeezing a SPONG) With out it you would try to remove water with 20 bar and then 50 bar etc going higher as tHE tank fills resulting in higher Moisture content in the tank THAN IF TEH SEPARATOR WAS SALWAYS at 140 bar untill the tank got 141 bar. 140 bar as I have read ; purges 99+ percent of the water. That should perhaps be volumn but that translates to RH also when decompressed through the regulator. With out the filter stack you would be breathing perhaps 1/5% RH at 1 ata. The desicant is what removes additional water that the mechanical separator can not do. You can help out the situation by cooling the final stage temp using a heat exchanger or 50 ft of hp pipe coil in a water bath befor hitting the final water separator. lowering the temp raises RH and causes more water to come out. You have 3 factrors --------- RH temp Pressure----------. RH become moot once teh gas first hits 100%. lower temp get out more water .., raise pressure get out more water.

The next phase in getting dry air is to have a large volume filter canister. the larger the canister the longer the gas is in fie filter stack. the longer time the more it filters. It is called dwell time. you can calaulate it by saying what is the cuft of the filter at psi and divide it by your compressor output. so I have a say 4 inch 3 ft filter can. it ( for arguments sake) at psi is 30 cuft. my compressor pumps 5 cuft. dwell time is 6 minutes. that dwell time is controlled by the back pressure onthe filter output just like teh compressor moisture is dependent on the PMV. You put a PMV on the final output of the filter stack for best air.
 
You did not waste our time. In fact you gave Rack321 an opportunity to give an excellent explanation of why the relative humidity of the ambient air is unimportant to the life of the filter and why the temperature of the air leaving the compressor is what matters entirely. Unfortunately it appears that not everyone understands that concept or perhaps it is just my perception that they don't. I am happy that you understand it. Thanks for your thread. Understanding physics and physical sciences is perhaps the most important thing we can do to help us grasp what we see around us. Your thread moved that forward and will for many years as others find it.

Well said, Ray. I thought I understood the concept. Now I think I understand it a little better. Certainly not a waste of time @ATJ
 
After reading the posts here & the sources referenced, it appears that as pressure goes up, ability of air to hold water goes down & by the time you hit 2,000psi, there is very little water left. That last little bit gets mostly absorbed by the molecular sieve. That needs to happen so that the air in your tank is well below 100% RH and free water is not available to react with the walls of your tank.

Since the 2,000psi air had almost no ability to hold water, the original humidity of the air coming into the compressor doesn't matter much. Whatever was there, ends up in the high pressure moisture trap, with just a bit left for the cartridge.

That left me wondering about the molecular sieve itself. I've started reading about it & found some interesting information. First, there are different sizes. 3, 4, & 5 being the most common, as measured in angstroms. The different sizes have different absorbent and exclusion qualities. MS is such a strong desiccant that it can pull water out of wet silica gel. MS can be reactivated by heat, vacuum, or chemical means.


The thing I haven't figured out yet, is which size should be used for filtering scuba air?
 
After reading the posts here & the sources referenced, it appears that as pressure goes up, ability of air to hold water goes down & by the time you hit 2,000psi, there is very little water left. That last little bit gets mostly absorbed by the molecular sieve. That needs to happen so that the air in your tank is well below 100% RH and free water is not available to react with the walls of your tank.

Since the 2,000psi air had almost no ability to hold water, the original humidity of the air coming into the compressor doesn't matter much. Whatever was there, ends up in the high pressure moisture trap, with just a bit left for the cartridge.
Wrong. All wrong. You need to think Pressure Dewpoint and not RH.
 
Wrong. All wrong. You need to think Pressure Dewpoint and not RH.
OK, so fill me in. what did I misunderstand?
 
The PMV (@140bar) DOES NOT remove 99.3% of the water in general as suggested in the article. It will only remove 99.3% of water at 100% relative humidity. The remaining water will be always be 0.7% of 100% relative humidity no matter what the original relative humidity was (unless the air was perfectly dry)
The PMV itself does not remove that last 0.7% of water.
The PMV keeps the pressure in the condensate, MS & AC filter at a level of 140bar or higher so the MS can do the job of removing that last 0.7% water.

Imagine filling a tank that has a start pressure of 50bar. After the compressor's last stage, the air is pushed straight into the tank, and it will take a while before a tank pressure of >140bar is reached. During that time, at those lower pressures, the MS is not able to filter out the last bit of humidity. Next, the more humid air will deactivate the AC that comes after the MS.
Wrong. All wrong. You need to think Pressure Dewpoint and not RH.
OK, so fill me in. what did I misunderstand?
Relative humidity only plays a major role during the first compression state. One of the goals is to quickly reach the dewpoint, followed by cooling down the air. The water will condensate in the first condensate filter housing.

At the next compression state, the air temperature will determine the amount of water that the higher-pressure-air can still hold. Cooler is better. And in that respect, a 4-stage compressor has an advantage over a 3-stage compressor.

After the last stage, the compressed air is first cooled, then runs through a condensate filter, followed by a MS and AC filter.
If the pressure is below 140bar at that point, the remaining water in the air is not condensing enough. That's where the PMV starts playing a role: it will block the air leaving the compressor. The pressure is built up relatively fast until it reaches 140bar. Any air above that pressure is allowed to pass the PMV.

Without a PMV, the humid air is saturating the MS very fast. How much humidity can be filtered, depends on the dwell time in the filter housing. Too humid = not all humidity filtered out. Too humid air going through the active carbon = inactive carbon.
If a hopcalite filter is used: too humid air and you can throw the hopcalite away.
 
In case it isn't already obvious, I'm just starting to get the hang of air purification for scuba use & I still have some things to figure out.

My questions today are in regards to the "eyeball" visual indicators that sense moisture & CO after the filter stack. I see that there is a variety of different indicator disks that can be put inside the housing to sense different things. The CO sensors all seem fairly similar. The moisture sensors, seem to be rated in various degrees of humidity. One reads at 10%, another at 40% & yet another has 3 different sections that indicate 3 different levels.

I basically have 3 questions here today:

1 - The moisture disks are rated in relative humidity. What percentage is considered acceptable? I see that -50 dew point is specified some places for what scuba air is supposed to be, but I don't see how that translates to RH in the eyeball gizmo.
2 - When you put a new filter in the stack & the air gets dry again, does the moisture indicator change back to blue? or do you need to replace the disk?
3 - when the CO disk turns dark, will it clean up again when it sees clean air, or does it need to be replaced?

I'm trying to decide if I should buy one of those eyeball things, so I'd like to get an idea of how they can be properly used & relied upon.

Thanks
 
In case it isn't already obvious, I'm just starting to get the hang of air purification for scuba use & I still have some things to figure out.

My questions today are in regards to the "eyeball" visual indicators that sense moisture & CO after the filter stack. I see that there is a variety of different indicator disks that can be put inside the housing to sense different things. The CO sensors all seem fairly similar. The moisture sensors, seem to be rated in various degrees of humidity. One reads at 10%, another at 40% & yet another has 3 different sections that indicate 3 different levels.

I basically have 3 questions here today:

1 - The moisture disks are rated in relative humidity. What percentage is considered acceptable? I see that -50 dew point is specified some places for what scuba air is supposed to be, but I don't see how that translates to RH in the eyeball gizmo.
2 - When you put a new filter in the stack & the air gets dry again, does the moisture indicator change back to blue? or do you need to replace the disk?
3 - when the CO disk turns dark, will it clean up again when it sees clean air, or does it need to be replaced?

I'm trying to decide if I should buy one of those eyeball things, so I'd like to get an idea of how they can be properly used & relied upon.

Thanks
It will eventually change back to blue, but it takes awhile. My moisture eye is not something I trust very much.
 
It will eventually change back to blue, but it takes awhile. My moisture eye is not something I trust very much.
How about the CO feature?
 
How about the CO feature?
I have a CO meter on the output. I have a regulator on the output and a flow control feeds a steady low rate to my output O2 sensor and to my CO meter. I suspect a dew point meter capable of testing air as dry as scuba air would be expensive.
 

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