Well I do need a filter

Jim, what device / method are you using to meausre the moisture content?

Thanks!

What do you consider dry enough? 

What do you consider dry enough?

First off I am not a authority on what is dry. I have been confused reading the forums with one saying you need filters and another saying you don’t. Thinking that I could get an answer to what is acceptable moisture. I ask by email Air Arms, Steyr, and Daystate. I didn’t get a number like I wanted. The best answer I got was breathing standards. I look up standards for diving air moisture and found, less than 24PPMV moisture. So I hoped for something close to 24. I bought a tank in 2010 and it has only had air from Sports Chalet until they closed then my local air gun dealer. I started my compressor and let it purge and come up to 3000 PSI then started the measuring the dewpoint the reading claimed exceeding 120 PPMV. I set the compressor to purge 5 minutes and let the compressor purge several times. Then retested getting  the 119 PPMV. Then I wondered what my tank would read. I started reading the tank air when it got to 20.1 PPMV I stoped my test knowing that it was pretty dry air. I claim to know nothing about what is acceptable for PCP air. In my opinion I want my air down to the 24ish moisture level. So I will get a filter and go from there. I am confident the numbers I got are accurate numbers. I thought I would post this because not many have access to a dew point analyzer. Maybe no one else gets confused by advertisers claims but I do.

I am not an expert on humidity and it's affect on tanks or PCPs.... However, when you fill your gun from a tank, since the pressure drops, so does the humidity, in both your gun and the tank.... The "wettest" the tank will ever be, is right after you fill it and it cools down to whatever temperature you store the tank at.... I would think that as long as the compressed air in your tank is not saturated with water at your storage temperature.... and therefore NO liquid water condenses in the tank to cause corrosion.... you should be OK....

That would be the only thing that would concern me, anyways.... If your humidity in the tank is only 20% AT TANK PRESSURE (and I would be shocked if that was the case), then that is certainly VERY dry air.... If on the other hand you are taking a sample of that air, and measuring it's humidity at atmospheric pressure, then IMO it's pretty darn wet, because as you expand the air from the tank to atmospheric pressure the humidity drops quickly.... That is why when you SCUBA dive the air seems so dry, because it is expanding to a lower pressure through your regulator and as a result you are breathing very dry air....

My question would be, is the air being tested by your instrumentation at 3000 psi when being tested?.... Here is another recent thread about moisture content in compressed air.... https://www.gatewaytoairguns.org/GTA/index.php?topic=139845.0
At the bottom of the original post is a link to a .pdf file that shows the ACTUAL moisture content (not the relative humidity) of air at various pressures, and the standard for Class 1 breathing air.... If I interpret that information correctly, at 4500 psi there is only 0.002 grams water per cubic meter for Class 1 air.... yet the air is not saturated until it has 0.06 grams / CM.... By comparison, at 1 atmosphere, air is not saturated at 68*F until it contains 17.5 grams of water.... If you have the ability to measure the Dew Point, according to that table, anything lower than -48*F at ambient pressure would not be saturated at 4500 psi.... If you can get to -50*F Dew Point, you should never see any liquid water in your tank, if I am reading that table correctly....

Bob

I think I found some answers, with the help of some research and this calculator.... http://www.dew-point.com/dew-point_calculator.asp

The required humidity standard for maximum Dew Point (atmospheric) for Breathing Air depends on the Class....

Class 1 = -70*C (-94*F).... This is 2.6 PPMV, or 0.002 g/m³.... Dew Point at 4,500 psi = -22.6*C (-9*F)
Class L (SCBA) = -53.8*C (-65*F).... This is 24 PPMV, or 0.02 g/m³.... Dew Point at 4,500 psi = 2.8*C (37*F)
Class D = -45.5*C (-50*F).... This is 67 PPMV, or 0.05 g/m³.... Dew Point at 4,500 psi = 18.2*C (65*F)
Class 2 (SCUBA) = -40*C (-40*F).... This is 127 PPMV, or 0.1 g/m³.... Dew Point at 4,500 psi = 28.8*C (85*F).... but at 3,000 psi typical for SCUBA, DP = 22*C (72*F)

Basically, the drier the air, the lower the temperature you can store your tank at without getting liquid water forming.... Therefore, you can look at the above choices and decide what is the lowest temperature you might store your SCBA tank at for prolonged periods of time, and you can know how dry your air must be.... The Class L SCBA standard should be plenty good enough unless you store your tank outside, in freezing conditions.... If you have it inside a heated building, even in a basement, then Class D air, should be just fine.... That has an atmospheric Dew Point of -50*F, or 67 PPMV of water.... I am going to use that for my goal.... Since SCUBA tanks are usually only filled to 3,000 psi, they can get away with Class 2 air....

Something else to consider.... If you fill your PCP to 3,000 psi from a 4,500 psi tank containing Class D air (67 PPMV), the Dew Point at 3,000 drops to just 12*C (53*F).... Yes, the air gets "drier" as it expands when filling your gun, so you can never get liquid water in your PCP if you fill it from a tank, unless you invert the tank and spray liquid water into it.... If you fill only to 2,000 psi, the Dew Point at that pressure is even lower, at 6*C (43*F).... I knew, intuitively, that you couldn't get water in a PCP when filling from a tank at a higher pressure.... but now I have the proof.... The only time you could have water condense out inside a PCP would be in freezing conditions.... and then it would evaporate again as soon as it warms up.... Actually, as the air pressure in your SCBA tank drops as you fill your PCPs, it will absorb more water, should there be a trace of it in the tank.... By the time you get the SCBA tank down to 3,000 psi, the relative humidity will be down to 60%, because the air can hold more water as the pressure drops.... This means that your tank (and your gun) will kind of "self-dry" as the pressure drops....

Taso, an 88 CF SCBA tank holds 88 CF of air.... The tank would only hold 550 CI (9 litres) of water, so you are compressing that 88 CF of air into only (550/1728) = 0.32 CF of space.... As I understand it, you don't really care what the relative humidity of the air is in your tank (or gun), as long as it is less than 100%, which means there will be no liquid water condensed out to cause corrosion.... Since the air can hold more water as the pressure drops, the worst condition will be right after you fill it, when it cools to ambient (storage) temperature.... If there is no liquid water in it then, it will never have any, unless you cool it down further.... and then the water will evaporate again as soon as it warms back up....

Hand pumping directly into a PCP is a different story completely, of course....

Bob

Jim, now that I have done the research, I have a better understanding of what the ATMOSPHERIC Dew Point, and the PPMV you are quoting actually mean.... and as you will see in my post above, they directly relate to the point at which water will condense out in your tank, depending on temperature and pressure.... 24 PPMV air is certainly VERY dry (Dew Point -65*F), you could store a 4,500 psi tank at 37*F and still not get water condensing out.... My personal goal has just become an atmospheric Dew Point of -50*F, or 67 PPMV, which is Class D air.... If I can get there, I am confident that I will never have liquid water in my SCBA tank....

Bob

Lots of good stuff going on this thread.  Nice to see people are getting their heads around the uselessness of thinking in terms of relative humidity - given that we are working at different pressures nothing is "relative" - and we are working in absolute humidty values of grams of water per standard cubic meter, or PPM . . .

First off to Bob - I am confident that you will be getting to class D air (or better) with the set up you are moving too (the large desiccant filter using silica gel), as I have verified that my set up with the same basic filter has done just that.  I put 10 cubic meters of air through my tank using that set up with a Shoebox compressor and found zero trace of any water when I vented the tank and inspected it.  You also have a solid understanding of moisture in air now.

To Jim (and pretty much everyone else)- you have to get a dryer; we all do for long term safety reasons with our tanks.  At a minimum, we need to to be able to know that our tanks will live a long full life without early corrosion (especially considering many with SCBA tanks and their own compressors will exceed the last hydro / rated life of the tanks).  The bottom line is that without a dryer the air leaving the compressor and entering a tank is going to be saturated with water vapor at whatever temperature and pressure the air is leaving the compressor.  Which is guaranteed to mean that some water will condense out of the air that has been added to the tank - and as we repeat that it will build up over time.

The challenge you face with an all in one compressor like the Omega is that the only practical way to dry the air is with a desiccant filter after compression, like Joe B's Alpha or the gold filter from China.  There is so much water vapor in ambient air that you will need a lot of desiccant to dry it before compression.  Even the air in your freezer has about 1.5 grams of water vapor per cubic meter in it - I suppose that if you drew the air out of your freezer and passed it through a desiccant filter like Bob and I are doing you would be able to get where you want to be without saturating the desiccant very quickly.

Bob,

That big filter full of silica gel will get you there regardless of the start point - the  only question is how long it will take before you have to recharge it . . . .

I too have thought of passing it though my freezer, at least in the summer time.  I run my compressor in the garage, so I probably average about 35F for my compressor dewpoint in the winter, but a higher temp in the summer (although the air cools in my manifold in the basement, so about 68F).  In the end it averages out to a reasonable level and I just verified that by drying out my desiccant last week, along with rebuilding the cylinders in my Shoebox.  My log and hour meter show that I ran it for 145 hours, and my calculations show I average about 6.6 cubic feet per hour through my Max, so that works out to about 27 cubic meters or air through my filter (plus maybe another cubic meter or two vented through the compressor when it was not running as the filters bleed down, but I am not including that as I don't know how much that is).  I weighed the desiccant before and after, and it was 50 grams of water that were removed when I dried the beads, so that works out to 1.9 grams of water removed per cubic meter of air.  I don't know how dry the beads were when I started, so I can't say for sure all of it came from the air I flowed through the Shoebox (that seems a touch high for what I would have expected), but it is in the ballpark . . . .

I will point out that the beads were still blue when I dried them, but definitely a light blue and not the deep blue they resulted in when I dried them.  Also, that 50 grams works out to 6% of the weight of the beads, so well within the saturation level so there was no rush to do it then - I was just curious what I would find so I did it to see.

Given that you are up in Canada, you could also do the same - just run the compressor outside and save the hassle of passing the the air though the fridge.  I love having the compressor in the garage and not in my shop.  It keeps the noise outside too.

Jim,

Neither Bob or I filter on the high side - we filter between the shop compressor and our Shoebox compressors.  If you mean the high side, I am not sure who you are asking.

On the input side, I run this as my desiccant filter, and Bob has ordered one very similar to it: https://www.zoro.com/wilkerson-dryer-desiccant-x03-02-000/i/G0780071/

I believe that I exceed the -45F dewpoint with this unit because the flow rate when used feeding the Shoebox is less than 1% of the flow rate the filter is designed to support at a -45F dewpoint.  So the dwell time for the air is off the charts, and I get air that is dryer than rated.  It would be interesting to test the air in my tank as you did, but I have no way to do that.  My only test was to open it up after much air had been passed through it, and I verified that that it was bone dry and that there was no signs of there ever having been water in it.

My filter is a lot smaller than Alan's.... but way bigger than the two small inline filters I have now.... The flow rate is, like on Alan's, MANY times greater than what goes through a ShoeBox.... I am thinking I should install valves on both sides to isolate it from the air when not in use to prolong the life of the beads....

Bob