To complete the story in one place, I'll add this here as a follow up to my post above - this explains
why we need to use active steps for moisture filtration with powered compressors . . . .
Very few people ever understand this topic properly – in truth it is fairly simple, but it is not common knowledge, and the typical ways of talking about it to “simplify” things makes it worse – specifically, the use of the term “relative humidity.”
Relative humidity (RH) is great for things like the weather report, where we want to compare the levels of humidity in air under different temperatures, but with basically constant pressures. I
n the realm of air compression, nothing it constant, so nothing is relative – we vary pressure greatly, and temperature too through the act of compression (don’t be fooled by the fact that our tanks start and eventually end at the same temperature – the air charge temperature during compression can probably approach 200F, depending on the compressor!).
We need to think and work in terms of
absolute humidity, meaning the amount of water vapor contained in a standard unit of air, like 1 cubic meter at atmospheric pressure and a given “room temperature". To facilitate it I will list a table of values of the amount of water vapor in grams that can exist in 1 cubic meter of air at 68 degrees F:
- 68F, atmospheric pressure: 17.5 grams per cubic meter (maximum amount, at 100% RH - likely less as would not fill guns with this air)
- 68F, 125 psi, 1.8 grams
- 68F, 2000 psi, 0.1 gram
- 68F, 3000 psi, 0.09 gram
- 68F, 4500 psi, 0.06 gram
So think of that first data point and put it into practice – if you have 68F air at a low 15% RH, then there will be about 2.6 grams of water per cubic meter of air. You can’t see it, but it is there. Compress it to 3000 psi, and about 2.5 grams of water will have to condense out to liquid, which conveniently equals 2.5 ccs of water (boy I wish we switched to the metric system back in the 70s . . . ). Even though we don’t shoot at only 125 psi, I did include the data for air at that pressure for two reasons: first to illustrate how the maximum carrying capacity for water vapor falls off rapidly, and second to show how much can be removed “naturally” by the first stage of compression for those using either a Shoebox or air booster pumps. Air is much easier to "dry" with desiccants at 125 psi than at ambient temperature (and easier again at higher pressures - but higher pressure is not needed for desiccants to work; it only impacts how much desiccant is needed based on the amount of vapor needed to be captured).
Here is the first thing to remember going forward: in almost every situation after that first data point in the list, the air is going to be at 100% humidity once the conditions stabilize. So you have to realize this fact: air coming out of a compressor is at 100% humidity (either relative or absolute for those conditions) regardless of what it was going into the compressor. This will be true pretty much anywhere on the planet, other than running a compressor outside in Antarctica in the middle of winter . . .
Now here is the where the second really important point comes in: change the temperature of the air charge and we change the amount of water vapor it can hold. And when we compress air it gets hotter - a lot hotter, especially with the small fast pumps like Yong Hengs (if the water cooled head is getting ~50C, the air that is heating that head will be much hotter than that). And that means that the air charge can hold more water vapor in it when it comes out of the compressor – water vapor that will condense out to liquid when the charge cools back to ambient. Here is data like above, but at higher temperature, like what might be coming out of a compressor:
- 150F, 3000 psi, 0.9 gram
- 200F, 3000 psi, 3 gram
and
- 150F, 4500 psi, 0.6 gram
- 200F, 4500 psi, 2 gram
Notice that at 150F the air is holding ten times the water vapor as at 68F (and even more if hotter than that). So that means that if we take “nice dry air” from our 15% RH room and compress it to 3000 psi for a direct fill with a Yong Heng, and assume the air charge outlet temp is 150F, we will get about 1.7 ccs of water to condense and vent out for every cubic meter we compress, but that still leaves 0.9 ccs to pass into the reservoir as vapor – of which about 0.8 ccs will condense out to water when the charge cools to ambient (Note: people often say “heat makes more water” but that is not correct – heat allows more vapor to pass and condense later). So without some form of active water vapor management, liquid water will end up in the tank or gun. Anything that cools the air charge down helps condense out more water, and that is why coalescing “filters” work well (if I were running one, I’d put it in a bucket of ice water to further cool down the air charge).
Obviously we don’t pump a whole cubic meter into our guns in one refill, but this illustrates what happens, and liquid water will build up over time. For most air rifles about 30 fills will use this amount of air, depending on reservoir size and start and end pressures. It can be what we pump into a SCBA tank on a refill though.
Also, this helps to illustrate why hand pumps – used properly – work so well. If we pump slowly, the air charge cools in the base of the pump before going into the gun. Add in frequent cool-down breaks for the pump – no more than about 50 pump strokes or so, then venting the line and letting it cool for 10-15 minutes – and the air charge stays cool enough that there really is nothing to condense out. Maybe a tiny amount, but it will likely flash to vapor again as the pressure in the gun drops (since lower pressure air can hold a bit more vapor). Done right, handpumping won’t lead to condensed water in guns, even in Hawaii
.
Happy shooting (and pumping)!
. They both said no Silicon oil which I found different. Edit: Since I guess I have to be specific I asked if I should put a few drops of silicon oil in the fill port(foster fitting).
