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Filling and Humidity

Discussion in 'Compressors, Boosters and Blending Systems' started by ATJ, Jul 9, 2020.

  1. iain/hsm

    iain/hsm Manta Ray

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    Are you quite sure that is exactly what he said. If it is then he is mistaken.

    However the reason I ask and offer a possible alternative suggestion is that 75% RH water vapour is a "magic number" it is also the calibration set standard that all relative humidity sensors are calibrated to. 75% at 70F
    Why 75% I guess is the big question and precisely why 75%

    It turns out that 75% is the relative humidity generated from table salt dissolved in warm water.
    And for those who like to learn something new each day. I guess today is no exception.

    May I also suggest you buy a compressor from somone who knows what they are doing.

    If you ever need to calibrate your RH sensor (we did this some years back on SB or DS with the Rix SA-6 oil free exped compressor).

    Load a small 6 inch bowl with ordinary table salt to the brim, add small amounts of warm distilled water a table spoon at a time and until the small grains appear to form a heavy white paste. Place the sensor 100mm above the salt level and place both bowl and sensor in a bigger plastic bag and seal it all up overnight in a room box or cupboard at 70F. Next day the magic number of 75% RH on your sensor will appear. If not your sensor is out of calibration and can be adjusted. Cheap effective accurate and magic.

    While you wait it also helps to use some of the spilt dry salt rather than waste it with a freshy squezed lime ice and a Reposado or Blanco of your choice. Iain
     
    KWS, AfterDark, couv and 2 others like this.
  2. AfterDark

    AfterDark Solo Diver

    # of Dives: 1,000 - 2,499
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    Funny I found this thread today. I just got a compressor and was filling tanks for the 1st time Sat AM with probably more than 75%. I purged the moisture trap more often and for a longer time, seemed like the logical thing to do with high humidity. I was wondering if it would reduce the life of the filter and now this thread.
     
  3. ATJ

    ATJ Barracuda

    # of Dives: 1,000 - 2,499
    Location: Sydney, NSW, Australia
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    Actually, this article gives a little credence to what he's saying.

    According to the article, the PMV (when set to 140bar) will only remove 99.3% of the moisture from the air. The remaining 0.7% has to be removed by the molecular sieve. This means the more moisture there is in the air before the PMV the more there will be after it and the more the MS has to remove.

    If we use the example in the article for 45ÂșC 100% RH results in 65.5g/m3 of water. After the PMV it still has 0.475g/m3. Now if the air was only 75%, it would start with around 50g/m3 and the MS only has to remove 0.35g/m3 of water.

    Obviously, people are still filling tanks successfully at or near 100% RH but it does appear that the humidity of the intake air will have an affect on the amount of water the MS has to remove.

    Is 75% a valid number? I don't think so. Is relative humidity the right measure? I din't think so. But it does appear that the less humid the air is, the better the system will work.
     
  4. ATJ

    ATJ Barracuda

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    Location: Sydney, NSW, Australia
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    Yes! Certain. He's said it on two different occasions.
     
  5. AfterDark

    AfterDark Solo Diver

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    Location: Rhode Island, USA
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    So the answer is to fill tanks in an air conditioned room with humidity control @ 50%.
     
    Yawana-Dive likes this.
  6. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
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    which still makes no difference after the 1st stage compression. there will still be condensate in the first mech seperator and be 100% form there on.

    on the topic of filling with indoor A?C air. one has to be careful what is i the air in the shop of house. in the end the A?C doesnt mean much to the final output but it does when it comes to what you are sucking and compressing such as smoke products. packing chemicals from merchandise. perhaps freon in the air if the AC is not tight natural gas unless the shop is all electric (hot water heaters). using outside air can be just as much a problem with auto exhaust (diesel) or fumes from surrounding businesses like refineries paper mills or chemical makers. either way the chem filters can make or break the consequences of a shop location. We have a mill in our area and whenthe wind i right its fumes are terrible. I dont pump on those days and I dont want to know what is i n the air either. my chem filters has water sieves monoxicon and carbon. I pump on days that I believe that what is going into the pump suction is covered by my filters. they certainly as far as I know do not treat acitones or other things like that. And again those things are often inside the shop along with glue fumes and other things. IMO bad idea to suck from an inside or confined source.
     
    tbone1004 and AfterDark like this.
  7. tmassey

    tmassey Manta Ray

    # of Dives: 500 - 999
    Location: Shelby Township, MI USA
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    I believe you are missing the point. It doesn't matter if it's 50% or 75% or 99% humidity at the intake. It will not change the amount of work the stack has to do. Please allow me to show you.

    Imagine a really big, really squishy, really absorbent sponge. Like one of those beige car-washing sponges. Now imagine that it is filled with 50% of its capacity of water. For even numbers, let's say it can hold a gallon, and we've poured a half-gallon into it. No problem, it's all in the sponge.

    Now squeeze the sponge to half its volume. At this point, no water will have leaked out. If the sponge had been at 100% of its capacity, then 50% of the water would have been squeezed out. If at 75%, then 25% (of capacity: 33% of what we put in) would have been squeezed out. But we were at 50% so we squeezed out no water. But the sponge is *now* at 100% capacity, because we squeezed the sponge and reduced its capacity.

    Now, when we squeeze it further, we will get water out, no matter were we started: 50%, 75% or 100. In addition, from here on out, we will get *exactly* the same (additional) amount of water out of that sponge. That's because no matter where we started from, at this point the sponge is 100% full. It doesn't matter *how* we get to 100%, once we get there we get the same amount of water out of it.

    The sponge is the air. The water is the sponge is the humidity in the air. Squeezing the sponge is compressing the air. The difference is, the compressor isn't compressing the air to merely 1/2 its volume. If we're compressing to 1500PSI, the air is now 1/100 the original volume. Even if the air were at 1% humidity in the beginning (and it's *never* that dry, even in the middle of winter when your furnace roasts freezing cold air to 75C or more..), it would *still* be at 100% humidity at that point.

    So whether you started with 50% humidity or 75% humidity or 100% humidity, you will *end* up with 100% humidity no matter what. The fact that you're getting water from your drains proves this. Where do you think it came from? It's the water that the compressor is wringing from the air-sponge. If we're draining water we're at 100% humidity at that point. If we're not at 100%, we're not getting water out from a *mechanical* process like our drains. Again, no matter what the incoming humidity *was*, if you've got water in your drains, you've got 100% humid air going into your filter stack. It's physics! :)

    The bigger factor is *temperature*. The "humidity" measurement is *relative* humidity. The *absolute* *capacity* of air to hold water (that is, how much water by mass is in the air) increases with temperature. So if you have 50% humid air at 0C and heat the air to 75C (like in the furnace example I gave above), you'll drop the *relative* humidity dramatically. But same mass of air. Same in reverse. If we have 50% humidity in warm air, it will contain *more* water by mass than 50% humidity in cool air. Same is true at 100%: warmer air will have more air by mass than cooler air, even though they're both at 100% *relative* humidity.

    So, forget intake humidity. You're going to exceed 100% humidity no matter what you do: your PMV *guarantees* that -- that is its entire *JOB*. You will get more water out of the drains, but that's *their* job, and you don't have to replace them like filters, so who cares. *Temperature*, though, *does* matter. If the air entering your compressor is warmer, it'll be even warmer after compressing. If the air is 10C warmer going in, it might actually mean that the air coming out is 20C warmer -- because cooling will be less effective, too. And because the *absolute* amount of water in 100% humid air is higher when the air is 20C higher (even though they're both 100% *relative* humidity), now your filter stack has to work harder.

    ETA: By the way, I am too lazy to look up a relative humidity curve. It very well may not be linear for either temperature or pressure like my example sponge math may imply. But it's irrelevant: the point is, once the air is at 100% it doesn't matter how dry it was *before*, it's going to be the same no matter what. It in no way changes anything about my example: this is just to address the ScubaBoard lawyers that want to nit pick everything... :(

    ETA2: By the way, that's why we need the filter stack for drying. Our drains will always leave us with 100% humidity, because they only mechanically separate the water. And the water will only "fall out" when it's above 100%. But we need air dryer than that. We could achieve that three ways: 1) Compress the air *even* more (like 10x more, which would be wildly expensive and even more dangerous), 2) Chill the air down really cold, so that it's still 100% humid but 100% of a much lower capacity. This is how commerical low-pressure comrpessors work: they cool the air to below freezing to knock a bunch of the water out. But their standards are much lower than ours -- we want -50F dewpoint, and that's really hard to chill things to... Or 3) Chemically/molecularly remove the water. And that's what our filter stacks do!
     
    rjack321 and AfterDark like this.
  8. ATJ

    ATJ Barracuda

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    Location: Sydney, NSW, Australia
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    That is NOT what that article says. That's my point. Or are you saying that the article is wrong?
     
  9. tmassey

    tmassey Manta Ray

    # of Dives: 500 - 999
    Location: Shelby Township, MI USA
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    The part I quoted of yours in my original post conflicts with both the article and what I wrote (which I believe agree). No matter what you start with (10% RH, 50% RH, 100% RH) you will end up with an identical level of water in your output. (That is, after your drains, before the filter stack.) Identical. 100% RH. Because the absolute amount of water in *any* of those intake RH's is guaranteed to be *much* higher than the absolute maximum capable of being in what comes out. So if you start with twice as much or 100x as much, who cares? You will throw away the excess (however much) in your drains and *end* *up* at exactly the same place.

    And in case you missed my ETA in the original:
    ETA2: By the way, that's why we need the filter stack for drying. Our drains will always leave us with 100% humidity, because they only mechanically separate the water. And the water will only "fall out" when it's above 100%. But we need air drier than that. We could achieve that three ways: 1) Compress the air *even* more (like 10x more, which would be wildly expensive and even more dangerous), 2) Chill the air down really cold, so that it's still 100% humid but 100% of a much lower capacity. This is how commercial low-pressure compressors work: they cool the air to below freezing to knock a bunch of the water out. But their standards are much lower than ours -- we want -50F dewpoint, and that's really hard to chill things to... Or 3) Chemically/molecularly remove the water. And that's what our filter stacks do!
     
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  10. ATJ

    ATJ Barracuda

    # of Dives: 1,000 - 2,499
    Location: Sydney, NSW, Australia
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    You are still missing the point that I am trying to make and what the article makes....

    Forget about relative humidity as it is a stupid measure....

    The PMV (if the article is to be believed) will only remove 99.3% of the water (and the maths makes sense as 139/140 is around 99.3%). So there will always be 0.7% of the water that was in the original air left after the PMV. This means that the more water there was in the air to start with, the more water that will be left AFTER the PMV but before the filter stack.

    The molecular sieve has to deal with this 0.7%. It can only do this by absorbing it. If there's more water going in there's more water for it to absorb - so it will take less time before it has absorbed all it can.

    So at the very least, the humidity of the air at intake will have an affect on how often you need to change the MS in the filter.
     

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