How humid is compressed tank air?

[pescador775]

Nah, Luis, if I printed some diplomas with tear off cert cards (fill in your name) I could charge $300. The key is showmanship, and I can do that, if I want. That is what is missing from my posts, and it is intentional. I don't have to do tricks, here; I don't have to list my credentials, display an impressive photo, etc-- I don't have to do anything, except be credible. Have you ever seen those people on public TV, investment gurus and nutritionists? I could give Andy Weill's stock TV lecture on "graceful aging" while standing on my head. The only one of those guys I couldn't beat is the one who teaches piano (the "easy" way). I never studied piano.

 

[pescador775]

50% RH for compressed breathing gas is way too high. To make sure the activated charcoal in the filter is working well you need the compressed gas below 30% RH.

CGA grade E specs are for 24 ppm (v/v), -65F dew point at 1 ATA which is about 20% RH at a typical backpressure reg setting of 1900 psi.

Here's a nice conversion chart someone sent me:
http://www.afcintl.com/pdf/Dew.pdf

Thanks, I need some cool charts for my upcoming lecture series on Diving medicine, exercise, nutrition, compressor repair, advanced jet ski, diving physics, spearfishing, and lobster cooking. If I throw in a biographical presentation of "Cousteau, the early years", I can maybe get Ken Burns to cough up some seed money.

 

[pescador775]

rjack, I put Mr Hyde back in the basement and I am ready to comment on your post. Thanks for the chart (really). I can always use more data and know what to do with it. About your comment on the perceived need to maintain dry filtration; charcoal is tolerant of humidity. It will function as a filtrant even when submerged. Charcoal is used in some water filtration systems. However, in a compressor filtration system, if the activated charcoal becomes thoroughly soaked, the soggy granules cannot interact with air properly and produce a funky smell of its own which enters the breathing air. So, we need to keep that stuff from becoming too wet but a modest rise in humidity has little or no effect on its ability to deodorize and clean air. There is a filter medium called hopcalite or "monoxycon" which is sensitive to moisture and the several media, particularly the dessicant, should be arranged in the cartridge to prevent this potential problem. In this regard, in the unusual event that vaporshell 13X should become water logged this chemical will produce heat, a lot of heat. I know of one condensator explosion due to dessicant heating. This is mainly a matter of common sense, eg drain the condensator before water level rises too high.

 

[rjack321]

Wet activated carbon only sorta works. Heck its used in water filtration, but that's a different application. The key is preventing volatiles from getting by. There have been deaths due to narcotic vapors breaking through expired filtration.

Grade E, the minimum quality gas suitable for breathing at more than 1 ATA, should have a RH% much lower than 50%.

24 ppm, -65F, and ~20% RH at pressure:

1) so regs don't freeze up
2) so steel tanks don't corrode
3) so the 13x isn't saturated and unable to remove oils
4) so the activated carbon stays dry and capable of absorbing narcotic vapors - an especially critical point when using mineral oils.

All in all, filters are cheap compared to all the heartache excessive moisture can cause.

 

[pescador775]

Hey, you're preaching to the choir. However, the theme of this thread has been to explore certain variables which may tell us when compressed air is, or is not, humid. Other posters offered up some current standards which, I concluded, if used as design criteria would produce humid, rust producing air in steel scuba tanks. Until the 1980's, many compressors sold in the retail market would only meet the minima listed in the British standard, about 35mg water/cubic meter (1bar). Also, even today, if the dessicant in a modern compressor is spent, same result. It is not a coincidence that 35 mg is about what any compressor equipped with a BP valve but not a chemical filter puts out... As you say, a modern compressor capable of meeting CGA E should produce air which is acceptably dry. As far as charcoal, as long as the charcoal is in good shape, not "spent" it should absorb the hydrocarbons like toluene along with small amounts of water vapor. The problems with volatiles arise when the compressor is over heated and the filter media are shot. Whether wetting contributes to this depends on the amount. However, I don't believe the amount of water allowed by a system designed to BS EN 12021 is anywhere near enough to contribute to charcoal failure. I've used compressors like that for years and no odor got past the charcoal even when damp (odor is a "volatile"). However, I'm not advocating anything here and I hope I've been consistent in that. Nevertheless, I can agree that it would be in the divers best interests to inquire as to the quality of the air he is buying or pumping. Personally, I am rather cautious in designing and modifying my own compressors' filtration systems. Years ago, I dealt with rusty tanks, and my 1974 Bauer was the culprit. In those days, even a back pressure valve had to be special ordered. Today is better but not always. I understand that Alkin does not offer the BP valve as standard equipment. Some manufacturers believed that their machines would "last longer" if the valve were omitted. Kinda backward to my way of thinking. I know of one fellow who claimed water draining from his fill line. Same thing with the RIX, and how. Trust me, I don't approve (but don't tell anybody).

 

[rjack321]

Even worse, the Alkins W31 has an erroneous filter life table in the manual (page 6). The temps in the left hand column are for seperator inlet NOT ambient. Hence the estimated operating hours are all wrong - too high. Damned dangerous.

Airtex, the USA importer/distributor has been informed (by me).

I have a second filter on my W31 and there's a 1900 psi backpressure valve after that.

Guess we got a little mixed up between what could be in compressed air and what should be in there :11doh:

 

[pescador775]

About Grade E air, can anyone tell me when CGA 7.1 (1997) specs for moisture in Grade E air were lowered from 67 ppm to 24 ppm? Under NFPA 1500, the firemen appear to have used 24 ppm as their criterion for breathing air/ moisture content since at least 1989. I'm not questioning the CGA number-- but just when was the CGA spec changed?

 

[pescador775]

The CGA wants 50 bucks for pampflet G-7.1. I found an open source and here is the gist of it, the part concerning "grade E" air:

Carbon Dioxide.........1000 ppm
Carbon Monoxide.......10 ppm
Oil mist....................5 mg/m3
Volatile Hydrocarbon...25 ppm
Water......................24 ppm


My notes:
Dew point at 1 bar: -65F
Dew point at 240bar: 30F
All fractions rendered volume basis (v/v)
Examples of volatile hydrocarbons are methane, toluene, benzene. Most likely sources would be external as also would carbon monoxide.
Last update to CGA G-7.1: 2004.

 

[pescador775]

I have developed some opinions (surprise) about the above CGA standard; mainly it is a lot of bull and is not suitable for diving applications. The carbon monoxide spec is way too high. For example, at a depth of 200 feet, the CO fraction would be equivalent to an exposure of 70 ppm, enough to make a diver dizzy. This is not a "SCUBA" spec.

The oil mist spec appears to be an artifact and for whatever reason, not subject to the same kind of scrutiny as the water spec which recently resulted in a modification to its present lower value. Look, when the diving community sees this stuff they interpret it as reality however improbable it may be. IOW, nobody is going to PP fill a tank with oxygen after it has been filled previously with "dirty" Grade E air. Just how much oil is actually in that air? Let's take a look at a simplified interpretation of the physics. An air compressor sucks in humid air and then, by means of a sophisticated "grade E" filtration system, removes most of the humidity. At the same time, the reciprocating action of an oil lubed compressor block is bypassing a small amount of oil into the same air stream. Downstream, the oil and water have formed an emulsion which is collected in condensators. Not all the oil water emulsion condenses out in these cans, a small amount passes through the system into the SCUBA tank. How much? One way to estimate the ratio of water to oil is to measure the components in the condensate. I haven't actually done this but would estimate that oil constitutes about 5% of total (I've emptied plenty of drain buckets). A well maintained compressor which is jamming a 100 cf tank will deposit about 0.05 gram (2 drops equivalent) water vapor in the Scuba tank. If (a big if?) the ratio of vapor is the same as the ratio estimated for the condensate drained in the compressor, then the total oil for any particular fill is about 1/20 meaning about .0025 gram (2.5 mg) is deposited in the tank. Since a 100 cf tank holds about 2.9 m3 of air this equates to 0.9 mg/m3, a far reach from 5 mg/m3. I'll take a look at some more involved physics using vapor pressures when time permits. However, I strongly suspect that the "standard" is overstated.

 

[pescador775]

Imagine a hotel room, the "non-smoking" type. Somebody rents the room and lights up a stogie. Next day, the patron checks out and the maid enters. She has run out of air freshener but airs out the room, changes linen and cleans up. A non smoker rents the room that night. This person is fairly certain that the odor of tobacco is in the air but is not sure, not enough to complain, maybe. Anyway, how much residual "volatiles" and particulates do you think are in that hotel room air? Forget 5 mg/m3 it's probably more like 5 molecules/m3. A human is not a hound-dog (well, most) but the human nose can detect very low levels.

Open a jug of compressor oil and sniff. The stuff has a mild aroma but nothing really to grab the nose. It certainly won't gag you. However, once the same oil is heated, compressed and aerosolized, it reeks, even in small concentrations. My guess is that it can be detected in concentrations of only a few molecules/m3, like cigar smoke. In a compressor, this odor occurs and is transported with the air stream. Try filling a tank directly off the condensator and then breathing the air from that tank. Yecchh. The odor is associated with molecules of oil suspended in a gas like state. It is not an actual gas and it should not be assumed that the odor is produced by some lighter fraction which has cooked off from the oil. That doesn't happen because the compressor head temperatures are just not high enough to crack synthetic compressor oil. The odor is oil, just oil. Now, instead of running the air line off the condensator, tap the next tank fill off the compressor's final filter. Where did the odor go? Where did the oil go? Remember, odor is caused by molecules; no odor, no molecules. Vaporshell 13X has an average pore size of about 10 angstroms which is good for adsorbing water but only the smaller molecules in oil. However, all the pores are not that size, some are larger. Activated charcoal has all kinds of nooks and crannies of varying size and it is known for its ability to adsorb odors, many kinds of odors from many sources. That's where it went, the odor and the oil.