I don't need no stinkin' vis

[DA Aquamaster]

Take one of your tanks and let the gas out quickly. Notice all the condensate forming on your valve. That same condensate forms some inside too and drips down into the tank. There are sometimes lines of rust inside from that happening.

I've heard this for years and it's just not the case. It's especially annoying as I hear it from shop owner all the time. Unless the tank is dumped incredibly fast, allowing the surface of the tank to get WAY below zero, it's just not an issue. As Awap states, the air in the tank should be dry enough and have a low enough dew point where condensation is a non issue. If not, it's an air quality issue. (Now, the qualifier here is that you need to leave the valve on and closed until the tank warms to keep moist air out of the tank until it rewarms.)

I have often seen marks inside tanks that are clearly drips, but they are more commonly a result of water trapped in the valve being forced into the tank during a fill - that is and remains the major method of introducing water into a tank, with the number 2 method being a compressor with an inadequate or failed moisture separator and filter stack.

Per the cited article, a tank left full and stored in the cold can experience condensation in below freezing temps as the dew point is higher in a tank at it's service pressure - but I never noted that as an issue when I lived and worked in the frozen north. We saw temps as low as 30 below zero and I recall an ice dive on new years day with air temps of -13 degrees F but condensation was not an issue, nor did we see any higher rate of rust in steel tanks that came in for hydro or VIP. Partly it's due to a good filter stack and drying system, but it's also an artifact of starting with air that is already cold and very dry. There is not much moisture in air that is maybe 10 degrees F at 10% humidity in the winter, so the moisture separator and filter stack are already working with comparatively low levels of moisture.

 

[jar546]

I've heard this for years and it's just not the case. It's especially annoying as I hear it from shop owner all the time. Unless the tank is dumped incredibly fast, allowing the surface of the tank to get WAY below zero, it's just not an issue. As Awap states, the air in the tank should be dry enough and have a low enough dew point where condensation is a non issue. If not, it's an air quality issue. (Now, the qualifier here is that you need to leave the valve on and closed until the tank warms to keep moist air out of the tank until it rewarms.)

I have often seen marks inside tanks that are clearly drips, but they are more commonly a result of water trapped in the valve being forced into the tank during a fill - that is and remains the major method of introducing water into a tank, with the number 2 method being a compressor with an inadequate or failed moisture separator and filter stack.

Per the cited article, a tank left full and stored in the cold can experience condensation in below freezing temps as the dew point is higher in a tank at it's service pressure - but I never noted that as an issue when I lived and worked in the frozen north. We saw temps as low as 30 below zero and I recall an ice dive on new years day with air temps of -13 degrees F but condensation was not an issue, nor did we see any higher rate of rust in steel tanks that came in for hydro or VIP. Partly it's due to a good filter stack and drying system, but it's also an artifact of starting with air that is already cold and very dry. There is not much moisture in air that is maybe 10 degrees F at 10% humidity in the winter, so the moisture separator and filter stack are already working with comparatively low levels of moisture.

As I stated, there is a lot of partial pressure ean filling around here and the tanks are often bled down too fast all the way to empty, then they sit waiting to be filled. The condensate that builds up on the outside, Ali's builds up on the inside as the tank nears empty until is is at ambient pressure. Every shop I have been in has quarterly air testing that passes a minimum of grade e and has a very strict filter program. Having empty tanks that were recently bled out outside in the 80+% Florida humidity does not help. I bleed my tanks slower for PP filling purposes, wrap the valve in a towel and fill them immediately upon emptying. Mine steel tanks are pristine. Most steels that I VIP need a tumble.

 

[NAM001]

You need better than -50 for the filtration to work 13x can get it below -100 If there is moisture n the tank one can only guess what other cntaminates are there when the filter is too wet to remove them.

I forget, what is the dew point of proper scuba fill air?

---------- Post added January 5th, 2013 at 11:26 PM ----------

fast venting chills the valve to below the dewpoint of the surrounding gas. so the valve chills to 50 and you are in 90% and 95f air,,,, you are giong to get condensate on the valve OUTSIDE. The inside of the tank should be far less than 1% humidity (probably .2%) ,corrected to atm psi and probably around 40% under presure as the desicant inthe filter will remove that much at 3000 psi. the valve really has to be cool to cool the tank walls below the dew point of 40% air when full and <1% air when near empty. One could only guess what a tank would do on a ice dive if the fast vent theory is true. I suspect tanks are filled with wet air from a poor functioning compressor. I have never seen the rust lines you mention. I am in texas on the gulf. The one time i had a rust line was on a tank was one returning from the hydro shop.
If these tanks are being filled on your compressor. look at the back presure regulator and see if it is working. It greatly effects the dwell time in the filter desicant. the longer the time (higher the presure) the more moisture it removes.

Take one of your tanks and let the gas out quickly. Notice all the condensate forming on your valve. That same condensate forms some inside too and drips down into the tank. There are sometimes lines of rust inside from that happening.


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[captain]

Can I assume you are talking about the dew point of pure o2 for partial pressure mixing?


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For all practical purposes commercial O2 has a moisture content of zero. It is produced by a cryogenic process that cools air down to the liquefaction point of -319 F and the oxygen, nitrogen and other gases are separated in a distillation process. Long before air reaches -319 F all the moisture and CO2 has been frozen out and removed.

Air separation - Wikipedia, the free encyclopedia

 

[redacted]

As I stated, there is a lot of partial pressure ean filling around here and the tanks are often bled down too fast all the way to empty, then they sit waiting to be filled. The condensate that builds up on the outside, Ali's builds up on the inside as the tank nears empty until is is at ambient pressure. Every shop I have been in has quarterly air testing that passes a minimum of grade e and has a very strict filter program. Having empty tanks that were recently bled out outside in the 80+% Florida humidity does not help. I bleed my tanks slower for PP filling purposes, wrap the valve in a towel and fill them immediately upon emptying. Mine steel tanks are pristine. Most steels that I VIP need a tumble.

As long as there is positive pressure in the tank, no moisture will be getting in unless it is pushed in through the valve either by a fill whip or even aided by the attachment of a regulator (not really applicable to an "empty" tank). Even when the tank reaches ambient pressure, moisture entry will be extremely slow. Leaving the valve open for a short time (hour) will still not allow moisture in as the air in the cold tank would be expanding as it warmed to ambient temperature. That is such a small opening, that it might take weeks for a measurable amount of moisture to be entering through that open valve. And that would be independent of whether the tank was drained quickly or slowly. If it were drained and the valve immediately removed (for a vis), then the amount of moisture entering and condensing in the tank would probably be effected by the rate the tank were drained. If your shops are like most, routine compressor maintenance is usually performed just before testing so the tested air will be at its driest and highest quality. The most likely culprit for injecting moisture into the tank is wet fils and user mounting a regulator without blowing out a wet valve. The problem is not because tanks were drained too quickly. The biggest problem with draining a tank quickly is the noise.

 

[ajduplessis]

The only real problem with moisture in breathing gas is badly maintained systems and filters (especially the water traps needs draining after every couple fills). I would have expected an scuba instructor to understand the dynamics behind this easily. Not sure what they teach their students??

 

[Peter_C]

The only real problem with moisture in breathing gas is badly maintained systems and filters (especially the water traps needs draining after every couple fills).

Do they even make an industrial compressor that doesn't have auto drains? I would hope not.

Auto drains sure are nice to have, as I don't have to babysit my compressor, nor run a timer.