Don't breathe tanks to zero?

[herman]

I'm not talking about monkeying with the ports. The original question was if you could contaminate a tank with moist air by breathing it down completely. You can't breath a tank dry -PERIOD- The seat will lock with spring pressure to at least the IP setting. Now if you want to hang around under water sucking on the regulator until you are gasping for air to disprove my point, knock yourself out.

If you want to check this out without turning blue...

install a set on a nearly empty tank. Push in the purge valve on the second stage until no more air comes out. You can suck on it until you turn blue, but at least you won't drown. Now, without closing the tank valve try to remove the first stage from the tank. Just joking, and I don't do jokes. Close the valve, remove the set. Take a plastic shopping bag and put it over the tank valve, pull it down to the neck and tape it closed over the valve. Open the tank valve through the bag. Did it pop? Did you jump? You judge.

You are completely backwards on how a reg works. The spring does not hold a first stage closed, it holds it open, IP and ambient pressure working opposite the spring close the valve. I suggest you buy a copy of Regulator Savvy and learn how regs work. Not only is it possible to drain a tank to zero it is actually possible to put a very slight negative pressure on the tank if you are really determined...done it.

As for the original question, as long as there is even the slightlest amount of pressure in the tank above ambient no moisture can get in the tank, even farther, as long valve remains closed, reguardless of the internal pressure be it 3000 psi or 15 inches of vacuum no moisture can get in. If air at 3000 psi will not get out, moist air will certainly not get in at ambient pressure. I think where this old scuba wives tale came from is emptying a tank and leaving the valve OPEN. In that case the tank will "breath" as it warms and cools which can draw in moist air that in that can deposit water in the tank and in time that water can accumulate in the tank but even in that case, it takes weeks, not minutes for enough moist air to get in to do damage.

 

[tracydr]

In all fairness, the DSAT Gas Blender manual and course teaches when you're blending helium mixes, if you didn't fill it, you start over with an empty cylinder.

It reads: "As a blender, if you did not personally mix the gases for the previous dive, you have no way of knowing what combination of gases are in the remaining mix without an oxygen analyzer and a helium analyzer. In this case, you must always start with an empty cylinder." -- DSAT Gas Blender Manual, p. 45.

As I neither dive nor mix helium blends, I've never done this. But as far as Nitrox goes, I never drain unless I need to bleed down to hit a specific mix.

Who would be blending gases without an oxygen and helium anylyzer? That's just a stupid statement from the book. Actually, that should be corrected or removed completely.

---------- Post added July 19th, 2013 at 08:10 AM ----------

Nobody flies with tanks here? (I haven't) Airlines all demand that the be drained and valve open to fly them. Ever seen the moisture on your luggage when it comes off a plane and lands in a hot environment? it's 40 below up there landing in 80 above.

I thought the valves had to be removed from the tank?

---------- Post added July 19th, 2013 at 08:25 AM ----------

The manual is telling the blender in that paragraph that he needs to know the residual mixture - analyze the residual gas, or start with an empty cylinder. Anyone blending trimix will have both oxygen and helium analyzers, so testing the residual mix is standard procedure.

That's not the way I read it. The manual says " you must start with an empty cylinder".

---------- Post added July 19th, 2013 at 08:41 AM ----------

Not sure if you mean an O2 analyser or a He analyser...

Anyway, you can blend trimix just with just an O2 analyser

I realize you can. But wouldn't somebody at a shop, receiving tanks and blending find it necessary to have both? Or, would they just do the calculations based on the O2 level? How do you know the percentage of helium in an unknown mixture, just based off the O2 percentage? Please remember that this is the Basic Scuba forum, not the Technical forum and I haven't had any Advanced Nitrox or other technical classes, yet.
Unless they always start with an empty tank, how do they know from the amount of O2 how much helium is in the tank? What about the nitrogen fraction?

---------- Post added July 19th, 2013 at 09:10 AM ----------

I guess we are just going to have to disagree. I believe everyone should plan a dive to breath their tank dry (or real close) so they will know how their gear performs under such circumstances. Had I not done that, I would not have know how well my 2nd stages performed as pressure dropped below IP or that one particular design of a 1st stage (Scubapro Mk7) essentially stops delivering breathable gas through the primary right ablout IP and I must switch to my secondary to access the rest of the gas in my tank.

What do your regulators do as your tank approaches empty?

We ran a tank down until my regs stopped delivering gas in the pool last summer. It's good for me to know that I only get a subtle, one breath warning before its all gone. At least in 3 feet of water, that is. I also use my husband's pony tank a lot without a gauge while diving in the pool and I know that regulator gives me a minute or two of warning before it stops.
I had my tanks Inspected last August, after several pool OOA sessions. ( we were doing DM training and using my AL 63 for buddy breathing with a 6'4" guy for long periods of time, the gear exchange and other fun exercises. My tanks looked fine, no evidence of water intrusion.

---------- Post added July 19th, 2013 at 09:19 AM ----------

Your class should have covered all of this. It doesn't get topped off with nitrogen, it gets topped off with modified grade E air aka oxygen compatible air. The point is the air has to be really filtered well to remove hydrocarbons that could build up inside the tank to the point of reacting with the oxygen and going BOOM.

So you watch them hook up a gauge and saw the percentage of O2? IMO that's not good enough. How do you you they followed the correct procedures for the device? How do you know it was properly calibrated in the first place? While nitrox is mostly thoery, math and dive planning, the critical performance skill is knowing how to analyze a tank so you know for an absolute fact what you are breathing. Otherwise it's a trust me dive.

YOU can use their device but the operative word is YOU.

Maybe he took the 1 hour resort class? Or the online version?
Either way, sounds like some remedial study is in order.

---------- Post added July 19th, 2013 at 09:28 AM ----------

Probably was covered in class and I forgot what exactly the mixer does. The class taught in the shop had us use their equipment and test a tank. The guy at the shop that I watched did the same thing I would have done on the same equipment. I just stood next to him while he did it.

Just using the tanks in the pool for now helping my girlfriend get through OP. i will get them refilled for the upcoming 60 feet drift dive. Probably check them myself for that. Hope she gets through class and can do the dive. I will be on the boat either way.


Sent from my iPad using Tapatalk HD

So you didn't actually analyse your own tank in the class? How long did this class take?
My nitrox class, back in the old days, took me several hours to read the entire book, more hours to do all of the practice questions and then the exam. Then, I had two dives using nitrox where I analyzed my tanks, figured out my MOD, logged my dives using tables,etc. I remember doing a ton of math problems and table problems in the workbook.
I've always wondered about the quality of these shorter nitrox classes and if students actually retain what they learn. Hmm.

Regarding analysing your own tank-most of the time everything will be fine but what happens when somebody screws up and gives you a tank full of 100% O2? Or, how about 7% O2?
And, what happens if you get a 40% mix when you thought you had a 28% mix for your 130 foot dive?
Or, you get air and you're diving well over the NDLs for air?
Do you know?

---------- Post added July 19th, 2013 at 09:46 AM ----------

I find breathing nitrox at depths of 60 ft and less really speeds up my deco stops.

Depending on where you dive the difference between nitrox and air fill cost is only a couple of bucks, if nothing else it's good for added safety.

There's that Florida nitrox mentality again.
here in AZ, nitrox can be up to $12 and some shops don't carry it. Many shops carry only 32%, no %36 or %40. The dive op that we love in Channel Islands ( for 3-4 day trips) doesn't have nitrox at all. Also, Catalina Dive Park doesn't carry it at their fill station, although the dive park depths would be ideal for nitrox (60-80 feet). Carrying all your tanks from town would soon become very old, since cars aren't available and the cost for a golf cart is prohibitive.
So, here in the West, we're still back in the "Dark Ages" when it comes to nitrox. I'd love to dive nitrox every dive, including in our pool but I get free air fills at our local dive shop and would have to drive out of my way to get $12 nitrox fills.

---------- Post added July 19th, 2013 at 09:51 AM ----------

A couple of bucks? I must dive in hell, here in hell a Nitrox fill is double what an air fill costs. I almost never use Nitrox. I got it mostly for trips like to NC where I just did 7 dives but only 2 on Nitrox. The weather was so bad we didn't know until after we left the dock it we'd be diving 60fsw or 130fsw.

I did breathe a tank down once. I was entangled but very close to the surface, 2-3 feet. It was a job so I was alone. After I sucked every air moulcule out of the tank trying to get free, I ditched the tank (no BC) and surfaced which of course is what I should have done in the 1st place, stubborn. The next VIP found rust on the bottom of the tank which was rolled. Cause? It was blamed on that incident.

Nitrox is almost 3x air cost here in AZ.

---------- Post added July 19th, 2013 at 10:06 AM ----------

In the long run, your body will be better off without the excess nitrogen. Most people feel better after a long dive on EANx as well (all else equal).

$15 seems high for a EANx fill. It costs about $7 for a one time fill or about $6 for a 12 tank fill card here. Oman was very similarly priced. What makes Texas so special?

Not a lot of divers diving in the local mud holes. The ocean acess in TX is really divable. So, they're diving is only slightly better than our diving in AZ. And, in fact, since we have the Channel Islands 7-8 hours away, San Diego Wreck Alley only 6 hours, one may argue we have better in the desert. S.E TX does have access to the N.FL caves but its a pretty long drive and from any other part of TX you pretty much fly for decent diving or drive long distances to marginal mud holes.
I know, I certified at Canyon Lake, TX while on a rotation in San Antonio and also lived just north of the border in Southern OK for 7 years. Most states at least have better freshwater diving. Many have Great Lakes or the ocean near enough, or nice quarry diving.

---------- Post added July 19th, 2013 at 10:19 AM ----------

Oxygen is a strong oxidant, but aren't the troublesome oxidants in the body various bioactive complex molecules? I don't think the science supports this argument.

Go to 20 feet and breath off 100% O2 for about 6 hours. See how your lungs feel.

 

[corvettejoe]

It's amazing how much crud gets continually repeated by the dive industry. I too have been told not to drain my tanks as "they risk water ingress". The same guy the next week told me to please drain my nitrox tanks out back so he could PP fill them from empty. *sigh*. I've also been told that water will somehow magically go against the force of pressure and end up dripping into the tanks if I drain them too quickly, that salt-water will somehow end up going backwards through my reg and into the tank if I breath it dry etc etc. I also guess someone needs to tell Faber that empty tanks are bad because all my factory fresh tanks turned up without even valves in them! My gosh they must be full of humid air eating away at the steel before I even got them.

If you're refilling a tank for a totally different gas, yes they may need to be drained. They also get drained for VIS inspections. Just as long as you don't pour water in it and keep it dry (don't inspect tanks in the rain LOL).

As for water getting in a tank... I've had it happen once. I loaned a tank to a friend (which I will NEVER loan my tanks ever again) and he went and somehow breathed it down to zero. There wasn't enough pressure to keep the water out ,so it leaked back through his 1st stage and into the tank.
I even heard water in my tank when I shook it (no lie.. I'll probably never know what he did to it). Anyway... it cost me a complete vis and hydro, the tank had to be whipped clean inside as there was rust in it. It was not cheap. All because he emptied my tank. I hope he got his reg inspected after that.

Also, have you ever seen what happens to steel brake rotors after a day or so of not being used? They start to flash rust very quickly.

At our school, we do an exercise in the pool where we breath a tank until the second stage reg cannot give air. This, however, does not mean that the tank is empty (0 ATM relative or 1 ATM absolute).
This excersise is intended to show to the students how does it feel an empty tank and how a second stage behaves when low on air.
A tank in this condition, still needs some minutes to completelly vent the air inside until zero relative pressure.

Shutting the tank valve off while a student breathes through the reg gives almost the same feeling and doesn't drain the tank.

 

[herman]

If you're refilling a tank for a totally different gas, yes they may need to be drained. They also get drained for VIS inspections. Just as long as you don't pour water in it and keep it dry (don't inspect tanks in the rain LOL).

As for water getting in a tank... I've had it happen once. I loaned a tank to a friend (which I will NEVER loan my tanks ever again) and he went and somehow breathed it down to zero. There wasn't enough pressure to keep the water out ,so it leaked back through his 1st stage and into the tank.
I even heard water in my tank when I shook it (no lie.. I'll probably never know what he did to it). Anyway... it cost me a complete vis and hydro, the tank had to be whipped clean inside as there was rust in it. It was not cheap. All because he emptied my tank. I hope he got his reg inspected after that.

Also, have you ever seen what happens to steel brake rotors after a day or so of not being used? They start to flash rust very quickly.



Shutting the tank valve off while a student breathes through the reg gives almost the same feeling and doesn't drain the tank.

Sorry Joe but that simply did not happen. To acomplish that, your friend would of had to 1: either pull a vacuum on the tank or take it under water (so ambient pressure was greater than the internal pressure of the tank) and 2: press the purge valve to open the valve while the second stage was under water. Short of it being intentional, the likelyhood of that happening is almost impossible. I don't doubt that there may have been water in your tank but how it got there was not from using the air with a reg.


While it's the "official" way to demonstrate OOA, it is no where near what it's like in real life. I have done a number real (controlled) OOAs to evaluate what really happens and a lot of the simulated cut the valve off ones (I am active DM so I demonstrate it in a lot of classes). What actually happens is the reg breaths fine until the tank reaches the IP of the reg (above ambient), at which time the first stage opens fully. Each breath after reaching the regs IP gets harder and harder, starting just slightly harder than normal and increasing with each breath. Depending on hose configuration, reg type and tank type, that can be a dozen or more breaths. As you near 0 pressure in the tank it starts to get really hard to breath from but you can still pull air until it's 0psi....and if you really try, you can pull a slight vacuum. In contrast, the simulated closed valve gives you 2 or 3 breaths then stops fairly quickly since you don't have the large volume of the tank to work against. I hear all the time that during as OOA the reg is breathing fine and just stops...that is pure BS, what really happens is the diver is not paying attention then panics when the regs does get hard to breath from.

 

[elan]

I did controlled OOG tests in a pool to get the feeling and it felt exactly like Herman described. The reg was Zeagle Flathead VI

 

[billt4sf]

15 pages, 144 replies. I had thought that the original question was a simple one....I guess nothing in scuba is truly simple...

- Bill

 

[corvettejoe]

Sorry Joe but that simply did not happen. To acomplish that, your friend would of had to 1: either pull a vacuum on the tank or take it under water (so ambient pressure was greater than the internal pressure of the tank) and 2: press the purge valve to open the valve while the second stage was under water. Short of it being intentional, the likelyhood of that happening is almost impossible. I don't doubt that there may have been water in your tank but how it got there was not from using the air with a reg.


While it's the "official" way to demonstrate OOA, it is no where near what it's like in real life. I have done a number real (controlled) OOAs to evaluate what really happens and a lot of the simulated cut the valve off ones (I am active DM so I demonstrate it in a lot of classes). What actually happens is the reg breaths fine until the tank reaches the IP of the reg (above ambient), at which time the first stage opens fully. Each breath after reaching the regs IP gets harder and harder, starting just slightly harder than normal and increasing with each breath. Depending on hose configuration, reg type and tank type, that can be a dozen or more breaths. As you near 0 pressure in the tank it starts to get really hard to breath from but you can still pull air until it's 0psi....and if you really try, you can pull a slight vacuum. In contrast, the simulated closed valve gives you 2 or 3 breaths then stops fairly quickly since you don't have the large volume of the tank to work against. I hear all the time that during as OOA the reg is breathing fine and just stops...that is pure BS, what really happens is the diver is not paying attention then panics when the regs does get hard to breath from.

I will probably never know what the guy did to my tank then That's what he told us... All I know is that it was returned to me with ZERO pressure and had water in it. How else can you explain it? (not saying you're wrong, I'd really like to know what happened, as this particular person would not honestly tell us, and that's the dodgy story he gave us).

I too have done controlled OOG on my own tanks ...and yes, there is a slight difference (last few breathes are hard to draw). But the fresh open water student isn't going to really notice if it happens for real. As you say, they are just not paying attention and will just attribute it to who knows what and suddenly panic. If they were paying attention, it never would have gotten to that point to begin with.

I've also experience hard to draw breaths on a tank BARELY cracked open...and I mean barely... if you breath normally, you can draw air quick enough to make those next breaths as harsh as an OOG tank is... of course air will keep coming, but you can simply shut the tank off after a few breaths to simulate the total OOG.

I just don't like taking my tanks to zero for any reason other than an actual emergency.

 

[Slamfire]

I realize you can. But wouldn't somebody at a shop, receiving tanks and blending find it necessary to have both? Or, would they just do the calculations based on the O2 level? How do you know the percentage of helium in an unknown mixture, just based off the O2 percentage? Please remember that this is the Basic Scuba forum, not the Technical forum and I haven't had any Advanced Nitrox or other technical classes, yet.
Unless they always start with an empty tank, how do they know from the amount of O2 how much helium is in the tank? What about the nitrogen fraction?

You measure O2 content before and after you add helium. Start with a tank that has no helium. Analyzing for O2 will also give you nitrogen content since we are disregarding the fraction of CO2 and trace gases that naturally occur in the atmosphere.

Then you calculate how many psi of helium you need to add to get your desired mix. Doing the correct calculations is crucial. Blending is not about randomly squirting in gases and then getting your trimix analyzer to tell you what you ended up with. Blending is a purposeful, calculated exercise were you have a very good idea where your mixture is going to be before you analyze it. The analyzer is just there to verify calculations and to let you know if there was an error during calculations or execution.

After you add helium, you analyze for O2 again. You know that the moles of nitrogen and oxygen inside the tank have remained constant and that any decrease in O2 concentration is exclusively due to an increase in helium. From that relationship you can deduce the helium percentage without a helium analyzer.

You can apply the same reasoning when mixing on top of a known left over mix even if it has some helium to begin with. However, it can get challenging after you have done several generations of leftover mixing on top of each other without ever having used a helium analyzer to verify. You could be compounding errors and end up getting farther away from reality with each new generation. This is where I see a real value in having a helium analyzer. But if you are starting at generation zero with no helium in the tanks, you don't really need a helium analyzer in order to get a reasonably accurate mix.

 

[Abdullah]

Who would be blending gases without an oxygen and helium anylyzer? That's just a stupid statement from the book. Actually, that should be corrected or removed completely.

---------- Post added July 19th, 2013 at 08:10 AM ----------


I thought the valves had to be removed from the tank?

---------- Post added July 19th, 2013 at 08:25 AM ----------


That's not the way I read it. The manual says " you must start with an empty cylinder".

---------- Post added July 19th, 2013 at 08:41 AM ----------

Read the whole paragraph. It says that if you don't have the analyzers that you have to start with an empty tank. If you have the analyzers you have to analyze the residual gas.

 

[Panama Jones]

15 pages, 144 replies. I had thought that the original question was a simple one....I guess nothing in scuba is truly simple...

- Bill

Bill....I have been on SB for 3 or 4 months now and yes, you may generate a lot of discussion on a simple question but:

1. A lot of the divers do some things that suit them better than other divers
2. A lot of dives use different equipment that may behave slight differently
3. A lot of the divers will not answer your question directly and answer such as "just don't breathe tanks to zero".

What I find is that with few exceptions, nobody on the boards actually provides totally wrong answers that will get you in trouble. Read them all and try for the answer that works best for you and then ......EXPERIMENT!