Regulator behavior at low pressure

[SBaircraft]

What happens to our equipment as we run out of air? More specifically, what happens to our equipment when the tank pressure drops below 140psi? Will the second stage regulator continue to function at lower pressures? Will any of the remaining air be useable or will our regulators be completely incapacitated?

-David

 

[rookers]

1. Depends on ambient pressure (how deep thou art).
2. Don't ever get into the situation where you have to wonder about this.

 

[redacted]

What happens to our equipment as we run out of air? More specifically, what happens to our equipment when the tank pressure drops below 140psi? Will the second stage regulator continue to function at lower pressures? Will any of the remaining air be useable or will our regulators be completely incapacitated?

-David

The first stage valve will stay open and the 2nd will get progressivly harder to breath from. A balanced 2nd will be easier than an unbalanced 2nd. With any reasonably sized tank, you will still have more usable gas than a spare air. I have tried it in a pool and it is really not too bad.

 

[Web Monkey]

1. Depends on ambient pressure (how deep thou art).
2. Don't ever get into the situation where you have to wonder about this.

What he said.

Terry

 

[DA Aquamaster]

AWAP pretty much covered it. A high performance/high quality balanced regulator will deliver virtually the last psi of gas in the tank before cutting you off. That is on one hand a good thing but on the oher it is a problem as it gives you basically no warning beyond the SPG indication.

In contrast an unbalanced first stage regulator becomes noticeably harder to breath as the tank pressure drops belw about 300-500 psi and becomes very hardto breathe once the tank pressure falls below the IP and not all of the remaining gas may be useable due to the slow rate at which you can inhale it. But the upside is that you will never draw what feels like a normal breath, exhale it and then discover you tank is totally empty when the reg gves you nothing as can happen with a balanced reg.

A related issue is that a diver with an" empty" tank and an unbalanced first stage reg will probably have enough gas remaining in the tank to inflate hos or here BC on the surface while the diver who has an empty tank and a balanced reg will probably not be able to inflate his or her BC on the surface. So again there are pros and cons.

With regard to depth, the regulator IP is relative to it. If you are at 100 feet you will have about 50 more psi of ambient pressure so the reg will act like it would act at a given tank pressure when that pressure is 50 psi higher than it would be at the surface. So in effect if your balanced or unbalanced esentially stopped delivering air at 100 feet, the remaining 50 psi would become available during your ascent. That is the reason that it is standard practice to keep the reg in your mouth and attempt to inhale periodically during an emergency swimming ascent.

Personally I think there is a geat deal of advantage of breathing your tank dry under controlled conditions (ie: in shallow water where you can just stand up when it finally stops delivering air) to see how it will perform under those conditions and to familiarize yourself with any warning signs. Contrary to popular dive shop/tank monkey myth, water will not enter your tank from doing this. To make that happen you'd have to empty the tank and then decend to where you have substantially greater water presure and then depress the purge valve to allow water to enter the system through the second stage.

99.9% of all water that finds it's way into a tank gets there because of either a faulty moisture separator on a compressor or because the fill operator did not use proper procedures to ensure there were no water droplets in the valve or in the fill whip. Failing to do that can result in those water droplets being blown into the tank during the fill.

Unfortunately, VIP and liability concerns being what they are, if you go into a shop with an empty tank they will probably insist on it being visually inspected so this is perhaps a practice best done before you tank your tank in for an annual VIP anyway.

 

[SBaircraft]

Thanks for explaining that guys. Perhaps I'll try it out in the pool some day. I prefer to never run low on air in the first place but it's helpful to understand emergency situations before they arise.

-David

 

[herman]

Welcome to the board.
As usual, DA's advise is great. It's well worth the experience to drain a tank or 2 down to almost 0....in a safe controlled condition. I have done it on several occasions (very shallow water and on purpose-VIP was due anyway) just to see how my reg would respond. My reg became harder and harder to breath until my tank pressure was less than 50 psi. At least for my reg, I have lots of warning something is wrong way before I am totally out of air.

 

[tstonedvr]

I have noticed on more than one reg that around 500psi the reg will go to a metallic shrill sound as you draw a breath, some kind of LP harmonics, I'm surprized that I can hear it as I have hearing loss across the top of all the frequencys so it may be that the bones in my body are involved....
Yarg

 

[DA Aquamaster]

Another advantage of breathing a tank dry under controlled conditions is to verify the accuracy of your SPG.

Most SPG's are calibrated at the middle of the 0 to 3000 psi range. This serves to minimize the cumulative error at the high and low ends of the scale but perhaps more importantly ensures the guage is most accurate in the middle the dive, - where the diver relies on accurate information to decide when to turn around and start returning to shore, the anchor line, the exit point, etc.

But what this often means is that a 200 to 300 psi error at the low and/or high ends of the scale is common. With regard to the low end of the scale, SPGs that read something higher than zero when the tank is empty are easy to spot and never get out the factory door. What is less obvious and much more common is an SPG that will read zero when there is still 100 to 300 psi left in the tank.

Normally a diver never lets the reading get that low on the dive as they will exit at 500-600 psi. But it is nice to know that a 200 or 300 psi pad exists for those rare occassions where you may be sharing air or wanting to finish a full 5 minute safety stop in a situation where you have less than 500 psi showing on the SPG. Knowing that as soon as the needle hits the peg means you still have a couple hundred psi and a few more minutes at 15' will let you take advantage of those few extra minutes. Conversely, if your SPG really means zero when it says zero, knowing you better be at the surface when that occurs is important too.

In my experience, electronic SPG's and the SPG functions of air integrated computers show greater resolution (5 or 10 psi displays) but they are usually no more accurate at the bottom of the scale. Overtime, they are also more likely to develop calibration issues than a mechanical SPG.