Easy tank pressure question...

[toodive4]

Just got back from North Queensland... no such law was ever quoted by our liveaboard crew... and since twice one of our ding-a-ling divers surfaced with NO air in his tank (zip , zero, nada)... I'd say there is no such law. He wasn't arrested, he didn't get a fine, hell - he didn't even get lectured. The captain did make the comment that in America - one, two, three strikes you're out at the old ball game... but I think he had been singing the song while drinking.

Had this happened in America our dive buddy probably could have hired Johnny Cochran and sued for being threatened by a foreign (Aussie) terrorist... as my good friend Dong King would say... "Only in America."

K

I got certified in Australia this summer, but am about to go on my first CA dive in a few days. I have a quick question regarding surfacing procedures;

In Queensland, it is apparently ILLEGAL to surface with less than 50 bar in the tank (an AL80 capacitiy is typically 200 bar, so 50 is a good chunk of air), and I was taught to start heading up with about 70. How does this equait to PSI? I know that the same AL80's hold 3000 psi, so should I be surfaced with 1/4 of that (750 psi) or is that just some QLD specific regulation rather than a guideline?

Thanks, and if there has been a thread about this in the past, please advise; I didn't run across any.

 

[Matthew]

That assumes you're breathing the tank down to zero psi doesnt it? Thats a bad thing to routinely practice, it will encourage moisture in the tank and therefore corrosion. ?

No, it assumes a worst possible scenario, in the unlikely event of which you would be more concerned with getting to the surface alive than possible damage to your tank.

 

[TX101]

AFAIK, there is no such law.

Apparently, there is some sort of "self regulating body" (don't know their exact name) in QLD that sets out "guidelines". These include: Always wear a snorkle, back on the boat with 50 bar, signing a dive sheet when exiting the water, no diving deeper than 30m, no reverse profiles, no decompression diving (this one is especially finny since every dive is a deco dive)...

In my opinion, most of these are BS and the diving outfit will enforce whatever they feel like at the time. For example,my instructor (a PADI course director) and her husband (PADI Staff Instructor) were diving in QLD and about to enter the water without wearing snorkles. The guy running the boat wouldn't let them and made them hire a snorkle each for $2/day. They just folded up the snorkle and put them in the BC pockets.

As far as the back with 50 bar goes, I think that's more for the dive outfit's benefit than yours because it means it dosen't take as long to refill the cyclinder at the end of the day. On one boat I dived off, there was a trainee DM giving the briefing who said "everyone back with 30bar". He got his *** kicked by the boat DM and the guy even said (and I quote) "You wouldn't get 3 breaths out of a cyclinder at 30bar". I almost volunteered right then to prove him wrong.

The reverse profile thing is usually rigidly enforced as well.. to the point that your are told to "drop to 25m and get your depth, then come back up to 10-15m for the rest of the dive". ie. bounce to 25m, then do the dive you were going to do. In my opinion, it's simply better to just do the dive you were going to do anyway..

The dive signing thing is a good idea, but they wont let you take off ANY gear or leave the back of the boat until you have signed the "sign-in" sheet and given them your final tank pressure, dive time and max depth (from your computer). Also, no one dives without a computer (computers available for hire, of course).

In reality it's all BS and next time I go to QLD, I'm going to attempt to find a boat with a more open minded DM... It will probably be some sort of "techie" boat but oh well.

 

[Snowball]

I agree it is all BS. Some of the rules are indeed legislations and rules imposed by the insurance companies, such as the sign on/off and tank pressures. The legislations came about after a few day boats had fatalities / left people out on the reef. The liability insurance side was after the largest insurer went bankrupt and the rest of the companies raised their premiums by several thousand percents (not kidding). Hence all the operators had to put in new rules and better record keeping to minimise their insurance cost.

Dive boats (both day and liveaboard) operating in QLD that cater for new divers tend to stick strictly to the PADI table, and if you exceeded the PADI table limits you get banned from diving for 24hrs (this includes if your computer went kaput during the dive unless you were carrying a backup or had gauges with readings that fit into the PADI table). Some of the boat operators will give you a "funny" look if you were diving with a computer. Some of the other boats enforce that everybody use a computer (mainly for insurance reasons).

The "bounce to a deeper depth" thing is also very common, as the operators need to cover their liabilities thus need to show in their log that they didn't allow people to do reverse profiles.

All being said, common sense does prevail. Everybody knows that most of the rules are BS. The crew usually will keep an eye on you for the first day or so. If they believed that you knew what you're doing they usually leave you alone, whether you're reporting back "real" figures or not for the log.

 

[cummings66]

That math in the pdf document looks to be taken and slightly modified from a posting Lamont made a while back, except he used a number closer to 2 instead of 1 which is the amount of air consumed by 2 divers under stress. It's called rock bottom time and the assumption is that at that amount of air you return because that's how much air you need to support both divers breathing off one tank worst case. It will leave you with no air left by the time your surface worst case, but you should be able to surface.

I would not use the 1 cubic foot of air number basis in the pdf however, I've donated air to a diver and the number shockingly did come to about 2 cubic foot per minute as he sucked the air down. Using Lamonts original figures for ooa is probably more realistic.

You'll find that you will return with a higher psi number than you would normally because you are considering the other diver and not just yourself. Lamont's post also had additional information in it that make his posting a bit better read.

I've been trying to find the link, I have the original post saved in .txt format but I'd like to know where he posted that in the first place. Here it is;

http://www.scubaboard.com/showpost.php?p=735077&postcount=33

I don't know if he's the first or not, but it's a good post IMO.

 

[markfm]

http://www.scubaboard.com/showthread.php?t=68203&page=4&pp=10&highlight=rock+bottom+time

Might be what you're talking about (Lamont's post)

 

[cummings66]

Yes, that's the post and one I think is very informative. It really seems to be a GUE based thing as roakey also posted something very similar before lamont did with only a few differences.

lamont added some things to it in other words.

 

[DA Aquamaster]

The general recreational diving rule used as taught to me in the early 80's was to start your ascent with at least 100 psi for each 10 ft of depth and add a 500 psi reserve. So if you were ascending from 130 feet you wanted to start your ascent with 1300 psi plus your 500 psi reserve - for the math impaired, that would be 1800 psi.

And of course as you came up shallower, the reserve needed decreased so for example at 60 ft, you could continue the dive until you had 1100 psi remaining.

In my experience, those numbers work pretty well even if you end up supporting an OOA diver.

You can go to great lenghts to plan for a worst case air share scenario where the hoover buddy runs out of air at exactly the turn point in the dive, etc, etc, but you are getting into the realm of catastrophising and it is very unlikely you are ever going to encounter a scenario that could not be handled by the simpler method indicated above.

Once you get into technical diving, air management gets more complex and more conservative - with good reason - but that type of air planning is overkill for recreational diving within the NDL's.

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Draining a tank to 200 psi rather than 500 psi or 750 is not going to cause condensation in the tank. Scuba air is quite dry (or should be if the compressor is well maintained) and the drop in pressure between 500 and 200 psi is not going to cause any more condensation than the change in pressure from 3000 psi to 2700 psi. To say otherwise is to promote an urban ledgend.

Where this is a concern is in circumstances where the tank is completely drained and then the valve is removed (for VIP, etc). The tank will be cold from the rapid draining of the tank out of the water (as air conducts heat far less efficiently than water) and when the valve is removed, relatively moist air from the room can enter the tank and the moisture can then condense on the cold inner walls of the tank. The remedy here is to drain the tank very slowly, close the valve when it is empty and wait until it has equalized to the surrounding temperature before removing the valve.

Theoretically you have more potential for condensation when your 100 plus degree from sitting in the sun tank gets plunged into maybe 60 degree water as you start your dive. If you have ever watched your SPG, you will note the significant drop in pressure due to the drop in temperature. The tank cools from the outside (ie: tank walls first) and the relatively warm air in it could cause condensation on the tank's relatively cooler walls. So if you have enough moisture in the tank to condense, that is when it is going to happen, not at the end of the dive. But again with a properly maintained and operated compressor, it is not an issue as the air becomes very dry as it is compressed.

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Rules that require a diver to be back on the boat with 500 psi, 750 psi, etc. are pretty stupid. What inevitably happens is that a diver gets back to the safety stop with about 600 psi and decides to cut it short in order to be back up the ladder with 500 psi. That 500 psi reserve coming back up the ladder does the diver absolutely no good at all and in that situation of trying to adhear to the 500 psi rule places the diver at risk.

I'd much rather see the diver do the full safety stop and come aboard with maybe 350-400 psi or even extend the stop if their buddy is still in the water doing a stop and come aboard with 250 psi.

Below 250 psi it gets a little sticky as some regs do not perform well at those low pressures. At the other extreme some regs perform very well at those low pressures and give no warning that you are getting your last breath. When you combine that with the fact that many spg's are not very accurate at low pressures and a 200 psi error is not unheard of, getting surprised and having to do an esa from your safety stop is a real possibility.

 

[MikeC]

Had this happened in America our dive buddy probably could have hired Johnny Cochran and sued for being threatened by a foreign (Aussie) terrorist... as my good friend Dong King would say... "Only in America."

K

You can hire Johnnie's firm but not Johnnie UNLESS you have a connection to the spirits. Mr Cochran passed away in March of this year.