Surface pressure

[CuzzA]

I think a big benefit of the safety stop, was to help ensure that the final portion of the ascent was slow. If the diver can and does stop at 15-20 feet, the chance for a limited-skilled diver to loose control of their ascent rate is greatly reduced and there are benefits from that - even if the stop itself is not that consequential.

It's not bad logic. Force the divers to think about slowing their ascents before even surfacing. As we know, going down is the easy part, coming back up requires some skill.

 

[Wibble]

100 ft / 33 ft/atm = ~3 atm

WRONG.

30m/100ft is 4 ATA

Each 10m/33ft is one Bar/ATA plus the one Bar/ata on the surface

 

[scubadada]

agreed. but i don't recall this being a post about solo diving. perhaps i missed that part.

The OP mentions nothing about buddy diving or solo diving. It is about surfacing pressure and gas planning. Those activities vary based on specific circumstances.

I essentially did adaptive safety stops until I got my Teric, with SurfGF, no guesswork. My SurfGF when arriving at 15 feet is often low and direct surfacing would be very safe.

 

[scubadada]

no seatbelt law in my state...

Ha, Live Free or Die

NH is the only state without a seatbelt law

 

[rick00001967]

No. Wearing a seat belt is the law. There is no law that you must do a safety stop, and as already pointed out, this didn't become a thing until just a couple decades ago. Which I'd say was based solely on protecting the absolute most prone diver based on age, skill and fitness from getting bent.

Anecdotally, once you've seen thousands of dives completed at sub 60ft depths with no stops, and know tens of thousands more are being done the same way, and you consider your own age, skill and fitness, you begin to look at safety stops a bit differently. Personally, for sub 60ft. dives at 30 ft. I just slow my ascent to around 10 to 15 ft. per minute and make sure I'm taking nice deep breaths expelling nitrogen.

Regardless, the argument was made that it's dangerous to arrive at safety stop depth with 500 psi and I argue that is ultra conservative. This would mean both you and your buddy have already ended the dive, there was no emergency, and now you're sitting for 3 minutes at 10-20 feet. Two kicks from the surface. And as I pointed out, even with one diver running out of gas, one diver with 300 psi in an AL80, you both would have to be huffing a lot of gas and could still do the 3 minute optional stop.

all valid points.
however, your plan does not account for an emergency happening at depth near the end of the dive.
if the plan is to hit the stop with 300psi, that means you would be pretty low on gas when on the bottom before begining the ascent.
if diving with a buddy, and that buddy has a need to share your limited gas supply, it could put you both at risk.
have you ever had two paniced divers breathing off one reg at 100 feet? the spg needle sure does fall quickly.
we all know diving is about managing risks. we all have a different level of comfort when deciding how to manage those risks.
imho, along with pretty much every diver i have ever trained with, planning to surface with such a low volume of gas is just an unecessary risk.

 

[Subcooled]

When I got my OWD cert, I was taught to surface with at least 50 bar left in my tank. No matter whether I dived a 10L, a 12L or a 15L. I gotta admit, I stretched that "rule" a little bit. As long as I arrived at my safety stop with 50 bar left, I was good. Or at least, I thought so.

Now, 50 bar in a 10L tank is 500 surface liters, while 50 bar in a 15L tank is 750 surface liters. So that rule didn't make sense. Over in Imperial country, you're supposed to surface with 500 psi left in your tank. That's about 35 bar, and for an Al80 (11 liters water volume), it's about 350 surface liters.

A standard SPG has an accuracy of some 10-20 bar. So, if your SPG tells you that you have 50 bar left, worst case you might have only some 30 bar left. For a 10L tank, that's some 300 surface liters or about 10 minutes of gas. On the surface, so perhaps 50% less at safety stop depth. If your Imperial unit SPG has the same error, you might be down to almost zero minutes.

Is it safe to arrive at your safety stop with only 500 psi/35 bar left in your tank? Frankly, I don't think so. And I think you should start thinking about min gas calcs.

A long time ago, before the age of SPGs, tanks had reserve gas handles. One would run out of gas once the tank hit 50 bar. You could then release the remaining 50 bar by pulling the handle. Could it be, that this 50 bar limit traces back to those days? Better procedures do exist, but then one has to start thinking.

What if you experience a reverse block and cannot ascend immediately? What if your buddy experiences the same problem? What if you get entangled in kelp? What if you do not dare to resurface because there's irresponsible jet ski riders above your head? What if you loose a fin and would like to retrieve it? All of these problems are solvable, given enough time and a peace of mind.

Simple answer: I am using 70 bar instead of 50. It also happens to be ~1/3 of my gas, which is beneficial in most gas sharing scenarios, too.

 

[Storker]

Every dive op that I've heard talk about this issue during the dive briefing has specified back on the boat, not starting a safety stop, with no less than 500 psi.

I guess that alleviates some of my concerns with the 50 bar/500 psi dichotomy.

Now, it's more than 10 years since I got my OWD cert, but I'm pretty sure we were taught "safety stop, 50 bar". And if SHTF you don't have to stay at the safety stop, you go straight up to that big, wonderful supply of gas called the surface. So you'll probably have enough gas left in your tank to inflate your BCD. Worst case, you can orally inflate it; we were all taught that during OW class, weren't we? Weren't we?

 

[Storker]

i don't recall this being a post about solo diving.

It most definitely isn't. I'm not dissing solo per se (to each their own), but solo is very much not for me. If not for anything else, I at least want the emergency services to have a decent idea about where to find my body so my family can get some closure.

All my posts are written by a dedicated buddy diver who doesn't dive solo.

 

[Storker]

A long time ago, before the age of SPGs, tanks had reserve gas handles.

And at least one of my tank boots has a slot to secure that handle

 

[lowwall]

Every dive op that I've heard talk about this issue during the dive briefing has specified back on the boat, not starting a safety stop, with no less than 500 psi. Most of them also talk about arriving at the safety stop with 1000 psi.

I've also heard many briefings where they said things like, "When you get to 1200psi, return to the mooring line and begin your ascent" or, "Ascend with the group or when you hit 1000psi, whichever comes first."

On most of my dives everyone was on AL80s, so there's no need to worry about specifying cubic feet/liters remaining. And the few on AL100s are using them because they are air hogs, so they'll want the extra gas anyway.

Also, when I surface, I'm normally going to keep breathing off the reg if it's rough or I've bought nitrox. I won't take it down to 0, but I'm not going to worry about breathing it from 500 to 400psi.