Nice dive and a lesson learned.

[htn123]

Just went diving with a buddy diving that I met on scubaboard. He is new to diving, about 6 dives total sofar.
He used the "bar" pressure gauge and I used the "psi" pressure gauge.
We went into the water fine, went down to about 60 feet and started crusing along a legde. Every 5 minutes or so I would check my PG and looked at him and gave an OK? signal and he looked at this guage and answered ok.
We saw an airplane, a big sail boat and a toilet in around 50 feet of water, every thing went great sofar.
30 minutes into the dive, my buddy went over and signal that he is OOA!!. I still had about 1700psi on mine, so we were ok. I gave him my long primary, and I used my bungeed octo. He was great, calm and cool all the way. We went from 50 feet to 18, deco for 3 minutes and went up from there. The whole time, he was great, no problem whatsoever. Well, I kind of have to breath real slow just to make sure we have enough for the deco and trip up. I surfaced with 1000psi left in my tank.
Turned out, he was all the way down to ZERO bar when he signaled OOA.
And thinking back we did not even talked about when to turn back while we were doing the prep check on the surface.

So big lesson learned: Never ASSUME anything, talk it out with your buddy on when to turn back.

I did have the long primary so before the dive I told him I will donate the long primary and that's what he got and it worked out great. And I told him next time, no matter what, when you get to 1/3 of your tank, it's time to turn back. Which for me it will be 1000PSI, not sure what it is for the bar equivalent.

 

[JMD123]

WHy would you turn back with 1/3 of your tank? That would leave you 1/6 short right?

 

[ShakaZulu]

WHy would you turn back with 1/3 of your tank? That would leave you 1/6 short right?

Nice fractions...........know what you are trying to say, but I don't follow the math??

Rock bottom times should be an essential part of every dive plan. Guess you/he can be lucky he was such a heavy breather.

 

[htn123]

WHy would you turn back with 1/3 of your tank? That would leave you 1/6 short right?

Hmmm, my plan is that when I start a dive, I will go against the current, used up 2000psi, then slowly drift up and flow with the current back to my entry point.

My theory is when first enter the water, the initial acclimatize to the water and kicking to dive down and swimming around, I will use much more air than normal.

On the way back, I will let the current and the bcd do the work and I just relax and enjoy the ride back.

Most of my dives are within the 60-70 feet. Worst condition, I can always surface straight up and do the 3 minutes deco and then surface swim back to the entry point.

Sofar, the lowest I have on my tank when surface was ~400psi.

So what you guys are saying is returning at 1/2 point (1500psi)? hmmm... interesting point... I can see the logic in it.... but I want to maximize my bottom time, don't want to get back to the boat with 1/3 tank left in it...

 

[Wendy]

think of it this way, you do your dive, reach your 1/3, do your turn around and your nice relaxing drift back to your start boat/the boat, and you buddy as an OOA situation, can you get you both back on that 1/3? Even if you do an emergency ascent from that point and surface swim back, you need to make sure you have enough air for both of you.

 

[voop]

Wendy,

While I agree on your condiderations, it does depend on what the dive profile was. If the profile in "straight down, mug around the anchor (i.e right under the exit-point) without incuring any deco-stop-obligation (I dislike the term NDL), thus spending the bulk of time on the bottom, then diving thirds likely might suffice. If the profile is different, a different gas-planning is required (an extreme, although not the only, example would be DPV's in caves....thirds ain't a good idea)

The bottom line: the reserve should suffice to safely egress both divers, should a problem arrise.

 

[JMD123]

Hmmm, my plan is that when I start a dive, I will go against the current, used up 2000psi, then slowly drift up and flow with the current back to my entry point.

I did not know there was a current. Does the current where you dive ever change. What would happen if you went down swam against the current and than it stopped or changed... I guess you could just surface and have a long surface swim. It makes more sence now that I know there is a current where you dive.


J

 

[SquattingRadishDM]

Does the current where you dive ever change.

I've been on quite a few dives where the current has changed during the dive. On some the current has slacked off, sometimes the current has really picked up and on a few the current has reversed completely. These were tidal currents when drift diving, its a good idea to check high and low tide times and try and dive at slack tide.

What would happen if you went down swam against the current and than it stopped or changed... I guess you could just surface and have a long surface swim.

Yes it could end up just being an inconvenience, but what what if you had a significant deco obligation and it took a while to surface, you could be too far to swim bck to a safe exit point. Not really a problem if you are diving from a boat where you could signal the boat to collect you but shore diving with no supervision this could cause a problem Thats why its a good plan to carry signally equipment; DSMB/lift bag, whistle, torch, safety sausage, mirror, dive flag, strobe, rescue streamer etc

So what you guys are saying is returning at 1/2 point (1500psi)? hmmm... interesting point... I can see the logic in it.... but I want to maximize my bottom time, don't want to get back to the boat with 1/3 tank left in it...

What ppl are suggesting is calculating you gas requirements for each dive you do, i.e. "Rock Bottom" ascent pressure, how much gas do I need to safely get me and my buddy to the surface? Its explained very well here: http://www.scubaboard.com/showpost.php?p=401869&postcount=65

 

[Kim]

There seems to be a misunderstanding of 'rock bottom' or rule of thirds here. You use 1/3 of your tank to go out - 1/3 to go back, and 1/3 for emergencies. You turn the dive with 2/3 left.
The point of this is to ensure that you have enough gas for both you and your buddy (and vis a versa) in the event that one of you loses all air at your furthest point in the dive to get out/or back to your exit. This can be in an overhead, or simply to an upline where you can't make a free ascent before due to current or other conditions. As someone said - using DPV's is different and requires more reserve.

 

[wsparks]

Isn't the rule of thirds used for dives with overhead structures? ie. caves, wrecks or dives involving decompression time. In a purely recreational dive wouldn't you subtract 500psi from your total cylinder pressure and divide the remaining psi for your turn pressure? Of course currents, cold water, working dives, etc could change your gas management even for a recreational dive.