Surfacing at how much air?

I'll preface everything below by saying that I'm newly certified and have my first salt water dive planned for Cozumel in a couple of weeks.

Since I will be doing mainly drift diving and as I understand it, we can surface and be picked up by the trailing boat whenever we like (namely, air gets low), I was wondering what should I aim to have in my tank when I return?

My instructor for my course seemed to imply that on most dives, you should start considering working your way to the surface/safety stop when your air gets below 500 psi. Does this sound correct, or should I be more conservative?

captains will tell you to be back on the boat with 500 psi in your tank.

I did a couple of drift dives in Cozumel this summer. The operation we went with had a DM in the water with us. For the most part the dive was time limited instead of air supply limited. What I mean is that the DM told us that the dive would be done at a given depth and would last about 45 minutes. He did indicate though that when anyone reached 600 or 700 psi we would all surface regardless of the time.

As it worked out, we ended the dives at about 45 minutes and one person in the goup had reached 600 psi at about that time also.

Drift diving is a very relaxing way to go.

Depends on your depth...

If you are at 80ft and you start ascending at 500 psi, you are definitely pushing it. I would aim to be hitting the surface at 500 psi

dallasM5 once bubbled...

should start considering working your way to the surface/safety stop when your air gets below 500 psi. Does this sound correct, or should I be more conservative?

Click to expand...

For a new diver that's a bit optomistic. But, as jbd said, your dive will most likely be controlled by who you dive with.

MD

Thanks, all for such quick responses! That's exactly the information I was searching for.

MD, just curious, but what did you mean by 'optimistic for a new diver'? I assumed that since I was a new diver, I would blow through my air really quickly and therefore that would be the limiting factor. But, it sounds like the time will still be the limiting factor.

Thanks again!

you may have answered your question to MechDiver.

Depending on depth, if you are still at depth and you reach 500 psi or lower before you start your ascent then you may not have enough air to do the safety stop since you are new and likely to go through your air supply quickly.

The operation we dove with made it clear that we would do the safety stop.

Our first dive was to the 80 foot range. All of us use air faster at that depth then we do at 30 to 40 feet.

During the dive briefing you should be told what the depth, time and air limits are. If you don't hear that info make sure you ask about it before getting in the water.

If you're a new diver, and diving to 80 fsw, then I would certainly not wait till you're at 500 psi before beginning your ascent.

Consider that it should take you two minutes to rise from 80 fsw to 20 fsw (more than that if I'm with you) ... then you've got a 3 minute safety stop.

That's 5 minutes from max depth to surface ... more than enough time for a new diver to completely consume 500 psi of air (especially from your standard AL80 cylinder).

The answer really depends on how quickly you consume air at depth. But if I were your DM, you'd start ascending with probably somewhere around 1,000 psi in your cylinder, and be on the surface with a 500 psi reserve.

Keep in mind that drift diving doesn't always produce predictable results ... downwellings and upwellings are common along walls, and can impact the amount of time and gas you require to ascend safely. Don't cut your margin too thin ... the consequences of miscalculating might cost you a chamber ride (if not your life), and it just ain't worth a couple of extra minutes of fish-watching.

... Bob (Grateful Diver)

dallasM5 once bubbled...
I'll preface everything below by saying that I'm newly certified and have my first salt water dive planned for Cozumel in a couple of weeks.

Since I will be doing mainly drift diving and as I understand it, we can surface and be picked up by the trailing boat whenever we like (namely, air gets low), I was wondering what should I aim to have in my tank when I return?

My instructor for my course seemed to imply that on most dives, you should start considering working your way to the surface/safety stop when your air gets below 500 psi. Does this sound correct, or should I be more conservative?

Click to expand...

No it does not sound correct to me. You should plan on being back on the surface with 500psi. If you are at 100ft (which happens often at Cozumel) and you still need to make a 5 or 6 min ascent (including safety stop) then you need to leave the bottom with more than 500 psi to be safe. I'd say when you hit 750 (rock bottom) to 1000 (comfortable) you need to be "thumbing" the dive unless it's really shallow.

There's nothing wrong with using part of your 500psi reserve if you need it but it is a reserve for the unforeseen and for emergencies. It should never be part of your plan to use it. For example, if you hit your safety stop and you have 450psi left (for some strange reason) then it's much better to do the safety stop and surface with 300 psi left than to skip the safety stop because you happen to be under 500. On the other hand, you should be aiming to hit your safety stop with more than 500 psi so you always have extra.

Think of it this way:

You can never have too much air...But you *can* have too little.

hope that helps.

R..

Find out what the DM wants for a signal when you hit 1000. If he is experienced and cares he will be fairly shallow (50-60ft.) when you hit 1000. I have never dove in Coz with anyone but Aldora and they come up sloooow and putter around in 50 ft. or so when the first diver hits 1000. The ascent is almost always very slooww. Then again they use steel 120's. Remember there is nothing wrong with staying ten feet above everyone if you are a newbie. Your air will last longer and your ascent will be slower.

dallasM5,

When you have the chance, do a search on ScubaBoard for the term "rock bottom" - this is a common term used to describe the minimum gas (or air) you should have left at a particular depth, before ascent to a shallower depth is required. There are a couple of good threads describing it in detail. Rock bottom means just that - the lowest pressure that you should ever reach at a particular depth, in order to be able to make a safe ascent to the surface, while sharing air with a buddy.

The idea is, if your buddy needs to share air from you for whatever reason, that you're guaranteed to have enough air to make a safe ascent, do a safety stop, and then surface. It's a little more complicated than saying "be back on the boat with 500psi". 500psi in what tank? A low pressure 72 steel, or an aluminum 80? Calculating rock bottom, and taking into account the tank size you are using, allows you to know what the absolute boundary is to "turn the dive". Diving any longer at that particular depth and using more of your gas supply means you are risking not being able to safely surface if you or your buddy has an out of air emergency.

Rock bottom calculations will give you a lower limit for tank pressure for a particular depth. It is unrelated to non-decompression limits.

cheers

Ben