Remaining air

[jonnythan]

There are lot's of personal preferences on this topic. For recreational purposes an easy guideline is the rule of thirds. Use 2/3 of your gas for the dive and surface with 1/3 as a safety margin. So for a 3000psi tank - you would surface with 1000psi remaining. This method does not require consideration for varying air consumption rates or tank sizes. (Those with a higher air consumption rate will be ending their dives in less total time to remain on target).

To more directly answer your question - it depends upon how much air you've used to reach the time (or depth) at which you would like to begin the return to the surface/mooring/anchor/beach/etc.

--Matt

So you drop to 100 feet... and you swim around......... what psi do you leave the bottom with in order to hit the surface with 1000 psi? If you leave at 2000 psi, your "third," you'll get up with way more than 1000 psi...... a reef dive or look-at-the-wreck dive isn't a cave dive where you will use about the same going as coming.....

"Thirds" has no place in rec diving and does nothing to answer the question posed. The only way it would be useful is if you're swimming out underwater in a straight line and you decide to turn around and head straight back when you hit 2000 psi. I don't think that happens often.

 

[Charlie99]

100 psi per every 10' of depth plus 300 psi pretty much duplicates my numbers.

my numbers are based on an OOA scenario with a combined SAC of 2.0 for both divers (your buddy who is fearing for his life may have a consumption above 1.0) and 1 min at depth, 30 fpm ascent to the surface and 1 min @ 30, 1 min @ 20, 1 min @ 10 for stops.

Sometime back I posted how I came up with the numbers I use ---- One scenario is normal ascent of just me, with a 20 to 30fpm ascent rate, and very generous set of stops and 500psi left in the tank. For the "share air with a freaked out buddy" scenario I am willing to do 30-50fpm, blow away the non-mandatory safety stops and accept the very slightly higher DCS risk, and don't require an extra 500psi in the tank upon surfacing. With a panicked diver I'd get within 15' of the surface before deciding on whether to go on up or hang for a while. Both sets of calculations came out pretty close to 1000 psi at 100', 700psi at 70'.

As for rule of thirds, you need to figure out just what is truly required and what your are willing to blow away as a requirement. One basis for the rule of thirds is that if you go into a cave using 1/3 of your gas, and then you or your buddy loses all of your gas; that you will have just enough to get back out if you each use 1/3 of your gas while sharing air back out. "Rock Bottom" calculations include consideration for an air sharing ascent and are more appropriate for non-overhead envirionments.

Even on a non-deco rec dive you may effectively have an overhead and need to account for it in your rock bottom calculations. For example, is your ascent pressure what you want at the upline on a wreck, or is it your minimum for starting an ascent from whereever you are at that moment?

Is your turnpoint pressure on a shore dive what you must absolutely have to be able to swim under a busy traffic lane, or is it just a case of being more convenient to return to shore underwater than with a long surface swim.

As a general rule, I set my turnpoint or ascent pressures such that, if it's just me using the air, then I have a nice, easy, elegant conclusion to the dive. OTOH, for the case of a catastrophic reg or tank failure that forces air sharing, I'm willing to accept the reasonably safe, although not optimum, dive conclusion --- such as doing a free ascent from a wreck rather than going back to the upline; accepting a long surface swim back in rather than the more convenient underwater return, surfacing in an area of boat traffic that I would normally avoid, etc.

The key thing is to think about different potential situations and what you need to safely end a dive with acceptable risk.

 

[redacted]

so on an open water recreational numpty dive to what, 75-100 feet, you want me to thumb the dive when my SPG reads 2000psi? screw that.

None are quite that conservative, but all seem to be safely on the conservative side. On 100+ ft dives, I'm usually NDL limited rather than air or gas limited. So I'm looking to be in the 50 to 75 ft range when my AL80 is down to 1000psi. I like to hit the 15 ft rest stop after a deeper short stop with at least 500 psi and don't mind using most of that up before I surface. By staying well within NDLs and making it a fairly slow trip to the rest stop, I'm usually prepared to shorten or pass the 15 ft stop in an emergency. But to recover an additional margin of safety, my buddy and I usually carry small ponys (13CF) to offset this somewhat aggressive planning.

I don't see the sense in paying $50 per dive with op supplied Al80 tanks and then leaving large amounts of gas unused. A plan and a pony (reserve) are key to getting your money's worth.

 

[P_Abercrombie]

"Thirds" has no place in rec diving and does nothing to answer the question posed. The only way it would be useful is if you're swimming out underwater in a straight line and you decide to turn around and head straight back when you hit 2000 psi. I don't think that happens often.

Gas Management should be a part of any recreational dive curriculum including Rule of Thirds, Rock Bottom calculations, etc. They need to manage their gas not only on a dive where no problems occur but especially in an emergency. I have seen recreational divers surface after a problem with too little margin in their tanks. Sometimes the Rule of Thirds, as conservative as it is isn't enough; they blow through it quickly. My ow students learn their SCR and how to use it utilizing 1/3's, 1/2's and rb.

To answer the original question: calculate your SCR rate; then plan your dive using one of the rules depending on the type of dive you are doing.

 

[Charlie99]

To answer the original question: calculate your SCR rate; then plan your dive using one of the rules depending on the type of dive you are doing.

Of course, one should plan your dive with both your normal SCR, and also plan using some assumed SCRs in an air sharing emergency. A combined SCR of 2 or 3cfm is not unreasonable for two divers in an excited state.

 

[P_Abercrombie]

Of course, one should plan your dive with both your normal SCR, and also plan using some assumed SCRs in an air sharing emergency. A combined SCR of 2 or 3cfm is not unreasonable for two divers in an excited state.

Absolutely. I usually have them do a resting and working SCR rounding up as we go. The calculations provide a couple of chances to do that.

 

[david.tolan]

snip...
Even on a non-deco rec dive you may effectively have an overhead and need to account for it in your rock bottom calculations. For example, is your ascent pressure what you want at the upline on a wreck, or is it your minimum for starting an ascent from whereever you are at that moment?

As best I remember there was an incident a few years back where one diver was Out of Air, and another gave him/her their octo.

They both headed for the ascent rope, and started to ascend. This resulted in I think a fatality and a serious decompression injury.

There is a risk of being swept away in a current if you do a free ascent, but you require a lot less air if you start ascending immediatly since you save the time to swim to the ascent point, and you are not breathing at max depth for as long.

Is your turnpoint pressure on a shore dive what you must absolutely have to be able to swim under a busy traffic lane, or is it just a case of being more convenient to return to shore underwater than with a long surface swim.

As a general rule, I set my turnpoint or ascent pressures such that, if it's just me using the air, then I have a nice, easy, elegant conclusion to the dive. OTOH, for the case of a catastrophic reg or tank failure that forces air sharing, I'm willing to accept the reasonably safe, although not optimum, dive conclusion --- such as doing a free ascent from a wreck rather than going back to the upline; accepting a long surface swim back in rather than the more convenient underwater return, surfacing in an area of boat traffic that I would normally avoid, etc.

The key thing is to think about different potential situations and what you need to safely end a dive with acceptable risk.

If there's been a reg / attention failure that results in sharing air, then I'm going to call two living divers at the surface my new "optimal"

 

[Chebby]

Too much stuff to remember for different tanks. That's the problem with air gauges - they show the pressure instead of how much air is actually in a tank. Now if there was a mechanical knob that lets you set the type of tank you have (hey, if you can set your air mixture percentage, this shouldn’t be hard either), the gauge would show the actual amount of air you have left. Wouldn't that be easier? I should sell this idea to manufacturers....$$$!

 

[matt_unique]

So you drop to 100 feet... and you swim around......... what psi do you leave the bottom with in order to hit the surface with 1000 psi? If you leave at 2000 psi, your "third," you'll get up with way more than 1000 psi...... a reef dive or look-at-the-wreck dive isn't a cave dive where you will use about the same going as coming.....

"Thirds" has no place in rec diving and does nothing to answer the question posed. The only way it would be useful is if you're swimming out underwater in a straight line and you decide to turn around and head straight back when you hit 2000 psi. I don't think that happens often.

Christ Johnny - what you should do is calculate your SAC rate and gas used to include time for ascent and decent and return with a margin your comfortable with. You could absolutely utilize a rule of 1/3's here with that calculation in hand. There are a million variables and situations to cover for these questions as you very well know. I was offering an easy, conservative method for figuring out what psi one should head back. Clearly a boat dive straight down and straight up (sans deco obligations) is best served by a different approach. The poster never said if he was swimming out and back from shore, etc. For an easy answer we need more information.

The real answer is to learn to calculate your SAC rate and utilize this information to determine total gas consumption.

--Matt

 

[jonnythan]

The real answer is to learn to calculate your SAC rate and utilize this information to determine total gas consumption.

--Matt

We certainly agree there..

BTW, there's no extra 'H' in my name.