Return to surface air volume

[Scuba_Noob]

I like playing it extra safe, so I always make sure I have at least 750psi for the safety stop. Depending on the depth, that might mean slowly ascending when I'm at around 850psi.

 

[TMHeimer]

My instructor for Deep course recommended that when starting the ascent a general rule is to have 100 PSI for every 10 feet of depth. 90 feet=have 900 PSI. When I do very shallow (15-30') solo shore dives I'm not too concerned if I return to shore with as little as 300 PSI or less. Don't like to run it down too far so as to risk contaminents entering the tank.

 

[g1138]

TSandM said pretty much what I was going to say.
What psi you decide to ascend at can be dependent on the tank you're using as well.

As a rule of thumb, if I'm diving above 50ft I practice Rule of Thirds. Rule of Thirds states you use one third of your air for your travel, one third for your return travel, and the last third as your reserve.
If followed exactly I'll end my dive with a third of my total air. So with an Al 80 filled to 3000psi, by the end of my dive I'll have 1000psi left. This 1000 was the air I might have needed if someone had run out of air or if I had hit an unexpected problem.

Normally, during my dives above 50ft, once I reach my reserve I'm within an area where I can ascend and make a short surface swim to my exit. So there are times where I'm just frolicking around until I hit 1000psi, then it's safety stop time. If I'm above my safety stop depth I usually ascend at 800psi just in case I need air for the exit. (This is using tanks with 50-80cf of air).

On dives below 50ft I plan my dives more stringently, calculating how much gas I intend to use for my travel, safety stop(s), and possible emergencies.
At the very least I always plan on being at the anchor line or ascending with 1200psi. (once again using 50-80cf tanks)
This may sound a like overkill by the math nerd, but really if you're doing similar dives to a max depth of 60ft you only have to calculate your gas usage once.

I should also say that I dislike running tanks below 500psi unless there's an emergency. I treat my tanks and rental tanks the same way.

 

[datadad]

In "normal" dives where you would do the deeper part of your dive first and you swim from a to b and backt to a
starting pressure - 5 or 600 psi = usable gas
usable gas / 2 = usable gas 1st half of the dive
Turnpressure = usable gas - usable gas 1st half of the dive

example:
3000- 600 = 2600
2600/2=1200
You should turn the dive at 2600 - 1200 = 1400 psi to return to your entry point

cascas, I understand the principle you're trying to convey, but there's a flaw in your formula. If you turn the dive at 1400 and you require 1200 to get back to your entry point, you'll only have 200 remaining in the tank. I think what you meant to say is this:

starting pressure - 5 or 600 psi = usable gas
usable gas / 2 = usable gas 1st half of the dive
Turnpressure = starting pressure - usable gas 1st half of the dive

example:
3000- 600 = 2600
2600/2=1200
You should turn the dive at 3000 - 1200 = 1800 psi to return to your entry point

Also (and I apologize for picking on you), this approach does not account for air consumed during your safety stop(s) on ascent, or air you might need to share with your buddy.

 

[ccx2]

Im trying to absorb all of this ,keep the opinions coming , Thanks.

 

[Mayor]

Gas management is taught in tecnical classes. non decompression diving in ow with no overhead is all usable gas. That said you still should plan for problems and have reserves. What they are is based on the dive, profile, and team resources. Do You have to return to the boat? Is it a drift dive? What stops are involved? Lost gas?

 

[cascas]

cascas, I understand the principle you're trying to convey, but there's a flaw in your formula. If you turn the dive at 1400 and you require 1200 to get back to your entry point, you'll only have 200 remaining in the tank. I think what you meant to say is this:

starting pressure - 5 or 600 psi = usable gas
usable gas / 2 = usable gas 1st half of the dive
Turnpressure = starting pressure - usable gas 1st half of the dive

example:
3000- 600 = 2600
2600/2=1200
You should turn the dive at 3000 - 1200 = 1800 psi to return to your entry point

Also (and I apologize for picking on you), this approach does not account for air consumed during your safety stop(s) on ascent, or air you might need to share with your buddy.

Correct.. I should have rechecked the calculations..
Usually a dive consists of two parts.. the descent /bottom part and ascent part. As in start the dive deepest part first and an ascent following the slope up ( if it is not a boat dive, then you would not use a "halves" rule ). During the ascent part you avarage depth will be (much) less than the first part of you dive normally allowing more than enough safetystop gas.
If an air sharing situation arises, all things change and you surface asap using your 600 psi if needed. Any DM or Instructor giving me a hard time because I surfaced gas-sharing and not having at least 600 psi in your tank will regret the day he/she got certified. I'll make sure that he/she knows I just saved his/her customers life and tell him/her to get lost...

Opinions may vary

 

[Colliam7]

Im trying to absorb all of this ,keep the opinions coming

What you are seeing is that there is a continuum of precision in gas management approaches, ranging from the more coarse (jump in the water, and be back on the surface with 500 psi), to the more precise (plan you dives before entering the water, with specific depths and times, and calculate your gas requirements based on YOUR calculated RMV, using a 'Rock Bottom' contingency approach). While your initial question was addressed to a position on the continuum toward the coarse end (at what pressure should I start my ascent, in order to be on the surface with 500 psi?), you can already see that asking that one question opens the door to a world of additional questions, and options. And, what you are seeing in the responses is not so much differences of opinion, as different people sharing what they have found to be the most useful / efficient position on that continuum, for them, AND sharing their 'rules of thumb' in doing so. Most recreational divers never progress beyond the coarse end (back on the surface with 500 psi). That isn't a criticism of those divers, just recognition of the reality of general open water training (and diving). So, good for you for asking the question. Don't let the volume of detail in the responses overwhelm you.

 

[DivemasterDennis]

If you are on an escorted dive, there is no real problem here. Just let the dive leader know your air level when they inquire, and if at some point you are under 1000 psi let them know. When planning and carrying out your own dive with a buddy, note psi at the start of the dive, and if you are moving very far from your point of entry, turn and head back to your exit point when you have 1/2 your air left. start your ascent with enough air (based upon your rate of usage, for a safe ascent and safety stop. If you are consuming air at about 50 psi per minute, then an ascent from 90 feet with a three minute safety stop is a 6 minute ride, so you need 300 psi. if you start your ascent at 500 or 600psi, you will surface with a safe amount. AT the risk of heresy, surfacing with more than 0 psi is a successful dive. But I too believe building in a margin of a few hundred psi.
DivemasterDennis

 

[ccx2]

Thanks Colliam7, I'll be the very first to say I'm not the smartest marble in the pile,I don't even know what continuum means, but I tried to drop out of my OW class because my instructor flew thru it and didn't fully explain why we were doing this or that when questioned,I told her I was going to drop out and study more on my own but stayed upon her insistence and finished / passed good enough for HER but not good enough for me. Some of the rest of the class may have understood well enough for there liking, they passed and got to get a card and go diving,others I think were just as confused or had unanswered questions as myself but continued on and got there card, I think that's scary for they will be my insta-buddy maybe on my next dive trip. I want ,as a buddy and for my own safety to know / understand things like gas management so when a DM/guide/instructor tells me to ascend with such an amount of gas I want to know and understand why, and I want my buddy to also understand. I hope on my upcoming trip to Mexico I can suggest to my insta-buddy that we do buddy check and air management discussion and not get the glassed over look of "let's just follow the DM".