Air Question

[DevonDiver]

As others have mentioned, to help you determine a definitive minimum psi, we would need more information. In particular, the tank volumes of the cylinders you (and your buddy) would be using, along with your (and your buddies) Surface Air Consumption (SAC) and the max depth of the dive/s you would be completing.

Many basic scuba courses will advise a diver to end/surface the dive with a minimum contingency reserve of 500psi. However, they have little advice for the diver on how to plan their ascent point, so that they can get to the surface with that minimum amount. Most divers just estimate/guess. Needless to say, estimating with your gas is far from the most prudent and risk adverse strategy...

Proper Gas Management takes a more accurate approach to your air supply needs. In particular, the concept of calculating a 'Rock Bottom' (absolute emergency minimum for you and your buddy) is a wise step for the prudent diver.

Rather than re-inventing the wheel, I will direct you to these excellent resources that forum members have already created:

http://www.scubaboard.com/forums/basic-scuba-discussions/295544-understanding-gas-management-2.html

http://www.scubaboard.com/forums/advanced-scuba-discussions/356687-deep-diving-gas-planning.html

Rock Bottom and Gas Management for Recreational Divers | Spherical Chicken

NWGratefulDiver.com

http://www.tsandm.com/gm/gm.pdf

 

[scadreau]

Thanks! The article is very informative on many things I never considered.

Bob's article is great! I now give a copy to all of my students OW - DM.

Scott

 

[Bogie]

There is the rule of thirds that has been described.

I don't end a dive on the surface with 1100 psi but for someone new it is the safest option.

There are so many variables.

Common sense and good judgement.

When you are new your air rate may change substantially during the dive so percentages are estimates. Check your air often all the time on every dive. When you check your air often it not only keeps you safe but teaches you about how you are diving. You will burn more air when you are tense or your buoyancy is not stable. Lots of other things effect your air rate, current, visibility, temperature etc. When you are new end a dive with extra air for an extra margin of safety.

 

[Charlie99]

The referenced articles are very good, but in the end, a reasonable rule of thumb for a diver within NDL limits using an AL80 tank is to start the ascent with a bare minimum of no less than 100 psi per each 10' of depth. As a new diver, until you have more experience on how much is needed, it is a good idea to add another 200 to 500psi for extra margin.

So for a shallow 30' dive on an AL80, you should be headed back up at 800psi. For an 80' dive, you should be headed back up at 1000psi to 1300psi.

While it may not be precise, the crude 100psi/10' + 300psi fudge is easy to keep track of during your ascent or during a multilevel dive to know that you always have enough air to get both you and your buddy back safely the surface.

Note that these are "ascent pressures" that are only about you being able to safely get back to the surface. You also need to figure out the "turnaround" pressure, which has to do with when you start heading back towards the boat.

 

[TMHeimer]

The referenced articles are very good, but in the end, a reasonable rule of thumb for a diver within NDL limits using an AL80 tank is to start the ascent with a bare minimum of no less than 100 psi per each 10' of depth. As a new diver, until you have more experience on how much is needed, it is a good idea to add another 200 to 500psi for extra margin.

So for a shallow 30' dive on an AL80, you should be headed back up at 800psi. For an 80' dive, you should be headed back up at 1000psi to 1300psi.

While it may not be precise, the crude 100psi/10' + 300psi fudge is easy to keep track of during your ascent or during a multilevel dive to know that you always have enough air to get both you and your buddy back safely the surface.




Yes, I agree with the 100PSI/10 feet idea--learned that in the Deep course. But the OP is at Vortex Spring. Heading up from 30 ffw with 800 PSI? The folks at Vortex will like that so they can do another air fill.

 

[Craig66]

Vortex is a very "benign" shore dive. I have taken a few courses there, and it also was my first dives after OW certification. The deepest spot is about 54 feet down the descent line. As long as you are not down there at that depth (where the little "cavern" is) when your air is getting low, I wouldn't worry. No tides, currents and constant water temp. I'd probably walk out with 200 (or even 100) PSI just to keep moisture from entering the tank.

Not sure that this is really advice that should be in the new divers forum. Whilst we all know that air does not stop being dispensed when one hits 500 PSI anything under 500 should be purely for contingency and should never be part of a dive plan. Perhaps when the OP is more experienced he can possibly experiment with hanging out at 20 feet enjoying the drift at 400 or 450 PSI just because he doesnt feel like getting back on the boat but as part of a dive plan that 500 PSI mark should be sacred

 

[txapacheguy]

I took my wife to vortex a few years ago right after our OW and AOW certs, and I was an air hog(still am to some degree)... At the relatively shallow depths we were at(we stayed out of the cavern), we both had at least 500psi after each of our two 45 minute dives. We ended those dives primarily because my wife was bored and cold.

I do not feel qualified to tell you how much air to plan to surface with. That is your business, between you and your dive buddy. The gas management articles and info referenced on this thread, or some continuing education might help to point you in towards making an informed decision of your own. I learned a ton about gas management from the TDI Advanced Nitrox and Deco Procedures classes when I took them earlier this year. Once you have some more experience, I would certainly recommend those classes, or some similar to them if you want to get down to the art and science of gas planning.

Above all else, before you plan to come up with less than 500psi onboard and no redundant air source, you might consider making sure your SPG is somewhat accurate all the way down to zero PSI , and/or at least your planned turn pressure, on land with a fill station close by.

Safe Diving!!! Enjoy your trip to Vortex!!!

 

[haumana ronin]

The referenced articles are very good, but in the end, a reasonable rule of thumb for a diver within NDL limits using an AL80 tank is to start the ascent with a bare minimum of no less than 100 psi per each 10' of depth. As a new diver, until you have more experience on how much is needed, it is a good idea to add another 200 to 500psi for extra margin.

So for a shallow 30' dive on an AL80, you should be headed back up at 800psi. For an 80' dive, you should be headed back up at 1000psi to 1300psi.

Yes, I agree with the 100PSI/10 feet idea--learned that in the Deep course. But the OP is at Vortex Spring. Heading up from 30 ffw with 800 PSI? The folks at Vortex will like that so they can do another air fill.

Doesn't that mean from 30ft, you head back up at 600?

We ended those dives primarily because my wife was bored and cold.

What is the hand sign for "I am bored" Is it different from "I am your wife and I am bored"

:zen:

 

[rye_a]

The folks at Vortex will like that so they can do another air fill.

considering that the OP and his buddy only have 2 OW dives under their belts I would still recommend that they err on the side of caution and end the dive with a little extra air in their tanks. There are worse things than paying for an extra fill.

 

[TSandM]

This is why I like being charged for the gas I breathe, and not by the fill . . . It was very pleasant to take my tanks in during our last MX trip, and not be annoyed that they were still pretty full. I think it encourages people to do foolish things, if they feel they have to get their "money's worth" out of a tank.