Safe tank pressure at which to begin accent.

[Professor Nemo]

Greetings all,

I am a semi-long term visitor to the board and have only recently created an account and officially joined the community. I have a question that will probably indicate my relative lack of experience diving but, as I wish to learn, looking foolish is often part of the learning curve. I have planned a few dives in the next two weeks. I will be diving on Al80s, 3,000 psi, with an EAN32 mix at a location which bottoms out at 109 ft. I do not plan to exceed 100 ft and may not exceed 90ft. I am not sure what my rate of consumption is; I confess that I have never actually calculated it. (I’m an English major so Math is not my strong point). In your combined and considerable experience, what is a conservative (i.e. safe) pressure at which to begin my assent? I know about the rule of threes but I have always understood this to apply to overhead diving (i.e. cave and wreck) but perhaps it would be a good rule to follow here as well. Dividing 1/3rd of my time to reaching the bottom and exploring, 1/3rd for the return trip, and 1/3rd to operate as a safety buffer might be the best logical solution for my situation, though, I don’t know if I would be unduly depriving myself of time that could be spent exploring the dive location. I realize that without knowing my consumption rate this is a difficult question to answer with any degree of accuracy as it relates to me specifically. However, I thought learning what your individual limits are and how you calculate them might help me make an educated judgment based on my own personal factors. I will tell you that I am a larger man who, while not unfit, is not prepared to run a marathon. In that regard my consumption will likely be more rapid than the average diver.

Thanks,

Josh

P.S. I apologize if this question has been asked and answered before. I did a few keyword searches but nothing seemed to provide the information I was looking for. If I have missed a previous conversation that would answer my questions someone please direct me to it so I don’t waste anyone’s time.

 

[Reku]

The "general rule of thumb" would be when you finish the dive you should have somewhere around 500 psi remaining. If that were the case I would probably start ascending around 900-1000 psi so by the time you get done with a 10ft-15ft safety stop you'll have 500 psi remaining.

You don't know your sac rating so these are just ballpark numbers.

 

[tbone1004]

DiveNerd - Rock Bottom Calculator for Scuba Gas Management

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Not knowing your SAC rate isn't all that important, but it isn't ideal. Using rough numbers, you need to technically start ascending at 1800psi. This gives a 200psi buffer, and follows rock bottom procedures for 2 divers, breathing 1cfm *not abnormal in a stressful situation* and gives you time to ascend properly. This is deemed too conservative by most, but I'd rather not perform a CESA. This will give you approximately 10 minutes of bottom time.

If you want to be aggressive and aren't planning on sharing air with your buddy, then you can start your ascend with as little as 1000psi and end up on the boat somewhere between 200-500 psi. That is up to you and your buddy on how you plan to work out your gas management strategy. I don't believe AL80's are a good tank choice for 100ft dives, but that is just me. If you go this route, it will give you roughly 16 minutes of bottom time but does not leave any room for error in an OOA event for you and your buddy to have a calm and normal ascent schedule.

 

[Reku]

My calculations don't account for a buddy at 90ft - I suggest you go with what tbone suggested.

 

[Hatul]

A rough open water (no deco) rule if you're diving with a buddy and may need to share gas, is to have 10* (depth in feet) + 300 to 500 psi. So at 90ft: 900+ 300 to 500, or 1200 to 1400 psi, depending on the size of tank and air consumption. I find this rule more useful than more precise calculations that you just can't do under water. It should be enough for a safe shared ascent with a safety stop.

 

[Hatul]

If you have an air integrated computer like the Atomic, Oceanic or Uwatec, there is more accurate formula based on the remaining air time, that works for gas sharing as well.

 

[KevinNM]

It's worth noting that the accuracy of mechanical pressure gauges is not so good at the very low end. So when it claims you have 100 PSI left you might have 100 PSI. Or 150 PSI. Or 30 PSI. Which is part of why you get told to be back on the boat with a couple hundred PSI.

 

[diver0008]

700 psi.

 

[tbone1004]

700 psi.

based on what?

 

[Reku]

700 psi.

That's cutting it a little close don't you think? I'm all for an aggressive dive but that's a bit much even for me.