Safe tank pressure at which to begin accent.

[diver0008]

based on what?

Recreational depth. Safety stop. 500 psi at surface.

 

[tbone1004]

so you're saying an ascent from 100ft, with a 3 minute safety stop at 15 ft, you're going to use 5cf of gas? As a quick fyi, that is implying you have a SAC rate of 0.3cfm. You are bloody delusional if you think that that is acceptable to recommend to someone. Also you are a freak of nature if it actually only takes you 5cf to get up from 100ft with a proper ascent rate and full safety stop, and you are also leaving basically no gas to assess a situation at depth and make a proper ascent while sharing air with a buddy

 

[diver0008]

You are bloody delusional

I clearly should have elaborated a liittle on the profiles I dive.

I plan the deepest parts of my profile before 1500, at which point I am turning around and heading back to the boat or shore exit. On the way back I generally gradually ascend and coast into my safety stop, which for me, at 700psi is more than enough. I like gradual ascents. Of course there are exceptions with shore exits that will be rough. Apologies for having raised any blood pressures.

 

[tbone1004]

are you saying 700 to get to the safety stop or 700 to leave the bottom? Leaving the bottom at 700psi just doesn't work under any math.... leaving at 1500 makes plenty of sense to get back to the boat at 500psi from 100ft

 

[seeker242]

100' depth with a single AL80 makes for a short bottom time if you actually keep a safe level of minimum gas for your buddy in an emergency. I personally would begin ascent at 1,700 psi. If you go less than that there is a good chance you will run out of gas, if your buddy has to share up from the bottom.

 

[redacted]

I plan to arrive at a SS with at least 500 psi. I work it backwards from there. SS are optional an can be eliminated in an emergency with minimal risk.

 

[gcarter]

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.

I am guessing by your post that this is a new profile / depth for you, although you have not explicitly stated so.

You can always do another dive. Your concern should about doing the dive and gaining the experience safely, not whether you will be leaving potential unused bottom time behind.

Rule of thirds is probably a good approach IMO. Even if you knew your normal consumption rate, as a new dive there will likely be some level of subliminal anxiety that increases your rate.

 

[decompression]

As tbone says, use rock bottom for a practical application of usable gas that will get you to the surface safely. Everything else is completely arbitrary. It's simple and it works.

 

[diver 85]

OP......Whatever you decide----have your Spare Air with you................

 

[boulderjohn]

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.

From what depth? That blows the most conservative estimate I have ever encountered out of the water.

Let's say that in the worst case scenario, you and you and your buddy are sharing air on ascent, you are each breathing 1 cubic foot per minute SAC rate. (That is an estimate a lot of people use.) That means if you were to ascend directly together from 99 FSW at 30 FPM, you would take about 3 minutes and consume 2.5 cubic feet per minute each--figure 15 cubic feet for the ascent. If you decide to do a safety stop while enduring an OOA emergency, add another 9 cubic feet. The gas exchange will take a minute, so at 99 feet, add 8 cubic feet. That's a total of 32 cubic feet for a bad situation at 99 FSW. So if your buddy goes OOA at 99 feet, you should be able to bring that buddy to the surface with a safety stop if you start the ascent with about 45 cubic feet left in your AL 80.

A common rule of thumb a lot of people use for a safe ascent is depth * 10 + 300. For 100 feet, that would be starting the ascent with 1300 PSI. Start the ascent from 80 feet with 1100 PSI.

Note that if you are diving a reef that gives you something to watch, that gives you a lot of options. If you are at 100 feet and start an ascent with 1300 PSI, you should reach 60 feet in about a minute, having used very little of that gas on that ascent. You can now explore the reef until you reach 900 PSI. If you then ascend to 30 feet, again having used very little of the gas in that one minute ascent, you can explore the reef until you have 600 PSI.

You will note that this contradicts the "be back on the boat with 500 PSI maxim." It does. That is because the "be back on the boat at 500 PSI" and "rock bottom" are two different systems, each designed to give you an adequate reserve to ensure that you and a buddy reach the surface safely.