RMV math…

[scubadada]

Hi @60plus

So you fully realize that some of us would like to be able to plan our no stop dives in a bit more detail than you do. How much gas do you need to make a normal ascent with a safety stop from 130 ft/40 m? Are you gas or NDL limited on a dive to 95 ft/30 m on 32% with the tank you have... I don't immediately see how your professional experience relates to diving.

Many basic divers simply swim down, and come back up when their NDL or gas supply dictates. That generally works just fine. Some of us plan it a bit more carefully.

 

[tursiops]

I have spent my entire working life working with pressurised gases for industrial use. For over 12 years I was the UK fabrication manager for a major international company that supplied the chemical, petro chemical and nuclear industries, I also had 13 years as a university engineering lecturer so I think it is fair to say I know quite a lot about gases and gas consumption and am well able to carry out calculations for complex dives IF I needed to. But this is the basic scuba section and I when I post I am just stating my opinion as applied to basic recreational diving. If someone wants to measure their SAC and calculate consumption before a dive then I have no issue with that. Knowing your SAC is a bit like knowing your cars typical gasoline consumption. Interesting up to a point but of little value if you can't complete your journey because your tyre blows out and you forgot to check the spare.

I'm very impressed that with all that background you provided max depth in your previous post when max depth has nothing to do with gas consumption.

210 bar air on entry for all dives. 32.8m and came out after 57 minutes with 65 bar. 41.9 m and came out after 49 minutes with 50 bar. This dive included 3 minutes deco. 20.9m, 45 minutes came out with 80 bar

Also, I find it disappointing that you are doing deco dives without knowing your gas consumptions rates or apparently planning your dives. Good luck with that. But mostly I find it really disappointing -- perhaps even dangerous-- that you post in the Basic forum that one does not need to know one's gas consumption or do any planning based on it to do deco dives.

 

[60plus]

I'm very impressed that with all that background you provided max depth in your previous post when max depth has nothing to do with gas consumption.

Also, I find it disappointing that you are doing deco dives without knowing your gas consumptions rates or apparently planning your dives. Good luck with that. But mostly I find it really disappointing -- perhaps even dangerous-- that you post in the Basic forum that one does not need to know one's gas consumption or do any planning based on it to do deco dives.

I posted my employment experience to show that it is probable I can do SAC maths and am familiar with gas consumption rates. I know that it is average depths that influence gas consumption but I posted the deepest point of the dive to show the type of dives I was doing. I do not intend to post "legal language" types of post to cover each and every eventuality. I did not say that you do not need to know your gas consumption, I posted that you do not need to know your SAC. Dive planning - simple example. Shore entry follow bottom due east until get to 20m. Turn left and follow bottom at 20m until first person reaches 120 bar, turn left and follow bottom to 12m, turn left and follow bottom at 10 to 12m until reach entry descent areaor start ascent when first person reaches 65 bar.

How much gas do you need to make a normal ascent with a safety stop from 130 ft/40 m? Are you gas or NDL limited on a dive to 95 ft/30 m on 32% with the tank you have

Again I could work out exact theoretical values and from 130 feet the answer is probably in the region of 120 to 130 bar but what would limit that dive in practice would be the amount by which I exceed my NDL and the proposed route back to the boat or shore. I know what I am going to say next is not regarded as safe but I would be concerned if the ascent had not started by 80 bar. I would be drawing other divers attention to my SPG and giving the thumbs up.
In answer to the second question on the first dive of the day I would probably be gas limited, on the second I would probably be watching my NDL but it depends on the dive profiles.
The use of a scooter would have a significant effect on WOB and gas consumption, as does current or stress and I believe that knowing this type of thing and how to change the dive to account for it is more important than knowing your SAC. My actual air consumption is usually higher on the first dive or two of a dive trip / holiday. Tomorrow will be my first drysuit dive since last September I know its a fair bet that my gas consumption will be a bit worse than when get used to the drysuit again.

 

[tursiops]

I posted my employment experience to show that it is probable I can do SAC maths and am familiar with gas consumption rates. I know that it is average depths that influence gas consumption but I posted the deepest point of the dive to show the type of dives I was doing. I do not intend to post "legal language" types of post to cover each and every eventuality. I did not say that you do not need to know your gas consumption, I posted that you do not need to know your SAC. Dive planning - simple example. Shore entry follow bottom due east until get to 20m. Turn left and follow bottom at 20m until first person reaches 120 bar, turn left and follow bottom to 12m, turn left and follow bottom at 10 to 12m until reach entry descent areaor start ascent when first person reaches 65 bar.


Again I could work out exact theoretical values and from 130 feet the answer is probably in the region of 120 to 130 bar but what would limit that dive in practice would be the amount by which I exceed my NDL and the proposed route back to the boat or shore. I know what I am going to say next is not regarded as safe but I would be concerned if the ascent had not started by 80 bar. I would be drawing other divers attention to my SPG and giving the thumbs up.
In answer to the second question on the first dive of the day I would probably be gas limited, on the second I would probably be watching my NDL but it depends on the dive profiles.
The use of a scooter would have a significant effect on WOB and gas consumption, as does current or stress and I believe that knowing this type of thing and how to change the dive to account for it is more important than knowing your SAC. My actual air consumption is usually higher on the first dive or two of a dive trip / holiday. Tomorrow will be my first drysuit dive since last September I know its a fair bet that my gas consumption will be a bit worse than when get used to the drysuit again.

Is your deco planned?

 

[60plus]

I would say anticipated and accounted for rather than planned. On the deco dive I gave in an earlier post it was expected. To some extent the amount of deco would depend on how fast we found what we were looking for and the how conservative our computer settings were. Rather than following the sea bed down from the shore we did a surface swim, then stopped until breathing rate had dropped. We then descended to about 6m and swam some more distance until we felt we we about the top of the wall. We did not follow sea bed down to minimise time at depth and nitrogen take up. When we descended and were about 30m short of the wall, at this point we would be at about 25 / 30m. Then dropped over wall to go to just over 42 / 44 m. It had been agreed that if we wanted to follow the seabed back to the shore max deco would be 5 minutes. If we wanted to spend more time at 42 / 44 m because there was something unexpected and interesting max deco could be extended to 8 minutes but we would have to ascend "vertically" with long swim back at 5 m and surface. Just had a look at log on my computer and we were at deepest between 11/12 to 17/18 minutes, by 30 minutes after steady ascent we were up to 6.4 m and spent rest of dive at about that. I cannot remember at what time the deco obligation dropped to 0 on my computer but it was not that long after ascent started.

 

[scubadada]

Hi @60plus

Resistance is futile, you will be assimilated.

 

[tursiops]

I would say anticipated and accounted for rather than planned. On the deco dive I gave in an earlier post it was expected. To some extent the amount of deco would depend on how fast we found what we were looking for and the how conservative our computer settings were. Rather than following the sea bed down from the shore we did a surface swim, then stopped until breathing rate had dropped. We then descended to about 6m and swam some more distance until we felt we we about the top of the wall. We did not follow sea bed down to minimise time at depth and nitrogen take up. When we descended and were about 30m short of the wall, at this point we would be at about 25 / 30m. Then dropped over wall to go to just over 42 / 44 m. It had been agreed that if we wanted to follow the seabed back to the shore max deco would be 5 minutes. If we wanted to spend more time at 42 / 44 m because there was something unexpected and interesting max deco could be extended to 8 minutes but we would have to ascend "vertically" with long swim back at 5 m and surface. Just had a look at log on my computer and we were at deepest between 11/12 to 17/18 minutes, by 30 minutes after steady ascent we were up to 6.4 m and spent rest of dive at about that. I cannot remember at what time the deco obligation dropped to 0 on my computer but it was not that long after ascent started.

The obvious question is, how did you know that you would have enough gas to do this dive, which you had never done before, and which might involve an unknown amount of deco?

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A ScubaBoard Staff Message...

Due to the slight complexity of the mathematics involved this thread has been moved to Advanced.

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[60plus]

The obvious question is, how did you know that you would have enough gas to do this dive, which you had never done before, and which might involve an unknown amount of deco?

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A ScubaBoard Staff Message...

Due to the slight complexity of the mathematics involved this thread has been moved to Advanced.

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The amount of deco was not unknown, we had decided its limits. Although I have not dived to the exact location before I have been diving this area since 2018 including a night dive to 38.5m where I was right on my ndl for quite some time so I am familiar with the gas consumption on the different approaches and exits to this region of sea bed. I just watch my computer and spg and if either gas or ndl looks like it may get outside what I am prepared to accept I amend or call the dive. I would have though that it is common knowledge among experience divers that so long as you do not spend too much time of decent you can spend a few minutes at 40 to 45 m on a 15L tank of air. I know of divers who even exceed 200 ft / 60m for a few minutes on 15 L tanks or air, but that really is pushing it. At the the other extreme, I was in a dive group today where one of the group (very skilled and experienced OC and CCR diver) who will not exceed 35 m unless on trimix because of concerns over WOB and possible resultant IPO.

 

[rsingler]

The amount of deco was not unknown, we had decided its limits. Although I have not dived to the exact location before I have been diving this area since 2018 including a night dive to 38.5m where I was right on my ndl for quite some time so I am familiar with the gas consumption on the different approaches and exits...

Are you really defending this practice???
Are you actually saying you never ever gas planned even one dive like this (because you don't know your RMV) and are just stair-stepping from prior successes? That is pretty close to the definition of Normalization of Deviance, IMO.
I'm really glad this thread was moved to Advanced. New divers shouldn't see this nonsense (not that they can't anyway).

 

[tursiops]

Whatever. I give up, Your ability to not answer a question far exceeds my interest in the answer. Now that the thread has been moved to Advanced, I no longer worry that some new diver will follow your dangerous and seat-of-the-pants procedures.