RMV math…

[60plus]

Just out of curiosity the following question. You arrive at the dive site with 220 bar in a 15L cylinder. The air temp is 22 Deg C. You dive to 20m and the water temperature is 4 Deg C so the 220 bar becomes 208 bar even if no air were consumed. Does anyone allow for this 6% "loss" of gas in their calculation?

 

[MikeinAA]

Thanks. I think I'll try and write a post for Basic that gives a progression from "trust your dive guide" to "come back with 500 psi" to ..... The key will be to encourage moving from the starting blocks without terrifying someone afraid of math or numbers. The key always is to give someone something to do....rather than telling them what not to do....


I'll have to reread that thread and see if it has gone in a useful direction or is it now just omnidirectional noise...

Please do.
I am likely your target audience. Newer diver. Received SDI OW cert 2 years ago. 30 some dives total. I am wanting to get better and learn. I read scubaboard often but rarely post.

So far my planning has been, never exceed ndl, start deep / end shallow, turn at half pressure, find the exit by 1k , safety stop by 700, out of water by 500. I started thinking something was off with this planning when I used a hp120 the first time. 500 psi has a different meaning than it did with an al80.

I have read the why surface with 500 psi thread and read about rock bottom pressure. I have calculated my sac (though at this point it is all over the place). I think I understand the concepts but can’t put it into practice yet.

 

[tursiops]

Please do.
I am likely your target audience. Newer diver. Received SDI OW cert 2 years ago. 30 some dives total. I am wanting to get better and learn. I read scubaboard often but rarely post.

So far my planning has been, never exceed ndl, start deep / end shallow, turn at half pressure, find the exit by 1k , safety stop by 700, out of water by 500. I started thinking something was off with this planning when I used a hp120 the first time. 500 psi has a different meaning than it did with an al80.

I have read the why surface with 500 psi thread and read about rock bottom pressure. I have calculated my sac (though at this point it is all over the place). I think I understand the concepts but can’t put it into practice yet.

Ray Bradbury (science-fiction author) was once asked how he wrote story. He said he worked on it a but, then would read it to his target reader, as if he were sitting across the desk from him, and see what the response was.
I'm working on the post right now. I'll "read" it to you in a bit, and see what you think! You've already given me some insights...
Thanks.

 

[inquis]

Does anyone allow for this 6% "loss" of gas in their calculation?

Always. Usually this is done during the fill, but sometimes by adjusting the plan (times/depths) before the dive. Additionally, I know/follow my Minimum Gas (Rock Bottom) pressures, which are naturally aligned with the ambient temperature.

 

[rsingler]

I should be able to get from A to C on a full tank if A to C is only 1 & 1/4 the distance between A and B.

Unless A to C was straight uphill, and you don't know the change in your gas consumption compared with flat land driving, because you haven't taken that route before.

Don't make yourself look more foolish. Yeah, I haven't dive planned ANY of my Bonaire dives, because I'm just like you in that regard - I know what to expect. And yeah, a 2-minute deco stop that appears on my computer at 80 feet may not be a big deal because I know it will clear.

But other divers read your posts. Divers with less experience.

Don't suggest to other divers that they don't need to gas plan. The first dive on the Spiegel Grove where there is NO PLACE to go above 75 feet except the buoy line that may be 200 feet away will put paid to your practice.

Unless that diver knows at what pressure he has to be at that line, or doing a free ascent to the surface, hoping that the boat crew sees him before he's 2 km away. You're beating a dead horse, my friend.

 

[johndiver999]

Was Eric Clapton a Scuba diver? did he calculate gas consumption?

PS my elder sister was an avid vinyl collector in the 60s

As is now probably clear I cannot understand this obsession with SAC. To go back to the gasoline car analogy. If on a full tank I can get from A to B and have 1/2 a tank left, then surely I should be able to get from A to C on a full tank if A to C is only 1 & 1/4 the distance between A and B. I do not need to know the mpg. What is useful to know is the factors that could vary that mpg, not its absolute value.

Your argument is growing tiresome and trivial. Obviously a diver who has done several hundred (or more) similar dives with similar gear and who pays reasonable attention to his computer and gages, really doesn't need to know his SAC or make any calculations- in order to do one more similar dive!

The whole idea of gas consumption calcs is to make sure there is enough gas to get the diver and his buddy to the surface in one (or two actually) pieces. IF we already know the answer to that question from a gazillion dives then calculations are not needed.

However, we are talking about newer divers who don't do many dozens of similar dives in a relatively short period of time. They don't have the experience to make good estimates or fly by the seat of their pants. The whole endeavor is for these people. I think reasonable people understand this fact and they also probably understand that most training is pretty inadequate in this regard.

The fact that (apparently) "be back with 500 psi" is still being taught is evidence of both a lack of good training AND an acknowledgement that flying by the seat of your pants will probably be OK for most recreational dives - even though we know that it is ridiculous- the question should be: when (at what pressure/volume) do I need to head up, not how much psi should I have on the boat ladder.

The math is not that hard, but it integrates a bunch of parameters that are not constant and it also requires the generation of some assumptions, so ANY mathematical solution/answer will always be subject to criticism and will incorporate a degree of ambiguity- however this does NOT negate the value of the process.

I dive solo, often within NDL, carry enough gas in a pony to make it to the surface and have a bunch of different tanks. I have not calculated my SAC in years - but I know about when I should leave the bottom. If I have a buddy who might need some air, I will reserve a bit more.

However, if I was doing a drastically different dive, you can be sure I would do a little scribbling on the back of an envelope. 1 cuft/min, ascent rate of 60 fps, zero minutes to resolve a problem on the bottom and round max depth up to nearest atmosphere - in reality the calculations are not that complex or necessarily precise - but they are valid and worthwhile.

 

[Kendall Raine]

Don't make yourself look more foolish.

Well said, but too late.

 

[Kendall Raine]

Your argument is growing tiresome and trivial. Obviously a diver who has done several hundred (or more) similar dives with similar gear and who pays reasonable attention to his computer and gages, really doesn't need to know his SAC or make any calculations- in order to do one more similar dive!

The whole idea of gas consumption calcs is to make sure there is enough gas to get the diver and his buddy to the surface in one (or two actually) pieces. IF we already know the answer to that question from a gazillion dives then calculations are not needed.

However, we are talking about newer divers who don't do many dozens of similar dives in a relatively short period of time. They don't have the experience to make good estimates or fly by the seat of their pants. The whole endeavor is for these people. I think reasonable people understand this fact and they also probably understand that most training is pretty inadequate in this regard.

The fact that (apparently) "be back with 500 psi" is still being taught is evidence of both a lack of good training AND an acknowledgement that flying by the seat of your pants will probably be OK for most recreational dives - even though we know that it is ridiculous- the question should be: when (at what pressure/volume) do I need to head up, not how much psi should I have on the boat ladder.

The math is not that hard, but it integrates a bunch of parameters that are not constant and it also requires the generation of some assumptions, so ANY mathematical solution/answer will always be subject to criticism and will incorporate a degree of ambiguity- however this does NOT negate the value of the process.

I dive solo, often within NDL, carry enough gas in a pony to make it to the surface and have a bunch of different tanks. I have not calculated my SAC in years - but I know about when I should leave the bottom. If I have a buddy who might need some air, I will reserve a bit more.

However, if I was doing a drastically different dive, you can be sure I would do a little scribbling on the back of an envelope. 1 cuft/min, ascent rate of 60 fps, zero minutes to resolve a problem on the bottom and round max depth up to nearest atmosphere - in reality the calculations are not that complex or necessarily precise - but they are valid and worthwhile.

Well said.

You make good points, but I think the concept here goes beyond simply calculating one's consumption rate on a typical dive/or factoring adjustments for an atypical dive. That's just the arithmetic. It goes to mindset (e.g. a commitment to maintain situational awareness in every aspect of diving from planning to execution to post-dive review). That awareness is the foundation (or should be), of technical training. That doesn't mean situational awareness is not relevant in recreational diving from beginner onwards, or that technical training is necessary to achieve and maintain situational awareness. Not at all. It's just that it takes time, effort and skill to teach it/learn it/practice it and is therefore antithetical to the evolution toward faster, cheaper, simpler scuba courses. Knowing one's SAC is just part of situational awareness. A technique I use is to guess at different times during a dive what my gas pressure is before checking it. I do that every dive, whether on a shallow reef, deep wreck or in a cave. If I'm off by more than ~100 psi that means I need to concentrate more on what's going on physically. It's a habit; one that helps keep my mind "in the game" and focused on my breathing, how hard I'm working and how I'm feeling. The times I missed that target were usually times of task loading/distraction. Being off on the guess is like a warning light on the dashboard telling me to break the cycle of distraction and refocus.

That's why 60plus' remark that knowing one's SAC/RMV was "pretty pointless" attracted so much attention. It was obvious from that post, and his prosaic follow-ons, that he didn't get the connection and is too arrogant to see it when laid out by others.

 

[BlueTrin]

Unless A to C was straight uphill, and you don't know the change in your gas consumption compared with flat land driving, because you haven't taken that route before.

Don't make yourself look more foolish. Yeah, I haven't dive planned ANY of my Bonaire dives, because I'm just like you in that regard - I know what to expect. And yeah, a 2-minute deco stop that appears on my computer at 80 feet may not be a big deal because I know it will clear.

But other divers read your posts. Divers with less experience.

Don't suggest to other divers that they don't need to gas plan. The first dive on the Spiegel Grove where there is NO PLACE to go above 75 feet except the buoy line that may be 200 feet away will put paid to your practice.

Unless that diver knows at what pressure he has to be at that line, or doing a free ascent to the surface, hoping that the boat crew sees him before he's 2 km away. You're beating a dead horse, my friend.

Out of curiosity, what’s the particularity of the Spiegel Grove? Is that next to a shipping lane or in an area which has strong currents?

 

[scubadada]

...I am likely your target audience. Newer diver. Received SDI OW cert 2 years ago. 30 some dives total. I am wanting to get better and learn. I read scubaboard often but rarely post.

So far my planning has been, never exceed ndl, start deep / end shallow, turn at half pressure, find the exit by 1k , safety stop by 700, out of water by 500. I started thinking something was off with this planning when I used a hp120 the first time. 500 psi has a different meaning than it did with an al80.

I have read the why surface with 500 psi thread and read about rock bottom pressure. I have calculated my sac (though at this point it is all over the place). I think I understand the concepts but can’t put it into practice yet.

You have a very good attitude. I will offer up 2 simple examples of using your RMV (or SAC) in volume/time, cu ft/min in imperial.

We are going to do a reef drift dive in Jupiter, FL. The operator has AL80s and HP steel 100s and 120s. I've done these dives many times and tell you to expect an average depth of 65 ft and a dive time of 53 min. What cylinder will you rent to be able to complete the dives with me? Say your RMV is pretty average at 0.5 cu ft/ min. You would need just under 79 cu ft of gas to complete the dive. A HP steel 100 would be your choice and you would end the dive with about 725 psi/21 cu ft.

Say you would like to carry a pony, either for solo diving or just to have redundant gas. You want to be able to cover any recreational dive to 130 ft. A very conservative calculation would be a minute at depth to figure things out, a normal ascent and a 3 min safety stop, all at twice your normal RMV. This calculation would give you a little over 22 cu ft of gas. You would probably choose a 30 cu ft pony. If all your dives were much shallower, perhaps a 19 cu ft cylinder would suffice.

Best of luck in your diving