How to calculate Minimum Gas / Rock Bottom / Reserve Gas
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- เผยแพร่เมื่อ 17 พ.ค. 2022
- Whatever you want to call it - Minimum Gas, Rock Bottom, Reserve - does not matter - but it matters a lot how you calculate and what conceot you follow. let´s have a look.
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“Rock Bottom” is all about asking a simple question... “When do I need to leave the bottom?” That’s it. That’s exactly what it is. If the worst case scenario happens at the deepest part of the dive, ask yourself, exactly how much gas will it cost ME to get 2 divers (myself and my out-of-gas buddy), safely to the surface, including all deco stops/safety stop, along the way. If diving in rec, then that would be to the surface. If diving in tec, that would be to the first staged cyl. If diving on CC, that would be 1/3 B/O till your buddy switches to your B/O, and repeat rotation. That’s Rock Bottom gas management planning in a nutshell. You take the one person on the team that has the highest SAC rate, and calculate their ascent/exit needs. That’s it. You don’t need “everyone’s” SAC, just the air hog’s in the group. So let’s do a simple rec example here: Let’s say you do a 30m (100’) dive. Let’s also say your buddy had a SAC of approximately 15l/min, you’re a little less, and you’re both using 10L (80cuft) tanks. If the air hog of the group ran out at 30m, how much gas do YOU need to get both you and them, breathing off of YOUR tank, safely to the surface, going no faster than the maximum safe ascent rate of 9m/min (30’/min), and completing your safety stop at 5m or 6m, along the way? I’ll spare you the math, because that would make this seriously long, but in THIS example, the short answer is, you need 2.3bar per meter of depth (or 10psi per foot). This would mean... If diving at 30m (100’), and your buddy either ran out of air or they lost all their air, you’re still within your NDL (diving rec in this example), and now you’re sharing air, you’d need appropriately 70bar (rounding up here) (1,000psi), to safely get you both back to the surface, assuming you travel no faster than 9m/min (30’/min), and you both complete your safety stop (which if you’re in your NDL, really is not necessary, but that’s a whole different topic), and it will take you both about 6 minutes to reach the surface (for a 3min stop), or 8 minutes (for a 5min stop). Therefore, according to this example’s dive plan, when you both hit your “Rock Bottom” gas pressure (70bar / 1,000psi), THAT is when it’s time to start coming up. THAT is what Rock Bottom is.
Out in Vis, Tek diving with Manta, and noticed a large inners pace explorers sticker on the back of a car on a Belgian plate. Hi Good buddy. Vis is simply awesome😎😎😎
much better changing intro music😄😄
Thank for explanation! It’s ever good reminder the procedure
Thanks for explanation, it is good as a reminder of the procedure. Could you add the multi tank scenario, where the reserve has to be in there in all cylinders used to the surface (though I know it is a bit more complex)?
Thanks for the video Achim
Nice calculations in theory. In reality one would take the out of gas diver to 5 metres and check SPG.
I learn from the best I learn from the worst. Your friend: Scuba Client.
Nice explanation 🥰
Looking well Achim, hope you're doing well.
Awesome to see how ISE does this and your absolutely right. But I wonder if many people make mistakes in these calculations. This way seems overly complicated.
@Achim, I have one minor quibble with this calculation. I was right with you up to the 3m mark, where you stated the 3min hang. So far, so good. But you didn't then add the extra minute to get you from 3m to the surface with your ascent rate of 3m/Min. So total surface time would be 12min, and not 11min. At least that extra minute would be in my calculations... (As after a 3min hang you aren't instantly at the surface.)
Theoretically yes, but due to the already mentioned fact that the average of 15 m is super conservative as you spend way more time above 15 then below I do not count that in. Keep dit simple and in a fashion where students can remember and use it:
Up to 50% with 10/min, then 3/min, and ISE min deco. Done
I have been trying to leave the imperial world and take my dive buddies with. I'm getting there. I just keep going through metric examples to solidify it, thanks. I have always just used 10ft/min (3m/min) ascent time from the bottom for min gas even though the ascent curve will be different. Both being 10min+1. That seems to work for all no deco ascents and is quick to calculate. Do you see holes in that?
You take different AMVs in the same dive. For the ascent you say 20l/min and corrected for stress this is 30l/min.
But when chilling at 30m it’s 15l/min. That in itself I find a bit weird. Did the diver change? 🤪
Also at 30m or at 15m I would say the consumption is different and it would be way more at 30m given the pressure difference? (4bar at 30m so that is 2 times more consumption then at 10m, right? What am I missing here?) I tend to say the dive is not possible with a 15l tank given the constraints set. Pls correct me if I’m wrong and thanks for all your content 👍
OK, if you listen carefully, I explain that 20 is a average and widely acknowledged as RMV. Times 1.5 in Stress makes sense, at least if you ever experienced it. So, for the critical part of the planning these are pretty conservative numbers. On the part of the dive that I can control, so the uncritical part I said clearly that I, personally, do not need more then 15, so I use that. I also say that you should use whatever fits YOU.
For the calculation part, it's all there, liters available divided by pressure and RMV, I don't know why you missed that. 🤨🤔
He's just playing it safe and asking you to play it safe with rock bottom. For planning phase you can plug your own number if you know it, and obviously if you start nearing rock bottom, its time to ascend.
You don't know what your out of air buddy is going to consume so use conservative numbers for the unknown. Plan for the worst. Hope for the best.
Good morning. I would like to read your book but haven’t found it here in the U.K. can you provide the tittle please ?
There is no English version. I actually deliver it as an English audio book on my patreon channel.
Would this calculation change if you dive in a team of three, i.e you have more reserve gas on the third diver?
nope. The odds of two divers in a team of three needing to gas share is very very unlikely.
I'd rather plan for only 2 and then have extra from the third diver (or not need them). Planning for 3 cuts your min-gas in half, but that's not a huge increase in absolute bottom time. Probably should be the team's call as to whether those few minutes are worth the risk.
@@pbillings808 that's pretty excessive.
The translation for AMV you're looking for is SAC rate I guess
Is it a big problem to ascend from 30m ? With a minimal stop at 6m... ?
Depends how long you have been down there, how fast you ascent, how much gas you have, how your body shape is, how old you are, how hydrated you are, how... ah, me again, sorry, just ascent, will work somehow ;-)
@@CoastalDevelopment take cf40 as stage, and relax of diving )
Have you ever had any conversation with Dick Rutkowski on gas ?
Yes 😅
@@CoastalDevelopment I did some training under him back in the early 2000's. Unfortunately when Divers Direct bought out Ocean Divers, they kicked him out of his place. All his training and historical stuff was gone. Very sad to treat the father of nitrox that way.
cries in Imperial 😢
I think you're extremely conservative; out of those 11 minutes needed for the ascent, the vast majority is spend at depth less then 15 meters, so the average must be less then 15 meters.
Besides the bottom gas and gas for an ascent while sharing gas, you should probably take into account the gas needed to go down;. You could get a little more margin if you ask for a "cave fill" (where they fill slowly, let the tank cool down, and top it up to 220 or even 230 bar).
Personally, I, and pretty much all of my regular dive buddies, dive with double-12 setups, which easily allows for 30 minutes at 30 meters (no deco limit for Nitrox), even if your SAC rate is higher.
so what would be your calculation? so we can discuss it in details and Achim can answer what he'd do differently...
Yes, it is conservative and yes, you could set the average at a more shallow depth. Where? 50% is simple.
Gas to go down is obviously included
Tank size and fill pressure has nothing to do with the calculation, it just changes the numbers. So the Calc is always the same, no matter if you take a Single 5liter with 20 bar or a double 20 with 300 bar :)
I don’t understand how Nitrox 32 had anything to do with the calculation? 😊
It doesn't. It's just the best gas for a no deco dive to 30 meters. Did I say it has something to do with it?
Could have done it on air for 20 but good practice is not to be in deco at 21 minutes when there's an emergency.
Mumbling