Calculating Search gas

[Rick Murchison]

Is it even physically possible to quadruple your breathing rate? That would be like me usually at around a .55 or a .6 going to a 2.4.

Absolutely. For example, my normal SCR is just under .5; in a little run in with a couple of big bull sharks, where I was moving as slowly and doing my damndest to remain relaxed as possible, I went through 1700 psi in a LP 85 in 16 minutes at an average depth of about 50 FSW... that's 1.37 CFM and that was without any real dragon or effort, just a mild adrenaline event.
Indeed, when heavy exercise is included, it is possible for the average human to go from a resting RMV of some 15 lpm to 150 lpm, a tenfold increase in consumption (or from about .5 to 5 CFM).
Rick

 

[UCFKnightDiver]

Your regs don't work below 200 psi...take that out of your equation

yes they do, I tried it they worked fine with no discernable breathing effort, dive rite balanced rg 2500's aka rg3000 all they way down till my guage was pretty much pegged on 0

 

[DA Aquamaster]

Your regs don't work below 200 psi...take that out of your equation

A good quality balanced regulator will suck the tank down to just about zero.

An unbalanced first and second will start to struggle once the pressure gets close to the IP - about 150 psi or so.

Where it gets dicey is the SPG. With some you will still have 100-200 psi left in the tank when the needle hits the peg, and with others zero may really be very close to zero. SPG's that read more than zero when unpressurized are easy to spot so they never leave the factory that way - but you need to check them now and then as an SPG that is 200 psi conservative on the low end of the scale may end up the other way over time.

In general an SPG is most accurate in the middle of the range but 100-200 psi errors on both ends of the range can happen. Most cave divers own several and can cross check them all at high, mid and low pressures, and then weed out the ones more than 50 psi off from the others.

But Richard is correct as unless you have not breathed a tank down to nothing and confirmed the accuracy of the SPG and the effectiveness of the reg at low pressure, you really can't rely on having that last 200 psi of gas.

yes they do, I tried it they worked fine with no discernable breathing effort, dive rite balanced rg 2500's aka rg3000 all they way down till my guage was pretty much pegged on 0

All your statement really says is that your reg breathed the tank down unitl the needle was almost on 0.

The important question is was there any pressure left in the tank? If there was still 100-200 psi in the tank, then that represents unusable gas you can't plan on.

Also, if the SPG was "almost" on zero, does that mean it reads 50-100 psi high? If so then it is telling you that you have more gas than you really do and it again represents unusable gas you cannot lan on.

Ideally the gauge reads zero when the reg stops delivering and when you take the reg off and crack the valve virtually no pressure remains.

 

[UCFKnightDiver]

this was the case, i let the gauge hit zero, and then opened the valve and there was virtually no air

 

[Rick Murchison]

As for emptying a tank, I can suck one slap dry with my 1972 Conshelf, though I start noticing increased resistance at IP, and I have to be mighty relaxed and "breathe through a straw" for that last 50 psi or so. With a Legend it'll give great performance down to the last four or five breaths, but it's still possible to breathe the tank completely dry.
Rick

 

[Lab Rat]

yes they do, I tried it they worked fine with no discernable breathing effort, dive rite balanced rg 2500's aka rg3000 all they way down till my guage was pretty much pegged on 0

Perhaps I should have said doesn't work well...It's a function of physics and not really open for a lot of debate. You'll notice that your LP hose doesn't have a great deal of internal volume...so breathing volume is made up for by having an intermediate pressure 140-150 psi (~140 in your dive rite reg) above ambient (you have to factor that in as well) at 100 ffw, that means you need roughly 60 psi for the 4 ata (45 psi gauge) and another 140 for the IP...hence my statement. (Edit: These are just rough numbers...it won't be exact but is meant to demonstrate a point.)

I am willing to bet that you breathed it down on the surface and were not struggling for breath/working hard. You can suck gas and get a little more...but that isn't really useable.

When we breath down stages at depth (saving the back-gas for reserve, something I don't routinely do), those stages become uncomfortable to breath from and get turned off by 150-200 psi.

More likely your gauge is off...at least it is off the 'right way'.

Richard

Rick, I know I have heard several folks say that, especially with older regs...one even claims that he can pull a vacuum on a reg. I suppose it's possible to get any valve to open and equalize on the surface...but this gas doesn't get supplied at a usable volume at depth and can't be counted on...long time no see by the way…hope all is well with you.

 

[DeepSeaExplorer]

Well, in the first place, I'm not going to be in that position, because I'm not going to penetrate that far. At least not without a couple of stages (which adds a level of complexity to search gas calculations not addressed here). But... if I were 800 (normal) psi from an exit, excited, with an excited OOA buddy and blown visibility, we'd better be mighty lucky and mighty calm and controlled to make it out on less than "6400 psi."
Rick

I was simply pointing out that your formula doesn't work. Multiplying by a random number like 8 means that you're turning the dive before you even hit the water.

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I have a pretty good idea how many PSI I need to exit and I usually have a stage bottle around P750 and an O2 bottle at the entrance for deco. The two extra bottles add a lot to the safety factor since I don't have to make it all the way back to the surface on back gas, but it also means I could be pretty far back in the cave. (All cave divers should place O2 on every dive.)

Me personnally, I would add a buffer to account for what I believe are the necessary factors and stay as long as possible. I have no issue violating thirds in a lost buddy situation, that's what it's for. I also don't worry that my buddy won't be able to borrow gas from me because 1) he probably has close to the same amount of gas that I do (assuming only a lost buddy situation) and 2) there are other types of assistance that s/he may need that I can help with, navigation immediately comes to mind, and 3) so what if he does need gas. If necessary, we'll skip breathe all the way to the exit or the first stage bottle, whichever comes first. If I simply exited the cave on thirds, or didn't do everything I could, in a lost buddy situation I would not feel that I had lived up to my buddy obligations, even if that means taking on some extra personal risk.

 

[The Chairman]

Is it even physically possible to quadruple your breathing rate? That would be like me usually at around a .55 or a .6 going to a 2.4.

Dude, quadruple is just the beginning.

 

[TSandM]

(All cave divers should place O2 on every dive.)

A bit of a hijack, but this seems a bit of overkill. Doing NoHoch on 6ths, with a max depth of 20 feet . . . placing O2? I don't think so. Honestly, even assuming I ever pass Cave 2, I don't think there are very many dives in MX where I would place an O2 bottle. I'll use conservative backgas management instead.

 

[Rick Murchison]

Multiplying by a random number like 8 means that you're turning the dive before you even hit the water.

No, it doesn't. You're comparing apples to oranges. The 8X number is the gas required to search for a lost buddy with blown visibility. It isn't a dive planning number. If that exceeds your gas supply then you cannot afford to search from that position under those conditions - you need to either get closer to the exit right away or go for help/more gas in lieu of a search.
Would you stick to thirds on a penetration where the vis was suddenly blown out? Or continue a penetration if your buddy had a total gas supply failure? Losing your buddy is an emergency that dictates an immediate modification and recalculation of the required gas. That may mean you can stay longer than 2/3 your original gas supply, or it may mean you need to retreat now. It depends on what you need to get out and the conditions at the time.
Oh, and 8 is not a "random" number pulled out of thin air, but rather a carefully considered multiple of required gas based on demonstrated human performance under stress and no visibility. 4X applies if the vis is halfway decent (you can see well enough to avoid any line traps.)
Rick