High SAC rate and low SAC rate

It's threads like this that make me realize why my "twice the gas required to exit" rule will never catch on... too much math :)

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Originally Posted by aainslie

It's threads like this that make me realize why my "twice the gas required to exit" rule will never catch on... too much math :)

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Lol!!! That would be really difficult! Lol


It's thread like this that make me realize people make things harder then they need to be.

Dive sidemount, can't have a total ooa from a single incident the odds are essentially the same as two bm divers both busting their manifolds.

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Sidemounting DOES make the odds against total system failure very high. But... It also increases the chances of 50% gas loss (as opposed to manifolded doubles).

So a tank failure near turn point can put you very critical on gas in side mount. Any delays (siltout line following,or just increase in SAC from loading your shorts with bricks) will put you on your partner's gas supply before you get back to your deco tank.

I side mount, and am very religious about tank matching. Even thought my side mount parter and I are diving with a total of 4 completely redundant systems.

Any body have any stats on the ACTUAL rate of sidemount system failures? Or incidents of DOUBLE system failures in a side mount team? (leaving no redundancy, and each diver exiting on a single tank?)

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Originally Posted by gschaut

Sidemounting DOES make the odds against total system failure very high. But... It also increases the chances of 50% gas loss (as opposed to manifolded doubles).

So a tank failure near turn point can put you very critical on gas in side mount. Any delays (siltout line following,or just increase in SAC from loading your shorts with bricks) will put you on your partner's gas supply before you get back to your deco tank.

I side mount, and am very religious about tank matching. Even thought my side mount parter and I are diving with a total of 4 completely redundant systems.

Any body have any stats on the ACTUAL rate of sidemount system failures? Or incidents of DOUBLE system failures in a side mount team? (leaving no redundancy, and each diver exiting on a single tank?)

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Sicne we're beating dead horses here... it probably doesn't. The tank is in front of you. Way easier to fix problems like leaking 1st stages etc that are difficult to fix when the tank is behind you.

WHACK! That poor horse. WHACK!

It is much easier to feather a valve in sidemount than backmount.

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Originally Posted by DA Aquamaster

I agree. People get wrapped up in too many digits when we are dealing with calculations that are only good to 2 significant figures and then they apply them to pressure gauges that are at best only accurate to +/- 50 psi and many are closer to +/-100 psi.

More often than not if I have LP 95s (tank factor of 7.2) and a dive buddy shows up with X7-100s (TF of 5.8) and everyone on the team is either filled to 3600 psi or is rounding down to that pressure, with 1,200 psi "thirds", I'll:

take the small tank's TF and round it to the nearest number (6 in this case) and then take it times the psi in hundreds of the desired thirds (12 in this case). Most people can do 6x12= 72 in their heads in either 1 or 2 steps. [6x12=72 directly, or (6x10=60)+(6x2=12)=72]. I'd then take the 72 and divide it by the larger tank factor (rounded down to 7 in this case). 72/7 gives you 10 and some change, so figure round that down to 10 and figure a 1,000 psi "third" for the LP 95 to match the 1,200 psi third in the X7-100.

If you do the math exactly, you get 966 psi rather than 1,000 psi, but that's within the range of gauge error anyway.

In any event, we'll usually back off another 100 psi or 200 psi from true thirds anyway, as thirds is a little aggressive in a low flow cave, especially with potential gauge errors. So we'll end up with 11x6= 66, 66/7=9 and change, and call it a 900 psi third in the larger LP 95s, or 10x6=60, 60/7=8 and call it an 800 psi third in the larger tanks with the more conservative 1,100 psi or 1,000 psi "third" used in the smaller tank to increase everyone's gas at the turn by 200-400 psi.

But then, I'm never bothered by having extra gas at the end of the dive.

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Here's an easier method that doesn't require a calculator. Keep in mind this only works if you are not diving full 1/3s, which I never do and do not allow my students to do. My general rule is penetration will not be more than 100 psi less than 1/3s and usually 200-300 psi less than 1/3s. So with 3600 psi penetration is anywhere from 900 psi to 1100 psi regardless of cylinder. Whether I choose to cut back by 100 psi or 300 psi depends on a few factors, which I won't get into here. The point is to always build in a buffer.

Once that is established then volume matching can be done. The diver with the smaller tanks gets to dive the penetration pressure decided above. The larger tanks get volume matched based on the rule of 10.

Rule of 10 - subtract the volume rating of the smaller tank from the larger tank and multiply by 10. Subtract this number from the turn pressure of the smaller tanks. This is the turn pressure of the diver with the larger tanks.

Example - 85s and 104s. Both start with 3600 psi. 85s will penetrate to 1000 psi. 104 - 85 = 19. 19 x 10 = 190. 1000 psi - 190 psi = 810 psi. Penetration pressure for the 104s is 800 psi.

So the 85s turn at 2600 psi and the 104s turn at 2800 psi.

This only works LP to LP or HP to HP. If you have someone diving LP 85s and someone diving HP 130s then you need to convert one set of tanks to the other. HP 130s are equivalent to LP 95s in volume so that is the easiest conversion and the HP 130s would penetration 100 psi less than the 85s (95 - 85 = 10, 10 x 10 = 100).

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Originally Posted by RN

HP 130s are equivalent to LP 95s

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Dude.

There was never an HP-130. There is an E8-130, but it's the same size as an LP-104. (It's an E8-119 that's the same as an LP-95.)

Dear Dude,

Russell still gets disturbed if the Sherwood Genesis trademarked "HP" name for their long since out of production 3,500 psi special permit tanks gets used to refer to any of the newer 3442 psi special permit tanks.

The reality however is that "HP" has become a generic term meaning any scuba tank in the 3442-3500 psi range. If Genesis were ever to take the issue to court the courts would most likely agree that it is now no longer a trademarked term due to the common use of the term to refer to an entire class of tanks rather than a specific product. Kleenex for example, always makes a note to refer to their tissues as "Kleenex brand tissues" rather than just "kleenex" to avoid kleenex becoming more of a common term for tissues and thus loose their trademark protection.

It's also worth noting here that the DOT considers all scuba tanks to be "high pressure" tanks as they hold over 800 psi, so Genesis using the "HP" name in the first place was pretty stupid as it was incorrect to expect it to be limited to only 3500 psi tanks, especially when 2400 psi tanks are also incorrectly called "LP" tanks.

In any event Sherwood is not complaining and I suspect Russell will eventually just adjust to the new reality. :D

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I agree with Russell though that the Worthington LP 95 has dimensions that are almost identical to the X8-119 (which actually holds 123 cu ft) while the X8-130 is 2" longer and 2 pounds heavier. The interior volume of the tanks are also very close with the worthington LP 95 holding .0353 cu ft per psi, the Worthington X8-119 holding .0357 cu ft per psi and the X8-130 holding a whopping .0382 cu ft per psi. So the tank factors are 3.5, 3.6 and 3.8 respectively. As an aside, the Faber LP 95 holds 95.1 cu ft at 2,640psi, or .0360 cu ft per psi, so it's an even closer match in terms of tank factors.

Russell is slightly less right about the LP 104 as it holds .0393 cu ft per psi and the LP 108 holds .0411 cu ft per psi, so they have TFs of 3.9 and 4.1 respectively - a bit higher than the X8-130.

When filled to 3600 psi, the Worthington LP 95 holds 126 cu ft and the Faber LP 95 holds 129.6 cu ft. I've noted however that if a shop fills LP tanks to 3600, the 3442 tanks end up closer to 3500 and if they put 3600 in the 3,442 psi tanks, the LPs seem to be at 3,800. Which is to say most shops won't overfill a 3442 psi tank by much at all and that a Faber LP 95 will hold it's own fill for fill with an X8-130 and the X8-119 will come up a few cubic feet short in comparison.

So in my experience in cave country, the cave filled Faber LP 95 and the X8-130 are for all practical purposes going to have the same volume of gas on the dive - which was Rob's point.

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Originally Posted by RN

Here's an easier method that doesn't require a calculator. Keep in mind this only works if you are not diving full 1/3s, which I never do and do not allow my students to do. My general rule is penetration will not be more than 100 psi less than 1/3s and usually 200-300 psi less than 1/3s. So with 3600 psi penetration is anywhere from 900 psi to 1100 psi regardless of cylinder. Whether I choose to cut back by 100 psi or 300 psi depends on a few factors, which I won't get into here. The point is to always build in a buffer.

Once that is established then volume matching can be done. The diver with the smaller tanks gets to dive the penetration pressure decided above. The larger tanks get volume matched based on the rule of 10.

Rule of 10 - subtract the volume rating of the smaller tank from the larger tank and multiply by 10. Subtract this number from the turn pressure of the smaller tanks. This is the turn pressure of the diver with the larger tanks.

Example - 85s and 104s. Both start with 3600 psi. 85s will penetrate to 1000 psi. 104 - 85 = 19. 19 x 10 = 190. 1000 psi - 190 psi = 810 psi. Penetration pressure for the 104s is 800 psi.

So the 85s turn at 2600 psi and the 104s turn at 2800 psi.

This only works LP to LP or HP to HP. If you have someone diving LP 85s and someone diving HP 130s then you need to convert one set of tanks to the other. HP 130s are equivalent to LP 95s in volume so that is the easiest conversion and the HP 130s would penetration 100 psi less than the 85s (95 - 85 = 10, 10 x 10 = 100).

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It's the same basic idea just a different approach. The idea in both cases is to keep the math simple enough to be done in your head and build in enough of a fudge factor to account for any rounding errors or inaccuracies.

With that in mind, I'll probably stay with rounding tank factors and backing off from true thirds, since I'm more familiar with it.

It's a great idea though and hopefully most divers can be comfortable with one or the other, or use the Mark Vlahos laminated chart method and do some form of gas matching.