PADI Gas Matching

[inquis]

The NACD method

Thanks for that. That's another group that uses what I initially termed the NSS-CDS approach. Sounds like TDI and NAUI are in the same boat.

 

[inquis]

I teach gas matching using the lowest volume cylinders and/or highest SAC rate. You can't assume one or the other as a hard and fast rule unless you are also controlling what cylinders the divers are using.

I must be misunderstanding you. You don't have to control what cylinders the divers are using. They use what the have. They also have their own SAC, and when someone uses up the penetration volume (same for everyone), everyone turns around. That someone WILL be the one with worst SAC.

Edit: I see you said one OR the other, so yes I think I see what you mean. The lowest volume AND highest SAC both play a role.

 

[Wibble]

With the NSS-CDS approach, I don't necessarily need to do their SAC on my tank and the other way round and then compare. The limiting case for time estimation is always the worst SAC on the smallest tank (regardless of who either belongs to. That's one of the reasons I like it -- it's easier.

ETA: Of course, everyone needs their own turn pressure, but individual SAC rates need not figure into that.

Say you’re diving with Mr G Guzzler and you’re frugal with gas, for the purposes of discussion, say you’re half of Mr Guzzler. Say also that his cylinders are twice the capacity of yours.

Should the excrement hit the propeller at the worst time with Mr Guzzler's gas lost, would there be enough reserve gas in your tank to get you both out?


Of course reality would never be this bad. But the planning requires this calculation just in case.

 

[inquis]

Should the excrement hit the propeller at the worst time with Mr Guzzler's gas lost, would there be enough reserve gas in your tank to get you both out?

Yes there would, plenty with both PADI and NSS-CDS approaches. In fact, we would exit with 33 cuft still in my tank (the smaller of the two). Details from the OP:

What about when the hoover has more air to start? Leave the SAC rates at 0.5 and 1.0 cuft/min, but swap the volumes to 99 and 198 cuft for Diver 1 and 2.

• Under PADI, Diver 1 hits turn pressure first after (99-66)/2/0.5 = 33 minutes and consuming 16.5 cuft. When the larger tank then dies, they end with 99-16.5-16.5-33 = 33 cuft.
• Under NSS-CDS, Diver 2 hits turn pressure first after 33 minutes and Diver 1 has consumed 16.5 cuft. When the larger tank then dies, they end with 99-16.5-16.5-33 = 33 cuft.

 

[lermontov]

this is part of a presentation i made up a while back - it may help

 

[inquis]

this is part of a presentation i made up a while back

That is a great visual breakdown of what I called the NSS-CDS approach.

 

[kensuf]

The NACD method (c2005) is to distinguish between the diver with the lesser gas volume and the diver with the largest SAC. the former controls the thirds computation; the latter controls the time allowed. So you get cuft from the smaller tank(s), and minutes (of penetration) from the worst SAC. The "turn time" is then calculated in terms of tank pressures for each diver.

The problem with "use the person with the worst SAC rate" method is it doesn't account for a situation where "the person with the best SAC rate" is struggling and winds up actually having the worst SAC rate.

Gas matching should be based on volume and use the guy with the smallest volume of gas. Even if everyone is diving the same size tanks, if there's different starting pressures, you have to match.

 

[Wibble]

Shouldn't the gas guzzler be forced to bring his/hers/it's own redundant gas -- perish the thought, a pony?

 

[inquis]

The NACD method (c2005) is to distinguish between the diver with the lesser gas volume and the diver with the largest SAC. the former controls the thirds computation; the latter controls the time allowed.

The problem with "use the person with the worst SAC rate" method is it doesn't account for a situation where [best SAC struggles]. [...] Gas matching should be based on volume and use the guy with the smallest volume of gas.

@kensuf, I agree the part of your post I bolded is not a viable method, but it's not what @tursiops (or I) was advocating. Penetration volume/turn pressure is based solely on volume. The gas is matched at that point without regard to anyone's SAC. The pre-dive time estimate (based on the worst estimated SAC) is just that, an estimate. If anyone's SAC turns out to be higher than that used for the time estimate, the turn simply comes sooner than planned with correspondingly less penetration.

 

[inquis]

Shouldn't the gas guzzler be forced to bring his/hers/it's own redundant gas

They do have redundant gas. Using a buddy's reserve is a backup to the backup (mandated by the severity of consequence).