PADI Gas Matching

[inquis]

The section about Gas Matching in the PADI Tec Deep training states one should use the larger gas volume to control the reserve amount. (In the text they also assume the diver with higher SAC rate uses the larger tank, a point I'll return to at the end.) Specifically, it states:
Take the volume of the larger gas supply and divide by three. This is the amount that must be reserved by the diver with the smaller supply.
Other agencies (e.g., NSS-CDS, based on various other posts, including this one from @Capt Jim Wyatt ) hold that one should use the smaller gas volume to control penetration (and indirectly the reserve).

Consider two divers with equal SAC rates of 1.0 cuft/min and volumes of V1=198 cuft and V2=99 cuft for simplicity.

Under PADI:
Reserve = max(V1, V2) / 3 = 198/3 = 66 cuft
Available Penetration Vol = Non Reserve Gas / 2 = (Vi - Reserve) / 2 = [(198-66)/2 (99-66)/2] = [66 16] cuft for [Diver1 and Diver2]
Unsurprisingly, Diver 2 reaches turn pressure first after 16 minutes, both turn around, and then Murphy strikes at the worst time and Diver 1 loses their gas. (Diver 2 losing their gas isn't very interesting since the larger tank is working.) On that return, they each consume 16 cuft off Diver 2's tank.
They end the dive with 99-16-16-16 = 51 cuft in the working tank.
(Same answer for a better SAC rate, BTW.)

So a worst-case out of air share returns with more than half the initial volume of gas. Doesn't that seem... excessive? Yes, I know consumption will likely change on the exit, no dilly-dallying, probably exiting with flow, etc., but ignore all that and compare apples to apples with what seems to be the more widely used approach.

Under NSS-CDS:
Penetration Vol = min(V1, V2) / 3 = 33 cuft
Both divers hit turn pressure after 33 minutes when disaster strikes. They each consume 33 cuft during the exit
They end with 99-33-33-33 = 0 cuft.
(Thus the oft-cited rational for Thirds being an extremely aggressive penetration strategy.)

What about when SAC isn't equal? Consider when Diver 1 has 0.5 cuft/min SAC, and Diver 2 remains at 1.0 cuft/min and still hits turn pressure first (smaller tank/higher SAC).

• Under PADI, they exit with 99-16-16-8.5 = 59 cuft.
• Under NSS-CDS, they exit with 99-33-33-16.5 = 16 cuft. (Yes, it pays to dive with calmer buddies... if you can talk them into shorter bottom times!)

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.

On the surface, PADI's "larger volume dictates reserve" (implying a smaller penetration volume) and NSS-CDS's "smaller volume dictates penetration" might seem like equivalent means to the same end. Clearly, the ends are not the same except for very specific cases.

So which approach do you use? Or perhaps something different than these two?
What agency taught it to you?
Do you even worry about Gas Matching?
Do you think PADI overdoes the conservatism? Please, I don't wish to get into PADI bashing in general -- focus on the Gas Matching topic.
Any PADI folks have any insight into why they chose that approach?

I trained with PADI but will be switching to what seems to be the standard -- and easier -- method for these computations. If I want more conservatism, especially when buddy's SAC is comparable to mine, I'll arrange to return with current/flow or only penetrate to 4ths (or less).

 

[Jim Lapenta]

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. If we had the option to say "Fred, you suck big time and those 72s aren't just going to work for this dive. Here, use these 95s."
Even then you still have to do the math based on RMV.
It is unique for every diver. If you have a regular team and you know the consumption of every team member and the cylinders they use, you still should be doing the math.
I devoted an entire chapter in my second book to gas management and included tables for some popular cylinder sizes that show volume of those cylinders down to 250 PSI.
Using that along with individual diver SAC rates it makes it a little easier to do the matching but I didn't cover every cylinder combination. I did include information and how to calculate that in an emergency the divers SAC rates are going to be elevated. Perhaps doubled or even tripled.
I'm a TDI instructor but I learned gas matching in my NAUI Tech Intro and Helitrox classes 14 years ago.
You can't use the larger volume as a hard and fast rule because what if the larger volume guy is the one who loses his supply and has the highest SAC rate? You've planned on his reserve and set the depth and duration on that. Now you have two divers that are screwed because the one with the smaller cylinders is now the supply for both. And the SAC rate for both divers is now doubled or possibly tripled.
You can't assume SAC will stay the same. Even going with the flow, in an emergency, it's likely the SAC of both divers will be elevated.
Just as an FYI, I also teach gas matching in my Basic Open Water class because you can't assume all divers will be using al80s. I had a mom and 11 yr old daughter this summer. Daughter used al63s because the 80s were just too big for her and our local spots all have 63s available.
Even at that mom was more nervous in the water (just in general and because she was mom with an 11 yr old she was trying to look out for) and not as efficient and as a result her SAC rate was higher. While gas matching was not strictly required for the shallow (less than 30ft) dives they were doing, it did have a very practical application for how much to budget for fills and how many times they'd need to lug tanks over to the fill station when we were figuring out times for dives and surface intervals to allow for fills to be done.
Mom was getting one 45 minute dive out of an 80 and coming up with 700 PSI. Daughter was getting two, end of first dive still had 1400-1500 PSI and still coming up after the second with 300-500 in the 63. Daughter could still do her second on the same cylinder. Mom needed a fill for the second dive. Even though mom was carrying a large cylinder, her SAC rate determined the needs more than the cylinder size.

 

[PfcAJ]

Use the same sized tanks.

Problem solved.

 

[tursiops]

The PADI approach is predicated on the hoover having the larger tanks, which is then your final example.

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.

So you think of it as a "very specific case." PADI thinks of it as the most likely case.

 

[Wibble]

What is the depth for "PADI deep tec" training? Is it beyond Trimix (60m)? Or is it an intro level?

Asking that as the experienced people who've moved through their technical levels on open circuit are unlikely to be gas hogs. Those would have been weeded out through the cost of the helium and this problem of gas sharing.

Fact is that if you're diving with others in a team, then you need to do your calculations on your gas for their SAC/RMV. Same the other way round.

 

[tursiops]

What is the depth for "PADI deep tec" training? Is it beyond Trimix (60m)? Or is it an intro level?

PADI Tec trains from air at 40m to CCR at 100m. The original course was air-based for 40-50-55m, rather like the TDI course through Extended Range. Then Trimix 65m came in, as a step toward full Trimix 90m.

 

[inquis]

you need to do your calculations on your gas for their SAC/RMV. Same the other way round.

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.

 

[inquis]

PADI thinks of it as the most likely case.

I would guess the NSS-CDS approach was around first (though it probably wasn't called that). If that's true, I find it strange that would PADI depart from what was already being done, in order to dream up something that was worse for those less likely cases.

 

[tursiops]

I would guess the NSS-CDS approach was around first (though it probably wasn't called that). If that's true, I find it strange that would PADI depart from what was already being done, in order to dream up something that was worse for those less likely cases.

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.

 

[inquis]

You can't use the larger volume as a hard and fast rule because what if the larger volume guy is the one who loses his supply and has the highest SAC rate?

I did that very example above.
PADI's approach (based on the larger volume) works fine, and in fact gives the same answer as the NSS-CDS (traditional?) calc. No one is immediately screwed (variable SAC rates not withstanding).