DPV gas planning

[PfcAJ]

On a deep one (and some shallow ones ) I wouldn't touch backgas at all. Stages only. And definitely bring a tow scooter.

[floridabob]

While we are all saying some very similar things, one of the themes I am hearing is that we would "use stages, not touch back gas". And then some throw a tow scooter in, too.

If I'm running the gas math right, that puts it some where around 1/10 gas for entrance and 1/10 for exit (with minimal overfill). Assuming single stage, double 104s. And a tow scooter. That dive would be better done swimming, if Im reading it right.

Just my thinking. If you are using a scooter, then you are either trying to extend your range or shorten the dive time. Or maybe just cause its fun! Whatever the reason, it seems reserving more gas than you would on the same dive swimming is uber-conservative. By more gas, I mean volume not 1/3rds, 1/6ths, etc.

In the stone age (only 15 years ago) my DPV class was either: 1/6ths or a tow scooter. Both were not needed. Local knowledge required, gas mgmt rules prevail and all other disclaimers.

Bottom line: I am fine with extremely conservative planning and I do not judge it--in fact, others consider me too conservative. I was once hammered due to my gas planning on a 10k dive in Manatee: "you dont need all that gas, thats too much logistics!". My comment: bring what you want but thats all you have--mine is not yours. On a dive, not much kills you faster than breathing water.

bob

[PfcAJ]

The thing about reserving a large amount of reserve gas on deep dives isn't about thirds or sixths or some fraction of choice. Its about time. Time to deal with issues at depth. Keeping backgas intact helps to ensure that you've got the bulk of your gas with you at all times, and its in a sharable and isolatable format.

Fwiw, a double stage (al80s) dive with 104s to 3600, using just stages is roughly equal to 1/6th.

You can't swim out of a deep thing. Hence the need for a tow scooter.

[patpicos]

@floridabob, care sharing the gas plan for manatee?

A quick napkin estimate of 2k on 1/3 stage that would lead to 5 Al80 + back gas kept for reserve. With that distance I'd also expect a tow scooter.

Assumptions:
2k per 3rd using ginnie flow (manatee is higher)
Amount of stuff being towed would also reduce efficiency...so the penetration distance may drop to 1500' per al80. This could increase the number of stages to 7-8

[PfcAJ]

half+200 on stages for me.

[floridabob]

@floridabob, care sharing the gas plan for manatee?

A quick napkin estimate of 2k on 1/3 stage that would lead to 5 Al80 + back gas kept for reserve. With that distance I'd also expect a tow scooter.

Assumptions:
2k per 3rd using ginnie flow (manatee is higher)
Amount of stuff being towed would also reduce efficiency...so the penetration distance may drop to 1500' per al80. This could increase the number of stages to 7-8

Should have started that with the fact we were on rebreathers, sorry. But, i'd not need that many stages and tow scooter would be required.

AJ--my statement about reserving more quantity than when swimming was based on: what posters were saying; not what I would do.

Example: Ginnie to Hinkle swimming uses less than 1/3 of 2 stages and again a bit less than 1/3 on 108s at 3600. Scootering using 900 psi in a stage. So, I was using me but extrapolating using others methodology: stages only. So, more in reserve on scooter than swimming was based on that w/time to poke around and get out.

Again, no argument here on reserving gas, just trying to understand.

[floridabob]

Manatee Dive. Total run time of approximately 300 minutes w/45-55 deco.

Team of 3 to 10k w/1+ hour survey time, 85 minutes to 8k, 50 minutes out. Yes, thats 8k from Friedmans but we get in at Catfish...so 85 minutes from Catfish to start of survey which includes:

1 AL 80 dropped at the "sign"
2 AL 80s at the crack
3 back up scooters dropped at the Siphon T
arrive at 8k about 85 minutes
Me: 2 95s at 3800; buddy --2 85s at 3800; buddy 2--2 LP 80s at 3800

We have done this dive multiple times and thats how its worked out. Quick gut check of all breathers dying and all scooters and we all swim out w/gas to spare.

Others doing similar dives use 2 scooters w/4 stages and 108/104s in OC.

Long and far but not deep. Yes, I love the safety of 3 back up scoots even though same say its overkill.

[RN]

I don't know; I think more complacency and poor planning.

I think it's both poor training and complacency and poor planning. I've heard of DPV classes in which instructors cover the in water skills but there isn't any discussion about gas planning. That's the most important part of the class IMO. I'm less interested in how well someone can tow someone else or match speeds while sharing gas than I am in how well someone can plan a dive so that there is sufficient gas to get out of the cave alive if something goes wrong.

But then there are those who get good training and learn all about proper gas planning. They end up going out and doing a bunch of dives with nothing ever going wrong and they start to think maybe their instructor was being too conservative. They can go a little farther and it won't be a big deal. Until it is. It doesn't help that we have scooter like the SS Magnus that can travel over 200 fpm and get really far back in the cave. While I could get 4000' in and out with my Mako I never attempted that without having enough gas to swim out. I just didn't trust it that much. A 4000' dive is nothing with my Magnus though. I still plan enough gas to swim out but I also need less gas to get in because my Magnus can travel twice as fast as my Mako could.

Oh, and buddy separation is all too common. The only reason I've ever had to withhold a card is for that reason. It's too easy to lose track of a buddy when you're travelling 150-200 fpm. It's one thing to have a short separation of maybe 30 seconds but anything longer than that is lack of awareness.

[RN]

Let's complicate things even more. Planning a dive that involves both scootering and swimming. Most of my dives are exactly that. In fact, I don't think I've ever planned a dive in which I've been on the trigger the entire time. I liken it to the express train . I take the train to my stop and then get off and do my dive. So now I have to plan gas to scooter in, gas to do my swim dive several thousand feet back, and gas to get out if something goes wrong.

[PfcAJ]

match speeds while sharing gas

Seriously? That's a thing you teach?

No wonder this is so complicated. Learn to tow efficiently. Link up, share from your buddy's backgas, and gtfo.