Scooter gas management

[rddvet]

I've been hesitant to ask this since passing Edd's scooter class, but FW's comment in another thread made me decide to go ahead. As I've said, I passed the class and I know many people are afraid to give their gas management plans open on the forum as it can serve as tool for someone not taking a scooter class to get into trouble. So you can also PM me.

I enjoyed Edd's class and it was very difficult, and as a whole I was happy. I was rather dispappointed in the gas management discussion. I completely grasp Edd's way of teaching scooter gas management (calculate gas needed to swim out from farthest penetration to simplify) and think it is safely conservative. At the same time, my original plan was to take the course in MX with an instructor I respect who teaches DPV/gas management similarly to the GUE protocols. Unfortunately due to work constraints I didn't have the time off to get down to MX. I did attempt to discuss alternative gas management plans during Edd's course as I've heard so many people discuss plans such as breathing the stage only while on the trigger. Obviously it gets more complicated when you plan a scooter/swim dive. Unfortunately, he didn't really want to discuss alternative gas management plans (which is his perogative as the instructor).

I'd like to hear how you choose to plan gas when scootering, especially when you add getting off the trigger to poke around somewhere. I personally am a fan of stage only while on the trigger, leaving full back gas for emergencies. A secondary question would be if you use stage gas only on the trigger, how do you calculate the gas used while swimming? Do you dive 1/3s or 1/6s in the swimming portion or calculate the gas consumed swimming out of your stage gas reserve?

Obviously this shouldn't serve as an alternative to a scooter class for anyone, but I was rather disappointed that it wasn't covered in more detail during the class upon request (no disrespect to Edd's class intended)>

[FW]

The "off the trigger" part is the bigger issue. IF you can use stages while off the trigger, then that is probably best. If your off the trigger passage is small, you need to save enough gas for emergencies. I scooter to Sweet Surprise, but limit my back gas use to sixths. If I am solo, then I take 2 stages, just to be safe. I can easily scooter there and back on one stage, but if I have a failure, then that isn't enough.

[PfcAJ]

Generally speaking breath the stages and keep backgas in reserve. If you want to swim somewhere it's cool IMO to use backgas so long as you reserve enough to get out of your scooter breaks. Having a buddy helps since he could tow you out. Having a tow scooter helps more.

With the exception of ginnie type stuff there's usually at least one tow scooter for the team, sometimes two.

[rddvet]

As for "breathing the stage" I have heard several different definitions. I have heard breathe half in and other half out (not going below 500psi), 1/3s on stage, and half plus 200. I would think that 1/3s is ultra conservative considering on a straight scooter only dive you'd have all of your backgas.

Btw my opinion matches FW and AJs but I've overheard so maby different opinions its nice to see them accumulated together


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[tbone1004]

so Edd's class has limited discussion on gas management because no one rule solves all, you need to calculate on every dive what that gas management plan is going to be based on your factors, and those factors change.

I run 2x the planned gas required for exit. That is typically kick speed, unless you have a backup dpv. For me that equates to 0.6cfm*50fpm*planned ATA. Quite a bit of buffer considering thirds is 1.5x, but I'm on a scooter so I can afford to carry more gas.

The "rule" of breathe only stage gas, only works to a point, and can break down VERY quickly, especially with the fast scooters. 200fpm vs 150fpm is enough to cause it to fail very quickly without a tow scooter.

If I'm doing linear penetration or DPV trigger only with no planned kicking, then the general rule with 2 stages is to breathe them to half plus two, and leave the LP121's untouched. It works well with the DPV's that we have, and the tanks that we have. If we add stages to that, or use smaller tanks, then that rule no longer applies because the backgas isn't enough. If you add a backup scooter, it may be too conservative and you're carrying excessive amounts of gear which may increase the risk of that dive. Using the variables that I have for a 90ft dive on 121's, the 121's are good enough for 3700ft of penetration on their own, or 4500ft of return only gas. Each AL80 is good for about 2300ft of DPV up and back. So if I'm planning a 4500ft penetration, I'd be able to do that with 2 stages, and not touch the back gas if I don't have a backup scooter. If I add the backup dpv though and only up the return speed to 100fpm, then the 121's are good for 6300ft of penetration AND return kicking. I would probably bring a buddy bottle for the pair of us, but there is no need to each bring 2 stages on that dive if we are also carrying a backup dpv.


My buddies and I each have a slightly different way of going about how we calculate it, but we always end up with in a hundred psi or so of each other. We have a printed table that gives us return gas required as a function of depth. We also have a worksheet that I wrote in excel that allows you to input things like planned penetration, planned avg depth, backgas kick time, planned return speed and sac, planned dpv speed and sac, exit safety factor, and backgas tank size, fill pressure, and a psi buffer *I dive Poseidons so I like to give everything a 200psi buffer just in case*. Lots of variables, but it will give you planned exit gas required, planned total gas required, number of stages required.
It will then tell you planned entry gas, exit gas, total gas, and minimum number of stages required, as well as your backgas turn pressure *based on kick sac, planned time, and the safety factor. I.e. if you set 2, it will allocate twice the planned backgas pressure to give you enough for an air sharing exit, or 1.5x which is thirds*.
It's not perfect, but it's great for rough cutting how many stages you should take on a dive, and experimenting with the value of a backup scooter, etc. It also allows you to input the variables and the number of stages you're planning on carrying and giving you a theoretical penetration limit. Great if you're going into a new cave or new line and you don't know exactly what you're going to do, but you know that if you allocate 10 minutes of kick time, that you shouldn't go past 5100 if you're only carrying 2 stages, etc.
I also put a tab for CCR gas management which is basically the same but only uses the exit values from the other sheet.

There is no cut and dry way to do it because none of the "rules" are all encompassing. It was one thing to write rules based off of a Gavin that could barely hit 200fpm if you were in a bathing suit and was pretty consistently about to maintain 150fpm in a tech configuration. Sixths works pretty well for that based on a 50fpm kick speed, same SAC rate factored for DPV and Kick, and a 150fpm planned entry speed. Sixths isn't good enough for something like a Magnus that can easily exceed 225fpm in the same configuration if you're going all out because you're going 1.5x as far on the same entry gas.

That is why Edd doesn't teach any "rules" to DPV gas management, because there aren't any. I know we had this discussion before your class, but the answer hasn't changed, and it won't.

The only rule is you need to plan how much gas you're going to need to get out from how far in you're planning on going, what safety factor you want to use *which is really the only thing that you will see any valid discussions about, particularly 1.5x vs 2x*, how much gas you're carrying, and subsequently how much you need to reserve. From there, divide the balance by two, and that's you're turn pressure. There honest to God isn't any simpler way to do that, especially with how fast DPV's have gotten in the last few years. You have to do the math....

[PfcAJ]

As for "breathing the stage" I have heard several different definitions. I have heard breathe half in and other half out (not going below 500psi), 1/3s on stage, and half plus 200. I would think that 1/3s is ultra conservative considering on a straight scooter only dive you'd have all of your backgas.

Btw my opinion matches FW and AJs but I've overheard so maby different opinions its nice to see them accumulated together


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Halfs is the way to go.

Imagine you had something seriously go sideways on your dive. You're scootering out on a stage bottle and when you hit the next stage (the one you dropped on the way in, which currently has 2/3 in it) you're going to have 1/3 left over. So now what? Do you keep it with you for that scrap of gas or do you drop it so you can go faster? If you keep it with you you'll have to stop again and switch to the 2/3 full stage. So you're adding an extra stop plus going slower from having two stages on you.

Halfs (plus enough to switch, 200psi-ish) eliminated that. You reach the 1st bottle just as you're about done with the bottle you're currently on. Switch, discard the empty, and boogie on out.

Thirds in a stage doesn't make a lot of sense. You can't share it. Keep your reserves on your back instead where it's in a format that's shareable, you can close valves, and it's with you at the far reaches of the cave.

[rddvet]

so Edd's class has limited discussion on gas management because no one rule solves all, you need to calculate on every dive what that gas management plan is going to be based on your factors, and those factors change.

Yeah, we had talked about it prior to my taking Edd's class. I got from Edd that there isn't a hardened fast rule and we went over calcualtions for gas needed for complete failures from max penetration. I also knew going into it that there isn't a hard/fast rule. That's why I keep my ears open and listen to people at dive sites as they gear up. The number of ways people calculate the gas they need or max penetration is all over the place, if they even plan it at all. I have thought of not touching backgas and only using the stages as a fairly conservative way to manage scooter gas as long as going into it you know that your backgas will be plenty to get you back home from your max penetration. The issue I had was I attempted several times to discuss these alternative gas strategies in class and it never happened. I finally gave up as it became clear it wasn't something he didn't want to discuss. My intent was not discredit his gas management but to get a better understanding of all of the ways people do it and why they are either good or bad. Because there's no set rule, my buddy who took a class with a different instructor will likely have an eniterly different gas management plan and I'd prefer to be fully educated as to why one may be more safely conservative than the other. That's why I asked here.

So far it's been informative.

[FW]

...I have thought of not touching backgas and only using the stages as a fairly conservative way to manage scooter gas as long as going into it you know that your backgas will be plenty to get you back home from your max penetration...

What do you if you are scootering beyond the point that backgas will get you out. GUE and KUR have good methods for that. They involve leaving safety bottles, and carrying backup scooters. Rebreathers also are helpful for longer dives.

[rddvet]

What do you if you are scootering beyond the point that backgas will get you out. GUE and KUR have good methods for that. They involve leaving safety bottles, and carrying backup scooters. Rebreathers also are helpful for longer dives.

I was leaving complexity of a dive beyond what backgas can get you back from out of it for the moment for simplicity. But agreed, just as tbone noted the modern scooters can get you beyond what you can swim back from quickly. So safety bottles would be a logical choice.


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[tbone1004]

yeah, the issue that stems from him not wanting to discuss any rules other than you need enough to get out at whatever safety factor you choose is to force you to do the math to make sure you understand how to do it. If you learn a hard and fast rule that applies to a Gavin, and decide to apply it to a Magnus/Xk1/pick whatever fast modern scooter you want, then you'll off yourself if you have a serious issue. With that, you have to develop your own rules.

The two things I can see being discussed as DPV gas management are what safety factor you use to get out, and what you breathe your stages to.
I personally will always breathe stages to half plus two regardless of if I'm kicking or not. The reason why was discussed by AJ. I want to have a stage, be able to breathe everything out of and get rid of it. I'll breathe my backgas until I reach the next stage. Lather, rinse, repeat until you're out of stages. Doesn't matter if kicking or scootering. If sh!t hits the fan, you want to get rid of as many bottles as possible to make your exit easier. Holy crap, AJ and I just agreed on something...
The other is the safety factor. I have a couple thoughts that haven't been firmly ingrained yet from lack of experience. My current believe is 2x safety factor if in a team of two, or if you have a backup dpv for the team, and 1.5x if you're in a team of three or have individual backup scooters. The thought being that the team of three and the individual backup dpv's help to pad the buffer a bit, so I'm comfortable diving to 1.5x. Otherwise, I'll carry the extra gas and know that I have an almost indefinite amount of time to get out, though I'll be quite cold/bored/hungry towards the end of the obnoxious deco. That obnoxious deco is something a lot of guys don't plan for either. You'll often see guys DPV in with two stage bottles and an AL40 for O2. Good luck completing your deco if you have to kick out....
This is all done with a somewhat elevated sac rate of 0.6 which is about 20% higher than normal kicking SAC, and about 50% higher than DPV SAC. It also will have the 300psi/tank buffer built in, so there's a lot of extra conservatism that gets thrown in aside from the actual numbers.