How do you calculate when to turn when using a scooter on OC?

[Dsix36]

I know that turning on thirds (or even sooner if conditions dictate) is the most common approch to OC cave diving but what changes when you scooter. Suddenly you get much further into the cave before you ever hit thirds. There is no way you have enough gas to swim out if the scooter dies. Knowing your SAC rate, swim rate, and gas amounts makes for easy calculation of the distance you can swim with given amounts of gas BUT Y0ou are using gas as you go into the cave (and this can vary from day to day and dive to dive) so how can you truly plan when to turn in a realistic manner that is acceptable,

Very simplified example: swim @ 50fpm, scooter @ 150fpm, SAC @ .5, 160 cf of gas @ 3000psi, 4ATA
Swimming =Thirds is at 2000psi in 20 minutes with 1000' of penetration
Scootering =Thirds is at 2000psi in 20 minutes with 3000' of penetration

Simple logic tell me that is an easy way to die so one must plan according having enough gas to swim out but it can never be a linear plan due to variables of each dive. Turning must be done on thirds of the lowest common denominator. Scootering to 1000' and turning will be way early of 1/3 of my gas supply and at the end of my dive is when I will finally reach thirds. Meaning I turned at sixths. Is there a simple magic formula to calculate proper turn time on the fly?

RB diving is simple since I never touch my OC gas and thus I still have full tanks at only penetration.

OK, let the keyboard warriors attack for asking for online training etc. I have thick skin and really do not care. It will be fun to see how the different replies tally up

 

[inquis]

Is there a simple magic formula to calculate proper turn time on the fly?

Amplify your desired Safety Factor considering the different speed/consumption values each way. (The principle behind Thirds is to have a Safety Factor of at least 2x the gas needed to return.) You just have to do the math to make that happen.

Your example would need to amplify the safety factor by 3x:

M = (150 fpm_in / 50 fpm_out) * (0.5 cfm_out / 0.5 cfm_in) = 3​

​

The turn number is T = (1 + M * SafetyFactor).
Using a Safety Factor of 2, you would turn on 7ths, giving 1700 ft penetration. (As an aside, kicking both ways gives M=1, and T=3rds for a safety factor of 2.)

The usual caveats apply, WRT to these assumptions -- no different than when kicking both ways, really. Unfavorable deviations from those assumptions effectively reduce the safety factor, so something larger than 2 may be wise.

The above is strictly for DPV penetration on back gas alone. Additional considerations are absolutely required for stage usage or continued penetration after dropping the DPV.

 

[FF / EMT]

What if you brought a backup scooter planning/ expecting one in the group to die.


you know what they say, 5 is 4, 4 is 3, 3 is 2 and bla bla bla....

 

[Dsix36]

Lots of good stuff here. To clarify, I am referring to being solo with no backup scooter, and no stages. My Cave DPV class was on a rebreather with bailout so turning was done at the lowest thirds of everything (bailout, scrubber, O2, diluent, light battery, and even computer batteries). At this point I am starting to think OC with a scooter is just stupid when I can just take my rebreather. Thirds on OC works for me since it is basically 2x what I need when most of my CC bailout is figured at 1.5x what I need.

 

[inquis]

Thirds on OC works for me since it is basically 2x what I need when most of my CC bailout is figured at 1.5x what I need.

The CCR BO is 1.5x after a major failure. The OC is 2x before a major failure, but possibly only 1x after that failure (assuming SM).

ETA: yes, losing both a reg and DPV is 2 majors, but you are solo...

 

[Whitrzac]

Dont forget about the added deco and thermal issues when dealing with a DPV/RB failure.

 

[kensuf]

Swimming out is unrealistic for most people whenever you're talking deep or more than about 2500-3000'.

 

[Dsix36]

Swimming out is unrealistic for most people whenever you're talking deep or more than about 2500-3000'.

This definitely the case for me right now since this whole OC sidemount thing is new to me. I am keeping it in the 1000' range at the moment but have goals of 3000'. I do not think that goal is going to be a realistic with my SAC rate and using aluminum tanks though, and will have to resort back to my rebreather.

 

[tbone1004]

I know that turning on thirds (or even sooner if conditions dictate) is the most common approch to OC cave diving but what changes when you scooter. Suddenly you get much further into the cave before you ever hit thirds. There is no way you have enough gas to swim out if the scooter dies. Knowing your SAC rate, swim rate, and gas amounts makes for easy calculation of the distance you can swim with given amounts of gas BUT Y0ou are using gas as you go into the cave (and this can vary from day to day and dive to dive) so how can you truly plan when to turn in a realistic manner that is acceptable,

Very simplified example: swim @ 50fpm, scooter @ 150fpm, SAC @ .5, 160 cf of gas @ 3000psi, 4ATA
Swimming =Thirds is at 2000psi in 20 minutes with 1000' of penetration
Scootering =Thirds is at 2000psi in 20 minutes with 3000' of penetration

Simple logic tell me that is an easy way to die so one must plan according having enough gas to swim out but it can never be a linear plan due to variables of each dive. Turning must be done on thirds of the lowest common denominator. Scootering to 1000' and turning will be way early of 1/3 of my gas supply and at the end of my dive is when I will finally reach thirds. Meaning I turned at sixths. Is there a simple magic formula to calculate proper turn time on the fly?

RB diving is simple since I never touch my OC gas and thus I still have full tanks at only penetration.

OK, let the keyboard warriors attack for asking for online training etc. I have thick skin and really do not care. It will be fun to see how the different replies tally up

Some high level simplifications.

You'll hear some stuff that correlates basically to sixths, and you outlined that with your DPV going 150fpm. That was fine and dandy if you were on a Tekna/Gavin/SS-UV that in a set of doubles with a stage or two really couldn't go any faster than that. You can pretty reasonably kick 50fpm so the math maths close enough with that. 1min in = 3mins out, if you give a full 2x safety factor then it's 1/7th, but 1/6th was deemed close enough. You'll see people spew nonsense about diving 104's with 2x stages and just using the stages, but that conveniently comes out to roughly 1/6. Rarely will the people saying that know the actual math behind it and dive it blindly which is annoying. Moreover it also doesn't apply anymore to the go-fast DPV's we have now and is a wonderful way to off yourself because they're blindly following a blue-goo saying and they don't understand why.

The problem now is that the DPV's can EASILY go 200fpm+ so the rules get a bit fuzzier. 1min in=4mins out, so 1/9th if you're going that fast. To those that just dive the stages and don't touch backgas, if you're on an XK1 then you should only be using one stage for penetration, the rest needs to be for bailout.

The last bit is one of the real problems because do you ever really know how fast you're going? It's one thing in big power caves where you get on the trigger and stay there and you generally know how fast you're going, but that breaks down quickly if you're on/off the trigger or slow down to go through twisty bits, etc. This does give you an extra safety factor though so it's not necessarily a bad thing, but you do have to do some experiments to figure out what the max speed you are comfortable doing on the DPV and then you can come up with your own ratios based on that speed, if that's how you want to do it.

Most of the time though, we do actually know most of the map for where we are going and I calculate DPV bailout the same way I calculate CCR bailout. 50fpm kick speed, 1cfm SAC rate, 10min buffer for stuff to go weird, and then you set your radius and your turn pressure. If you're screwing around in Ginnie for example, just set a rock bottom pressure for each thousand foot radius and then you can head to the back, screw around, hit pressure, then screw around some more meandering your way back in.
If you're using the DPV to get somewhere and then kicking afterwards, then same math applies. You know your rock bottom pressure to get back home from where you parked the DPV, so you have (current gas-rock bottom gas)/3=kick around gas.

 

[Manatee Diver]

Swimming out is unrealistic for most people whenever you're talking deep or more than about 2500-3000'.

My instructor used Ken's article as the only written materials for my Cave DPV class. And honestly is part of the reason that while solo I keep within 2K of the exit until I finally get a second tow DPV I can trust.

I came up with my own idea of calculating OC turn pressure based on the idea being I should always be back to my scooter with more than twice the gas I need to exit the cave swimming. Once I get a second scooter I will probably switch back to slightly reduced thirds.

 

[lukeb]

@Dsix36 thank you for asking this question. I have wondered about this for ages, and every time I asked someone I got a different answer!!!

(Contemplating a DPV using OC in the future)