Well a few comments...(from an OC perspective)
- 200fpm isn't going to happen with 4-5 stages with any scooter. 125ish for this dive would be reasonable, even on the high side.
- Motor compartment is sealed on scooters, heat buildup is a very real issue. Some scooters can handle running that hard, but older SS's and Gavins cannot. Rotating scooters to keep them cool is something to consider.
- I would consider using helium in the bailout to avoid doing setup dives...let it float behind you and tow it. Setup, diving, then cleanup would IMO justify the cost of helium, with it being a 45min drive from most dive sites. If you're doing a survey in the back, or laying line, this benefit diminishes.
Good comments. They reflect my relative inexperience with DPV's.
A question...
- Are you using your swimming SAC to calculate how much gas you'll need while scootering out?
- Why add larger tanks to a CCR? Even a 40cu ft is an extremely small amount of time, and at the risk of adding additional failure points I'm not sure I understand the benefit.
- What settings are you looking at for vplanner? Using +2 and 120min@80ft I get 47min without o2.
- Would looking at this dive as a 160min round trip (scooter speed correction) cause you to bring o2 for deco?
Thanks for your reply. I was mainly interested in seeing what people would choose when they perceive their life is on the line. I find it interesting how some CCR divers carry TONS more gas than an OC diver would have diving 1/3rds, and others are going at the razors edge.
1. Yes. I realize swimming SAC is considerably higher than scootering. I wasn't sure about actual speed of DPV and thought that increasing the SAC would sort of buffer the difference on a slower speed. I also intended to check the numbers on a double failure of CCR/DPV so left SAC rate close to a known OC swim rate with 3 bottles. I've also found my breathing rate on CCR is different than OC and it takes a bit for me to change my OC pattern back.
2. Adding a larger amount of onboard bailout is one way of carrying extra gas or reducing the amount of bottles you're slinging. Instead of carrying an 5th 80 bailout, I could make that last bailout bottle a 40 and make my onboard bottles a 40. The 40/40 combo would give me slightly less gas than an 80/19 combo but reduces the drag of slung bottles slightly and I'm already carrying more gas than "needed." This would be more of a benefit if you're using a bailout valve to access the onboard gas. I don't use a BOV, but mention the idea because some people do. As you noted, it adds extra failure points with an OC reg, but I think that failure is relatively remote.
3. I used v-planner at +2 on VPM-B. The first half of the dive I planned as a CC dive with a 1.1 setpoint and the second half on 32. Recalculating the whole thing on OC I show a total of 33 minutes of deco without O2.
4. I'm going to bring O2 regardless. My reasoning behind calculating without having O2 is considering that as part of a lost gas scenario. Make sure I have enough bailout to complete the plan without O2 in the event I don't have access to it.
As a generalization CCR bailout should be somewhere close to 1/3's for an equivalent OC dive. You're only using the gas for return at max penetration/deco in an emergency. Considerations are going to be how much OC is going to impact your deco and how much gas it's going to take to cover your exit + that additional deco. In your scenario, the difference in OC/CCR deco is negligible because the PO2 of 32% is damn close to the set point I'd use for CCR.
One of the big benefits to CCR in caves is extended dives at lesser penetrations. Lets assume instead of going to 10k, that you only want to go to 5k but you're going to spend an extended time there because you want to do some surveying at that point, studying fossils, or whatever. Your dive times between the two dives will be roughly the same, the only thing changing is the distance. For the OC diver, they would still need to plan the same amount of gas. CCR will allow you to cut that plan considerably, because they only need enough gas to exit from that point not for breathing while they are there.
You've got CCR experience, how about adding in some comments on your planning, or comments on mine?
