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IP Allowance in gas planning
On our recent intro course, in terms of gas planning we were taught to calculate turn pressure as follows:
Starting pressure-10 bar (IP)
Nearest divisible based on 3rds/quarters/6ths
Resultant number from above taken away from start pressure gives you turn pressure
So for a 210 bar fill the following would apply:
200/4 (if using quarters)=50 bar
210-50=160 Turn Pressure
160-50=110 Surfacing pressure
Emergency gas=100 bar and 10 bar left over for the IP to run the reg
I got into a discussion on SB recently clarifying some details on stage usage and options that had been posted and it would appear that the 10 bar IP idea is an anathema in Mexico and the US, they rely on the fact that roundings or conservancy (quarters or sixths) provide for this contingency already, or in the case of stages the half plus 15/20 bar rule leaves the 15/20 as contingency for IP.
Just wondered what views on this were TBH.
Depending on your reg, that 10bar will probably only work if you're at the surface or very near it, so it's probably a meaningless reserve. Regs definitely work at 20bar when very shallow... Personally, I don't bother taking it into account (but then I'm not cave trained). Using your 210bar example, I'd have 70 out, 70 back and 70 reserve, so 140 turn and 70 on the surface. For a 200bar cylinder, it would be 60-60-60, so 140 turn, 180 surface and 80 reserve. Easy numbers for quick math. I'm not one for complex "battlefield calcs"!!! I guess for caves and the like where you cannot cut the route back short, you might use something different though.
End of the day, there's many ways to skin the cat. Just make sure you always have more than enough.
Doesn't it largely depend on the kind of dive you're planning? Whether you're swimming up current or down current makes a big difference to your conservatism. Similarly your prior knowledge of the site; the general conditions; your own 'state'; your team's experience, etc.
Taking thirds and static water, "in theory" you won't have enough gas to get out if you're sharing gas with someone at the very end of the penetration, i.e. one third in, one third for you out, one third for the person you're sharing with. Sure, you loose the IP, but the bigger issue is you're unlikely to be breathing calmly after the total merde totale frappe le ventilateur moment at the extreme penetration endpoint. Of course you'll be gaining some time as you'll leave your lines and markers, and you won't be hanging around to tie on jumps, navigate, or take selfies.
Hence quarters and sixths. The question here is does one actually need to take into account the IP if using more conservative gas planning?
The chances of a complete gas failure at the end of the penetration is minuscule, if not zero when diving sidemount. Total gas failure on two completely independent cylinders could only be from a double failure (why did you continue when one failed?) or bad gas (would normally be noticed earlier? was it analysed?). There's more of a chance of failure on backmount: three(ish) O rings on the manifold; clout the rock as backmount's more exposed...
Interesting questions though.
Originally Posted by JonG
If your conservative in your planning its a non issue. If you get down to that 10Bar you screwed up and probably didn't plan on screwing up, so again its a non issue
Never worked it like that my self but then I dont do a lot of cave diveing so my ultimate FUBA planning is suck it dry, then go up and sort it on the boat. Obviously not an option in an OHE which is why I plan a lot more margin of error in in OHEs
Personally, I've never factored in the IP but I do routinely round down to make the maths easier in my head.
I don't blindly follow 1/3rds or 1/4s or 1/6s (or GUE C1 2/9s) for a dive, I try to calculate the best turn pressure based on a load of factors. It's important to understand that 1/3rds on one dive may be MORE conservative than 1/6ths on a different dive.
Take some extreme examples :
A dive in a big, long cave but you are only swimming in a short way to stop for a good while and do a survey job. On my first dive in a group of 3, I would probably dive standard 1/3s. Once I'd exited the cave and established how much gas it takes to get out from the survey location and I was totally comfortable with the navigation, I could try a different strategy for the next dive to the same location. If it only took 20 bar to get out, I could reserve say 5x this, giving a 100bar turn pressure. On the face of it, this is diving worse than 1/2s(!) but is actually quite conservative compared to....
A dive in a long shallow, cold cave as a team of 2 with a deeper section near the entrance (eg. Cabouy). You decide to dive 1/4s in backgas. You have been diving for several days, comfortable in the cave, drysuit is warm, SAC rate is really good, lots of kick and glide. You can swim a long way very easily. Some kind of compound problem occurs just before turn. Squeezing through a gap, break something on the manifold and tear a drysuit sorting yourself out. Getting out sharing gas may actually be impossible.
The other thing to consider is a time-based reserve. For really short dives with small tanks, 1/3rd or 1/4s is dangerous. Absolutely no way I'd do a dive with 10mins in, 10mins out, 10 or 15mins reserve. I'd be bricking it if I lost the line. 10mins in, 10mins out, 60min reserve is far more sensible.
Last edited by Steve Clark; 07-04-2019 at 03:24 PM.
This does reinforce that my decision to use OC for ohe was a good one for me personally, it's stretching my head for sure and tonight in the quarry was up at about 17 lpm which is above what I thought my SAC would be, but it was bloody cold without the recycler and I was properly pratting about with line and a stage so may come down on a simple dive, but useful as a base level for planning.
Ian was teaching us gas management as part of a mine course so no flow considerations and were working on 6ths for turn pressure.
I do take it into account for the sucking it dry purposes above, above 10bar the first stage should be applying positive pressure so you just need to inhale. Below 10 bar you are needing to apply negative pressure to the first stage in order to access tank pressure.
Whilst you can still breathe the tank the effort required to do this from your lungs is exponentially higher with each lung full.
Good test for this is gentle jogging on the spot breathing from long hose in living room, its a piece of piss with positive pressure from first stage (well positive pressure from the tank behind the regulated 10bar first stage) but the moment it drops below 10 see how quickly you feel out of breath despite still being able to take a lung full.
If you are on deco on a line in open water then you are not moving you can use the last 10, in a cave situation more often that not your egress is still going to require physical effort to reach the entrance even if its a few metres away - therefore the last 10 will allow exit with the equipment supplying gas as it is designed and no last minute requirement to over exert lungs.
I can go into the dive knowing my plan is spot on, in practice applying a real world scenario to it the plan will not match it for hundreds of reasons.
It really is a tiny thing in a plan but i include it for those reasons - im sure we can all agree that if you get to the end of a dive and end up 10 bar short or 10 bar left over then it wont be because it was included in a plan or not, real life just wont match it that closely.
If you are relying on the last 10 bar in all the cylinders you carry ,to get you out of the water , you need another cylinder.... or reduce the dive profile/time.
That is if you can find an SPG that can measure 10bar and you know your SAC to within 0.2%! Its spurious precision - the 1/3 1/4 etc is there to deal with all the kinds of uncertainty you can mention plus all the "unknown unknowns"
Originally Posted by bubbleless
You illustrate an old saying I heard somewhere long ago and repeat often to my students--"In technical diving (not just caves), we measure with a micrometer, mark with chalk, and cut with an axe." Nothing can be predicted with that kind of precision.
Originally Posted by Energy58
In time you will make quick estimates of those things and do a lot of rounding. When you do, you will hopefully realize that the purpose of the exercise is to make sure you have enough gas in case of all but the worst possible emergencies. If that realization truly hits home as it should, those quick estimates will give you more gas than your precisely measured predictions. I imagine it would be singularly unpleasant to run out of gas in an overhead, and I sincerely wish to avoid it.