First I miss why most CCR's are dived on 1.3 PO2 if the ccr works. This has to do with CNS that lasts 3 hours to get to 100% and most scrubbers officially last only 3 hours.
1.2 is also used quite a lot. 1.4 in ccr is not used a lot.
First we do open water ocean diving, no cave diving.
There are some advantages and disadvantages on which PO2 you use at depth. Let's say we go to 100m for 20 minutes . If you use a 1.3 or a 1.2, this makes already a difference in the total divetime.
Put it in a diveplanner, diluent 10/70 (DIR-gas with also a lot of disadvantages), it will give you on 1.3 a divetime of145-150 minutes (calculated with Multideco, 40/75). So 59 minutes at 6 m. You stay the whole dive on the loop.
A 1.2 setpoint for this whole dive will give you 70 minutes at 6m. So the 1.2 has for sure disadvantages.
This means, diluent switches can have advantages. All will be learned by your instructor when you do a full trimix course.
On OC I use the 1.4 on deep dives, but that is only at bottom. As soon as you go up, there is no 1.4 anymore. Then you switch to a 1.6 on the first stop where you switch, but the 1.6 will become less than 1.6 on the next stop. So with the gasswitches you go to 1.6 for a while.
A bailout gas means you go up from the deepest point in the dive with oc bailout. Then you choose for the decogases on 1.6. The bottomgas is not that important because you don't dive there anymore as bottomgas, it is just a bailout to breath and you go up.
So if you have 10/70, ean50 and 100% as bailoutgases (ignore icd as 10/70 to ean50 is not the best choice, ignore a travelgas between the bottommix and first decogas to make it easier), you take the 10/70 oc after 20 minutes bottomtime. Then you go up. The PO2 lowers because of the 10% oxygen. But at 21m, you switch to ean50 and at 6m you have 100% at a PO2 of 1.6. Because of the low PO2 of the first bailout, your total divetime will be longer.
But if you use a 12/65 instead of a 10/70 as bailout, the PO2 when breathing the 12/75 will be higher, and the total bailouttime and divetime will be less than using a 10/70.
So a 12/65 is a better bailout gas. But if you take a travelgas, the choice for a 10/70 as bailout can be more valid as a 10/70 also can be used as diluent if the 3 liter cylinder drains for whatever reason.
You can also choose for a poormans mix as diluent, 6/72. This means if there is a high PO2 at bottom, you can lower the PO2 quite fast. And high PO2's at depth are more tricky than in shallow waters. So the 6/72 vs 10/70 discussion is done in a course also.
OC diving, a 12/65 is for sure a better mix at 100m than a 10/70. It gives a lot less deco, the PO2 is still within the 1.4 norm. Some people choose a 13% oxygen also.
In a cave you can use if the tunnels are constant at depth (p)scr bailout. This means the bailoutgas must have a right PO2 to maintain a PO2 in the loop.
So if the cave is only 20m deep, you can take air as bailout gas. No problem, you must calculate how much is needed from the longest penetration distance, and you drain the tanks while breathing it oc.
But if you take an ean32 as bailoutgas, you can use the ccr in some cases as gas extender. This option fails with air as bo gas.
Also if you go deep in a cave and the tunnel is horizontal, a higher PO2 but max 1.4 can have advantages. So a 6/72 at 100m in a cave and 10 minutes swim back at 100m means oc a lower PO2 than a 12/65. The 12/65 will give you less deco. Where the 6/72 a good diluent can be, the 6/72 can be not the best bailout option.
And don't forget to think about safetydivers if you do ocean diving and go to 100m or more, oc and ccr. In a cave you can probably do setupdives, in an ocean it is more difficult.
