What happens when you descend on a CCR?

[Diving Dude]

You sure that's not 'Knob of the lobster realm'

Far more appropriate

[Janos]

Originally Posted by Diving Dude

You sure that's not 'Knob of the lobster realm'

Far more appropriate

Guilty as charged

Janos

[AM1]

Hello Mal.

I haven't read all the thread so apologies if I'm repeating what others have said.

I wanted to respond to your question about diving at 1.3. There is no absolute rule that you must dive at 1.3, personally I prefer to keep the PPO2 down for the working part of the dive then up to 1.6 on deco and none of this takes any significant effort. In fact on deco I flush the lungs with oxygen very regularly as this mitigates rebreathing inert gas.

The ascent and descent stuff is just practice and experience although is a bit more task loading on manual units upon ascent, the real danger time is long surface swims.

[Phoenix]

Originally Posted by AM1

<snip>

The ascent and descent stuff is just practice and experience although is a bit more task loading on manual units upon ascent, the real danger time is long surface swims.

This risk is unit dependent, the original boris electronics will maintain a SP of .40 at the surface so you "shouldn't" be at risk as long as you have O2 left, the current version with "autobreathe" works even if the unit is "off" AIUI.

Ian

[AM1]

Hi Ian, I did specifically mention that risk in alignment with manual units.

[NotDeadYet]

Originally Posted by Paul Evans

Why worry about the spike on decent, do a Scamahorn slide

You can only do a slide on an eCCR, can't do it on a mCCR. A slide relies on the rebreather being smart enough to not inject any O2 while your body burns the rising oxygen off. On a mCCR it's adding O2 all the way down so by the time you hit the bottom the ppO2 may well have gone off the scale and it will stay there. It takes a surprising amount of dil to flush it down to a manageable level, it's got to be far less efficient than staying on top of it on the descent.

Personally, I run my RB on pure O2 anywhere above 6m. The shallows/surface are the worst place to be on a rebreather, both from basic physics and the fact that there is more going on around you than any other part of the dive. If I'm on pure O2 then I don't have to think about what the loop is doing. On the descent if my displays are reading 1.5-1.6 (happy cells) at 6m then I dil flush and descend. If the dil is <11% O2 then I let the ADV add dil to about 10m then I fully flush there where it's safer. I then let the ADV do it's job, I generally choose a dil to suit the depth so by the time I hit the bottom I'm close to setpoint and can then boost it manually. I don't know if that's the most efficient gas use but I'd rather keep away from hypoxia than save 10bar.

Depending on my dive I'll use either 1.0, 1.1 or 1.2 as my setpoint. I find 1.3 a little high for long dives, I feel it afterwards. I'm also a bit happier shunting it up on ascent if I want out the water faster if I've been on a lower setpoint during the dive.

Cheers,

Stuart