What happens when you descend on a CCR?

[Mal Bridgeman]

I don't pop into this forum too frequently but I was reading the thread about the surface swimming thread and it has made me wonder what happens when you descend on a CCR....in say the first 10m.

Can anyone explain what happens in terms of maintaining the loop volume and the ppO2. Do you have to do this yourself or is it automatic?

TIA
Mal

P.S. You have to presume I know nothing of RBs

[Nick Kay]

I'll let someone diving an eCCR comment on their kit, but from an mCCR perspective:

Lets say the loop is at 0.5B on the surface - cos you've injected O2 manually to bring it to that level, then as you descend, two things will ordinarily happen:
1. The ADV (Auto Diluent Valve) will inject Dil(uent) because the loop volume will decrease, i.e. 2 x the pressure, so 1/2 the volume. That'll decrease the FO2 (and therefore the ppO2). However...
2. You're descending so the ppO2 will increase

Plus, you can manually inject O2 to increase the FO2/ppO2

Hope that helps

[David Pye]

My two penneth is regarding loop volume.

On a standard inspiration RB (without an auto-diluent valve), as you descend, the loop volume will decrease as the ambient pressure increases, meaning you can get less and less of a breath. You would normally press the diluent add button on the counterlung to add fresh gas to counteract this.

If you have an ADV fitted, or are using a KISS , you just take a breath and it activates it, adding fresh diluent into the loop.

[Depth-junkie]

Originally Posted by Mal Bridgeman

I don't pop into this forum too frequently but I was reading the thread about the surface swimming thread and it has made me wonder what happens when you descend on a CCR....in say the first 10m.

Can anyone explain what happens in terms of maintaining the loop volume and the ppO2. Do you have to do this yourself or is it automatic?

TIA
Mal



P.S. You have to presume I know nothing of RBs

Depends if you have an ADV (automaic dilutent valve) fitted or not. If you do then it acts baiscly the same as a reg, as soon as it feels negaive pressure against the diaghram it adds in gas. Basicly works the same as sucking on a reg(diaghram sucks in on inale and pushes needle valve and opens up the gas path in turn adding gas)

If no ADV you have to manualy add in the Dil via the counterlung buttons (when you feel negative pressure I.E cant suck in any gas-push the button on the inhale lung)

The use of an ADV basicly gives you one less thing to do, and leaves you hands free for the decent.

As for the PO2 on decent, it obviuosly goes up inline with the added ambeint pressure. But the addition of Dil keeps it within safe levels.

For whatt its worth i switch up to high setpoint (1.3pp02) @ 10m on decent even for deep 90m+ dives as the addition of Dil keeps my PO2 in the 1.00 region on the decent, it also allows me to spike the PO2 above setpoint on the decent to make sure cells are not current ltd (just hold my breath for a few meters and make sure cells reach 1.4-1.5).

So by the time i reach the wreck i am all set to go, and not have to faff about with switching from low to high and current ltd test.





ATB
Gareth

[Depth-junkie]

see we are not that bad, ask a decent question and get a decent answer







ATB
Gareth

[Mal Bridgeman]

Originally Posted by Depth-junkie

Depends if you have an ADV (automaic dilutent valve) fitted or not. If you do then it acts baiscly the same as a reg, as soon as it feels negaive pressure against the diaghram it adds in gas. Basicly works the same as sucking on a reg(diaghram sucks in on inale and pushes needle valve and opens up the gas path in turn adding gas)

If no ADV you have to manualy add in the Dil via the counterlung buttons (when you feel negative pressure I.E cant suck in any gas-push the button on the inhale lung)

The use of an ADV basicly gives you one less thing to do, and leaves you hands free for the decent.

As for the PO2 on decent, it obviuosly goes up inline with the added ambeint pressure. But the addition of Dil keeps it within safe levels.

For whatt its worth i switch up to high setpoint (1.3pp02) @ 10m on decent even for deep 90m+ dives as the addition of Dil keeps my PO2 in the 1.00 region on the decent, it also allows me to spike the PO2 above setpoint on the decent to make sure cells are not current ltd (just hold my breath for a few meters and make sure cells reach 1.4-1.5).

So by the time i reach the wreck i am all set to go, and not have to faff about with switching from low to high and current ltd test.





ATB
Gareth

OK so what I am understanding is that with an ADV the descent load on the diver is not a great deal different to an OC diver .... just clear your ears and inflate your wing to compensate for the buoyancy shift.

On the ppO2 front, if you have a ppO2 of 1.3 at 10m how do you get it down by the time you get the wreck? .... Does the automatic addition of diluent keep it down?

Once I have understood this, I am likely to ask questions about what happens on the ascent.

Before I do though, why a pp02 of 1.3? What is the wisdom behind that?

Cheers
Mal

[David Pye]

Well, the training agencies set their own recommended PPO2 limits for RB diving. According to BSAC SDPs, these are 1.3 for the duration of the dive, 1.4 for deco.

Hopefully, there's some sense behind these, rather than just arbitrary limits

As for the PPO2 changing with respect to depth during the ascent, assuming you weren't consuming any O2, it would rise as you descend. Obviously, if the diluent were pure Nitrogen that was being added, the PPO2 wouldn't change at all, but as air is 21%ish, it would creep up as diluent is added. As I've never actually dived one, I can't actually answer the question from a practical perspective, only from a physics-ish one.

But I'll stop being an armchair RB diver in a couple of weeks when I do my CK MOD 1

[Digger]

Originally Posted by Mal Bridgeman

OK so what I am understanding is that with an ADV the descent load on the diver is not a great deal different to an OC diver .... just clear your ears and inflate your wing to compensate for the buoyancy shift.

On the ppO2 front, if you have a ppO2 of 1.3 at 10m how do you get it down by the time you get the wreck? .... Does the automatic addition of diluent keep it down?

Once I have understood this, I am likely to ask questions about what happens on the ascent.

Before I do though, why a pp02 of 1.3? What is the wisdom behind that?

Cheers
Mal

Personally I use 1.3 because it's 3 hours before you hit CNS. It's also conveniently not far off what you get if you run bottom gas at 1.4 and deco gas at 1.6 on OC. So you're not in the water massively longer or shorter than guys carrying a sensible number of gases.

It's also what a lot of ECCRs run on, for whatever reason, and so you might as well stick with them

Basically it's as high as the ppO2 can be without being a tox issue. If you ran your unit at 1.4 then you'd run out of O2 clock before you ran out of scrubber life (in theory) and if you ran it at much less you'd have the opposite problem.

You should really dive a few of them, in the interests of balance of course.

Digs.

[Mick F]

Originally Posted by David Pye

Well, the training agencies set their own recommended PPO2 limits for RB diving. According to BSAC SDPs, these are 1.3 for the duration of the dive, 1.4 for deco.

Hopefully, there's some sense behind these, rather than just arbitrary limits

As for the PPO2 changing with respect to depth during the ascent, assuming you weren't consuming any O2, it would rise as you descend. Obviously, if the diluent were pure Nitrogen that was being added, the PPO2 wouldn't change at all, but as air is 21%ish, it would creep up as diluent is added. As I've never actually dived one, I can't actually answer the question from a practical perspective, only from a physics-ish one.

But I'll stop being an armchair RB diver in a couple of weeks when I do my CK MOD 1

Armchair or not, that's a pretty good answer.

From an Evolution viewpoint, as you descend add dil, manually or through ADV. Gas is good.

On the way up, nice & easy does it & allow the PPO2 to catch up to the setpoint.

[Depth-junkie]

OK so what I am understanding is that with an ADV the descent load on the diver is not a great deal different to an OC diver .... just clear your ears and inflate your wing to compensate for the buoyancy shift.

On the ppO2 front, if you have a ppO2 of 1.3 at 10m how do you get it down by the time you get the wreck? .... Does the automatic addition of diluent keep it down?

Come on Mal, for some one who is trimix trained and dives techy twins you are asking a silly question. Or if you are being serious and cant really figure out that adding Dil (low 02 content), wether it be air/trimix/heliair into a bag of a set high 02 content that inturn it will drop the oxgen content. I would ask for a refund on your courses.

Once I have understood this, I am likely to ask questions about what happens on the ascent.

Before I do though, why a pp02 of 1.3? What is the wisdom behind that?

Cheers
Mal[/color]

The 1.3 is used basicly because it gives a 3hr dive time before CNS reaches 100%, and also this is what the inspo scrubber is rated to.

So for most dives a 3hr RT is more than enough for BT and deco time on most dives, whilst optomising P02 levels.






ATB
Gareth