CCR/SCR - deco obligations?

[MATTBIN]

Excuse my ignorance on this one.
If I dive OC then I know I can work out my deco either by table or let the computer track it for me based on the gas I'm using e.g. 32% Nitrox, which of course has a varying PP depending on depth.
SCR - I assume its the same
CCR - how do you work it out given that the pp02 stays constant at whatever the set point is, in theory anyway. So do you cut a schedule based on a PC programme, what happens if you dont stick to the plan exactly. I expect the VR3 can be plugged into the loop and monitors pp02 and then calcs the deco obligations, but what if you a) dont have one or b) didnt have one in the past, how did you work it out then. What happens if the VR3 is monitoring and goes bang! How do you know what your deco obligations are. Do you have deco obligations on CCR if you set pp02 to 1.4?

These questions have been circulating my brain for a couple of days and although I doubt very much I'll go CCR I'd like to know none the less? Maybe others on OC would too.

Cheers
Matt

[nigelH]

Originally Posted by MATTBIN

CCR - how do you work it out given that the pp02 stays constant at whatever the set point is, in theory anyway.

In essence you use constant ppO2 tables rather than constant FO2 tables.
Most of your deco software will cut them (click the CCR box) and I printed a set of DDplan tables for my favourite mix and laminated them and stuff them in a pocket for the dreadful day when the computer packs up and my buddy is on a very different mix.

Deco software resolves the fractions of inert gases into partial pressures to do it's sums anyway so we are actually making things easier. I don't use a loop sensor on a VR3 as I prefer it to run on what I tell it I'm using and get the three cells voting rather than base everything on one other cell that might just want to go do its own thing for the afternoon.

Before I had the VR3 I set the Aladin Pro Nitrox to whatever my 1.3bar ppO2 worked out as at the bottom and took a slight penalty on deco stops where it though I was on a much weaker mix. However overstaying a stop on CCR is boring but doesn't burn significant extra gas.

I doubt very much I'll go CCR I'd like to know none the less?

Questions are the first symptom, then denial. You will be assimilated in time. Don't worry.

[Mark Chase]

Its exactly the same for the main dive plan as OC. you plan the dive using a computer package or Partial Presure tables and you tell the computer what PP your going to use at which stage of the dive. 0.7 for decent 1.0 -1.4 for the bottom phase and anything up to 1.6 for the deco phase. You can change the PPon the unit during the dive.

Just like OC if your not using a wet computer you must stick to the plan. If your using a wet computer it has to be able to work with PP02 and will need to be able to switch at least once during the dive if the depth is serious. If your working with a Nitrox computer you can run it on an equivelent gas that would give your set point at max depth and if its a multi gas computer you can switch to richer gasses on ascent to speed up deco and keep it more in line with the actual deco requirement on the unit.

Bail out is planed for OC gas in the normal way.

Hope this is clear if not then feel free to ask

ATB

Mark Chase

[NotDeadYet]

Hi Matt,

SCR is different. Depending on whether it it active (like the Dolphin) or passive SCR (RB80) then the ppO2 will vary throughout the dive and there are formulae to work out for each type of unit. Once you know the loop gas then you plug it into deco software and treat it is open circuit. The ppO2 dwindles on ascent.

[MATTBIN]

Originally Posted by nigelH

Questions are the first symptom, then denial. You will be assimilated in time. Don't worry.

Not a chance, the financial director says I cant have one!! Reckons they are dangerous, ha! what does she know. Maybe an scr though, like a motorbike is safe.

I have a feeling that comment above is going to come back and haunt me sometime in the future.

Thanks for the answers guys. Things are a bit clearer now.

Matt

[NotDeadYet]

I'd reckon CCR is much safer than SCR.

Go on, you want it and you know it...

[MATTBIN]

You lot are as insiduos as that other bunch of zealouts!!

Matt

[nigelH]

Originally Posted by MATTBIN

I have a feeling that comment above is going to come back and haunt me sometime in the future.

Don't worry. We'll just get the moderators to set it so you can't edit it and then save the URL away.

Originally Posted by MATTBIN

You lot are as insiduos as that other bunch of zealouts!!

Hey..... We don't say we're the only way. We're not always right. Just come and see. Come and try. Just one dive. Just one breath. You know it makes sense. You know you want it. No pressure. You can stop any time you want to.

[Brass Monkey]

Originally Posted by MATTBIN

You lot are as insiduos as that other bunch of zealouts!!

Matt

Resistance is futile……

Just try it once, it wont hurt you, you'll feel great. Tell you what you can try it for free the first time, just to see if you like it.

The yellow box will promise you many things, beware before you “follow the yellow box road”. "Lions and Tigers and Bears ho my". If I could get my hands on the guy the first said NITROX to me I’d slam him in the sack and he'd deserved it. There was a time when my kids used to have shoes.

Dave.

[MATTBIN]

Scr question, given that SCR's inject at a constant rate (active types) and that hard work will effectively deplete the Oxygen faster than is being replenished does this mean that the effective %mix is lower therefore any deco calculated at the original mix % will be compromised, unless there is a manual addition valve and a pp02 monitor in the loop that requires the user to crank it up a bit.

Matt