How long would it take for your pp02 to go below 0.16?

[ahar]

Morning all

In a moment of boredom I was wondering how long you could continue merrily breathing along wihout adding any 02 to the counterlungs before your pp02 would become hypoxic? I know these things are dependent on workrate and metabolism, but I was wondering about a rough figure in average conditions.

Assumptions:

Average diver at 30m mooching around a wreck with no current. Average workrate and constant depth at all times.

Starting pp02 is 1.3 - no 02 or dilutant is added at all (no matter how many times it beeps ).

How long before the pp02 drops to 0.16 or below?

If you want you can imagine a catastrophic failure of both the dil and the 02 bottles on a KISS without the diver ever looking at his guages, but it's more of a theoretical question about metabolism rather than a question on failure modes. Failure modes I'm sure would fill up a thread with several hundred posts

[nigelH]

Not quite the answer to the question but sitting in the pool tuesday night at 0.7bar on the YBOD I turned off the oxygen. 9 minutes later the pressure in the hoses and regs was exhausted and the SPG read zero so the solenoid could no longer supply O2 so the ppO2 started to drop. About another nine minutes later it hit 0.4 bar and started beeping and flashing lights at me which is what I wanted to see so I turned the O2 back on.

Since the effective volume of my lungs and the counterlungs is constant the ppO2 represents a specific quantity of O2 regardless of depth so the graph will always slope off at this rate for the same activity level (not doing much).

Nine minutes to drop 0.3bar so I would say quite a bit longer to drop from 1.3 to 0.16 but then I'd be finning about a bit. 20 minutes maybe?

Here's the dump. Time across the bottom and ppO2 up the side.
You are getting three traces each from the two computers.
The other trace at the bottom is depth so you are seeing my hand move up and down looking at the handset on my wrist.

(edit)Actually the plot is distorted by the ADV kicking in and adding 18/40 to the loop as I use up the O2 and decrease the loop volume to the point that it can no longer supply a full breath. Naturally this effect is much much larger at 3m in the pool than 30m in the sea so the steepening of the curves is an artifact of shallow where the 18% O2 dil dilutes the >30% (0.4ppO2/1.3abs) loop gas.(/edit)

[ahar]

Interesting, thanks Nigel

From your graph, and factoring in the extra requirements of finning around, cold etc we could assume 20-30 minutes for the pp02 to drop to hypoxic levels. That's quite some time.

[DaveB]

I was a little confused by this to start with and had to think about it, and I'm sure your logic is find. However, subjectively the ppO2 seems to drop more slowly at depth. However, I think this is just a result of the fact that the volume of O2 injected is so much less at depth, although the mass of O2 must be the same. As Nigel said, the ppO2 is contant, so there is the same mass of O2 in the loop independent of depth, and you burn the same amount per minute, so the drop should be the same.

This does give you lots of time to work out what is wrong, which is great, but remember that a low ppO2 at depth rapidly becomes hypoxic if no O2 is added to the loop on ascent, so initial reactions on low O2 should not include coming up.

[nigelH]

Originally Posted by DaveB

As Nigel said, the ppO2 is contant, so there is the same mass of O2 in the loop independent of depth, and you burn the same amount per minute, so the drop should be the same.

To try and quantify that I'm guessing the loop volume, including my lungs, is about 5 Litres (or it might be as high as 8L) so at 1.3bar ppO2 I have 6.5 surface Litres of O2 so to get down to 0.16bar (0.8sL) I have, even at the heavy burn of 1L/min, nearly six minutes. Now my consumption without factoring in the injected DL shows me using 0.3 bar in 9 minutes so less tha 0.17L/min (shorty in a pool but not finning) or 0.27L/min if the higher value for the loop volume was better.

This is almost the snag with CCRs. They are so efficient that you get bored looking at "1.3 1.3 1.3" time and time again and get sloppy so when it does start to fade you miss the problem until it is well set in. Conversely if I do fail to look at the readout and see the problem I have been really dumb and Darwin got me. Thankfully I'm a bit of a geek and neurotically look at the gauges quite often. This isn't such a bad thing.

[Mark Chase]

I think the big issue is being missed here.

The percentage of 02 metabolized is fairly constant and it takes a long time to breath down a loop on a CCR. People often talk about " running the CCR on manual" as if its something hard but it isn't its very easy to do because the PP02 drop is so slow.

The problem with ccr and 02 is ascent. You go up and the PP02 drops like the proverbial stone.

Its very easy to drop below life supporting levels of PP02 during ascent on a CCR and on a SCR. It also happens pretty fast. Ask any KISS diver.

ATB

Mark Chase

r

[tom]

Originally Posted by ahar

If you want you can imagine a catastrophic failure of both the dil and the 02 bottles on a KISS without the diver ever looking at his guages,

What's up - trying to prepare for diving with Janos sometime soon?

[ahar]

Originally Posted by Mark Chase

I think the big issue is being missed here.
The percentage of 02 metabolized is fairly constant and it takes a long time to breath down a loop on a CCR. P

That's why I asked the question - I wondered how long "a long time" was!

[ahar]

Originally Posted by tom

What's up - trying to prepare for diving with Janos sometime soon?

You never know, Mrs Janos may have beem doing some revenge kit fettling!

[Mark Chase]

Originally Posted by ahar

That's why I asked the question - I wondered how long "a long time" was!

Fair enough but for the sake of every one reading this its perhaps prudent to mention that your PP02 will quickley if you ascend. The 10min breath down time is for static depth only.

ATB

Mark Chase