Achieving max O2 window

divingtoors

HI There

We took our KISS converted IDA-71's for a day of "deep" diving in our favourit quarry, as the sea conditions were totally blown out, and had some time on a shotline to check various things on the IDA's, especially the PPO2 meters and the change of PPO2 in relation to time.

One thing that I cannot understand, as it happened to both me and my buddy, was the inability to reach a 1.6 Bar setpoint on our 6m decobar, where we spent some time "playing with the setpoints.

The maximum we could both reach was apprx 1.5 bar PPo2.

Why would this be?

We weren't at sea level (0m), we were propably about 150m up, which I dont think will make such a big defference...
It was a freshwater dive, so does that mean our Suunto's didnt read the correct depth, due to the lower density of Fresh water, and we werent actually at 6m, but shallower ?

Any other explanations as to why we could not reach the desired 1.6 bar PPO2 at our 6m stop.

Regards

Johan

wedivebc

A couple of things come to mind. It is common to get less than ideal flush at 6m since depending on how the Rebreather is contructed it may take a while to remove all inert gas.
As far as the depth calibration I believe Suunto are calibrated in fresh water to that shouldn't matter, in fact even if it wasn't it is responding to the same pressure your sensors are regardless of what the actual depth is.
I also find moist sensors tend to read a little lower than dry ones for some reason.

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Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die

nigelh

6% other in an 6L loop is 360ccs.
About a cup full.
You really aren't going to get better than that with out a vacuum pump and a big pot of O2.

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nigelh

Drmike

Even in seawater with correct depth it should be very hard to get 1.6 @6m if your calibration is correct. If I get 1.5+ Im happy. If I get 1.6 then I wouldnt be happy and would assume my cal was wrong.

When you 1st get to 6m stop you are still off gassing a lot into the loop and thats diluting it down with each breath, you also have 100% humidity in the loop - more dilution, and its impossible to suck a vaccum in your loop (whilst still on it) to remove all gases. I usually wait for some time at my 6m stop before trying to bring the ppo2 up to 1.5+ otherwise it just wastes a lot of gas. Haha I recall once even accidentaly running our of O2 during a 6m stop due to excess flushing/cell checking luckily I was on a deco bar with surface supplied O2 handy (Since then I put a p-ported 2nd stage on the surface supplied rig )

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Cave diving is a sport
Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

Yankee Rebel

iirc its almost impossible to get a true 1.6PPO2 at 6meters. the US Navy iirc agrees with this, and their flushing procedure on their LAR V has reflected that as well.

in order to get a true 1.6 PPO2 at 6 meters it will take ALOT of flushing and ALOT of O2 to try and flush out every nook and cranny of your loop. if it was me. 1.5 would make me plenty happy, and id run my tables accordingly, most likely with a lower setpoint just to compensate for that.

fwiw the USN EDU has also determined that the residual Nitrogen contained in the body is about 1 liter at sea level if the body is saturated at sea level, iirc.
later part obtained here:
bmj.com Rapid Responses for Wilmshurst, 317 (7164) 996-999
listen to others though, they know what they are talking about. im still learning as i go, but i hope this is of some help.

divingtoors

Thank you guys...

The info is great, and makes a lot of sense....

Regards

Johan

Padowan

Even if you've not been diving (and breathing an gas with inerts at pressure) in order to maintain 1.6bar at 6m, you need to have offgassed the surface Nitrogen (0.79bar saturated before start) so as not to dilute the loop. This is why the Navy protocols require such a high time flushing.

I'm not sure exactly what volume of N2 there is in a body saturated at 0.79bar on the surface, but compared to a loop volume of maybe only 6L, I think it could well be a significant amount.

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nigelh

Using a lot of guesswork and solubility numbers for lipids I'd guess about 0.72 grams of disolved nitrogen in a 70Kg person so about a litre.

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nigelh

dteubner

Making various assumptions about weight, percent body fat etc it works out to be about 1.5 litres at 1ATA. I made a simple calculator for inert gas volumes at Research etc Excess gas volume calculator - about 5th link from the top.

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Dave T

Hanlon's Razor - Never attribute to malice that which can be adequately explained by stupidity.