What O2 flow rate to use with KISS valve/CMF orifice???

marekm

Hi everyone,

I went for a dive in moderate current today with my O2 CMF homebuild rebreather. The O2 addition was set to 0.5LPM (which is my resting O2 consumption), and thus I was expecting to press the O2 add button once every 15 to 20 minutes. In reality I was pressing the button once every 3 to 5 min. On the second dive of the day, which was a relaxing drift dive with very little activity, I pressed the button only once every 10 minutes.

Is this amount of manual O2 addition normal? Or am I doing something wrong that prevents me from having to press the O2 add once every 10 to 20 minutes?

Should I increase the O2 flow rate to above my resting O2 consumption so that it matches my average diving O2 consumption? And would this mean that rising PO2 when standing still would be something I just accept?

Thanks in advance to anyone who can set me straight on this.

-Marek

decoweenie

Marek,

First of all, don't believe everything you read on the Internet.

Just like seeing 20% larger, time and depth stretch as well underwater for a lot of people...

Having to add manual everything 10 mins at low work pace is a very good set-up. If you do deco dives, the setting might be perfect to hold your set-point - and that's what you want.

Flow a little less is better than flow a little more than needed!

It helps to train you to monitor PO2 as well as gauging your metabolic rate. Flowing too high and you will waste diluent to drop it down. All you have to do is add when flowing a little low.


.

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enkelkijken

I advise to set the O2 flow this way that at rest it kreeps up slowly.
at the bottom of the pool, put yourself at rest, set PO2 between 0.6 and 0.7
wait for 5 minutes, (no manual injections offcourse :-) PO2 may kreep up 0.1 bar, PO2 should not decrease

paul

marekm

Wow, talk about a fast response, thanks for your advice guys. Seems like KISS valves/O2 CMF orifices work differently for underwater photographers versus the average diver . Guess if I was standing still in one spot for 20 minutes waiting to take a photo of something, I would not need press the manual O2 add too often.

Given my resting O2 consumption was measured when I was sitting on the couch in a warm room watching TV, it may be a bit lower than what I need when standing still and shivering in a cold wetsuit underwater.

Thanks again.
-Marek

decoweenie

Almost all of my dives are deco dives, and I get pretty relax during deco (i.e. to the point of almost sleeping ).

And almost never have to add manually unless my depth changes - but our water temp is usually warm except for the Winter months.

About the response... Yes, I have work hard now to regain the reputation-leading-spot lost to DrMike after his Meg-sensor thread...

__________________
"...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..."

- Rebreather World PM

marekm

In that case, have some green for your efforts.

bendomatic

hey marek, rickyboy had a similar problem initially with flow rate. he also had a very low couch o2 consumption rate of about 0.4-0.5 L/min. you could try, rather than a sitting on the couch, walking easily for a couple 100m. at least this gives you an idea of what your low activity consumption rates are. and gives your neighbours a laugh! I seemed to have lucked out and got my flow rate spot on the first time i set it up.
rachel

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