scrubber safety margin

jmurba

Hi all,

I dive a rebreather using a dolphin loop with a dolphin canister.

recently my buddy surfaced and was sick (nausea), this was certainy because he had had a hard night before the dive (big party).
however I peronnaly suspect a slight co2 problem cause he was at the end of his scrubber life.

So I started looking for accurate info concerning the dräger scrubber.

I am used no to exceed 200 minutes on my scrubber (I dive in 13° to 20° celcius water on the french riviera)

I found this dräger pdf

http://www.aqualung.com/technical_li...20Duration.pdf

It says that with a vo2 of 1.33 in a 4° celcius water the breakthrough happens after 250 minutes.

it means that contrary to what I thought I am not very far from the limit .

arround me many get up to 300° on their dolphin scrubber !!!

I have absolutly no intention to push the limits I'll won't exceed 180' in the future.

But my question is : if I never got any co2 hit where I was so close to the limit it must be because of the fact I don't dive at 4° but at 13° in winter and 20° in summer time, what is this effect of temperature on the scrubber life ?

I know there is an effect of course, but how longer a scubber can last when the diver dives in a 10° warmer water then the test water temperature ?

mabye isn' it "politly correct" to ask this question cause some may use this info to push their scrubber

as for me I just would like to know my safety margin.

how far I am to the break ?

what does it means when the co2 concentration reaches 0.05 % ?

does it mean that at 0.05% of co2 the "burning front" of the sorb has reached the bottom of the cannister and that in minutes there will be absolutly no more absorbtion ?

sorry for the long message and numerous questions

regards

jean mi

__________________
when will I be able to think about something else then spending hours underwater, when will I be normal ?

http://web.mac.com/jmurba/Site_2/home.html
http://web.mac.com/jmurba/Site_3/home.html

Brad_Horn

Jean,

You might be able to find some NEDU reports on testing the LARV (Mk25?) scrubber on Rubicon.

Actual tables for scrubber use, would likely be in the USN Diving Manual either version 5 or 6.

Additionally, a fair number of your questions are answered by presenters during the Jan 2008 DAN Tech Conference. Specifically during the Rebreather-Workshop section.
DAN Divers Alert Network

Regards
Brad

jmurba

thanks brad I'll have a look to all of that
regards
jean mi

__________________
when will I be able to think about something else then spending hours underwater, when will I be normal ?

http://web.mac.com/jmurba/Site_2/home.html
http://web.mac.com/jmurba/Site_3/home.html

WD8CDH

I don't watch my scrubber usage by time, rather I use O2 consumption corrected by scrubber temperature. Since CO2 production is directly proportional to O2 metabolization, I feel that scrubber life is more closely tied to O2 consumption than to time, especially with variations in work loads.

__________________
Ron Schroeder
Former Commercial Diver, now Lazy Diver

sdussault

As a general rule I change out the Dive sorb every 4 hours. From the previous post how do you calcualte when to change the Dive sorb O2 consumption corrected by scrubber temperature?

jmurba

How do you count the time ? the diving time on your computer or do you add to this time the pre-breathing time and the surface time where the computer does not count the time passing?I try to evaluate roughly the pre-breathing time and surface time and take it in account in the scruber use timeregardsjean mi

__________________
when will I be able to think about something else then spending hours underwater, when will I be normal ?

http://web.mac.com/jmurba/Site_2/home.html
http://web.mac.com/jmurba/Site_3/home.html

WD8CDH

Conversion of O2 to CO2 is nearly 1 to 1 depending on diet and exertion but is almost always between .8 and 1.2 to 1. That RATIO is a lot more constant than CO2 PRODUCTION per unit of time. Once I know a particular scrubber's lifetime at a particular set of conditions and safety margin, I use that particular amount of O2 used as my limit rather than the time. So if I am using up my O2 faster, I know that I am using up my scrubber faster too.

For example, if I know that my scrubber will last until my filled O2 tank is is down to 900psi with a light load, I know it will still be good if I breath the tank down to 900psi with a heavy load.

I do not discount for extra loop purges etc. I just take that extra O2 use as a better safety margin.

In cold water, I just allow for 75% of the pressure drop sins I assume only 75% effectiveness of the scrubber.

__________________
Ron Schroeder
Former Commercial Diver, now Lazy Diver

jmurba

hi,
how do you find the co2 absorption capacity of your scrubber?
do you simply get the datas on the sorb datasheet ?
how do you take in account the influence of your cannister design on this absorption capacity ?
I mean theoricaly one kilo of a given sorb has a given co2 absorption power
but this kilo used in a radial cannister will not have the same capacity then when used in a donut shaped cannister .
for canister design seems to have an impact on the canister efficiency (as far as i know and I don't know mutch about canisters).
this why there are breathing machine taht are used to test the different canisters and not only the sorb itself no ?
I think....but I am ready to change my opinion that if time is taken in account by the makers and EC testers it is because this critéria is the better on provided the tests are beeing made at high exertion levels and in cold conditions which is the case.
I am not trying to criticize your methode of course I just try to better my technical culture as for co2 scrubbers.
regards
jean mi

__________________
when will I be able to think about something else then spending hours underwater, when will I be normal ?

http://web.mac.com/jmurba/Site_2/home.html
http://web.mac.com/jmurba/Site_3/home.html

WD8CDH

Hi jean mi,

Very valid question. In a way, the method I use for determining scrubber useful life is more of a way of maintaining a CONSTANT safety margin over different conditions rather than an absolute time limit. Once you know the scrubber's life under ANY particular condition, when you translate that to O2 used, that amount of O2 consumption will use up about the same percentage of you scrubber even at different workloads. It will even hold true with other divers running the same rebreather configuration since everybody has a similar O2 in/CO2 out ratio. It may not be perfect but it is far closer than going by absolute time. The MARGIN part of Safety Margin is for the slight differances anyway.

If you go by time only, you have to go by worst case. That could give you not enough Safety Margin if you underestimate you maximum workload.

The baseline capacity could be from sorb datasheets with an efficency guess, Rebreather manufacturers specs if they publish the conditions or from field tests.

Over the years, I have used this method and compared the sorb color indicator point in a clear scrubber to my O2 usage and found it very consistant, even when other divers used my rig.

__________________
Ron Schroeder
Former Commercial Diver, now Lazy Diver

firemandiver

It may be a little too simple, as I probably waste good scrubber material, but I never exceed 2.5 hours on my scrubber, regardless of dive conditions. I think of it as an extra safety net. I'd hate to bet my life, or well being, on $5 worth of kitty litter!

Thanks, FD.