Evolution Scrubber Duration

grahamsp

I have been diving the Evolution since July this year and received my own unit two weeks ago. With just over 20 hours to date, every dive I do confirms the conclusion I came to after my very first try dive; CCR’s in general and the Evolution in particular are a much better way to dive. And in general, I can say that I like almost everything about the Evo apart from the harness.

That said, I do have one beef and it is to do with the scrubber duration limits prescribed by APD. Am I alone in thinking that Section 3.6 of the Evo manual is less than helpful? It says that the scrubber, packed with 797 grade Sofnolime (1 – 2.5 mm granule size) is good for two hours’ use based on a 3 – 4 degrees C water temperature, a breathing rate of 40 litre/minute and a CO2 rate of 1.6 litres/minute (according to my calculations, this is equivalent to an oxygen consumption rate of 1.76 litres/minute!).

I’ve picked up another bit of data. Molecular Products’ technical data sheet (01/1099A) tells us that Sofnolime 797 absorbs more than 140 litres of CO2 per kg; APD’s limit implies a maximum uptake of 91 litres per kg of Sofnolime, 65% of the data sheet figure.

OK. So far so good. I now know that, if I do a deep dive in bitterly cold water, working at rate that I suspect even Arnie Schwarzenegger at his physical peak would have had trouble sustaining, my set is good for 2 hours when packed with Sofnolime 797. Whilst I understand that all of this allows APD’s lawyers to sleep well at night, it does nothing to help me to understand the risks that I am taking if, for whatever reason, I choose to use the Evo beyond the APD prescribed 2 hour limit. I think APD’s approach is akin to Boeing limiting 747 range data to flight conditions the aircraft will rarely encounter; not wrong but not terribly helpful for everyday flight planning. For example, I don’t think it is unreasonable to ask how the scrubber performs at 50 metres in 15 - 22 degree Celsius water, conditions much closer to those I experience diving off Sydney.

And talking to other CCR divers doesn’t help much; they have no more hard data than I do but in general they seem happy to stretch scrubber durations and to use a coarser grade of Sofnolime than that specified by APD to reduce WOB on deeper dives. That’s the benefit of experience of course but I think it would be much better if the variables affecting the decision were more explicitly stated. As things stand, I really have no way of making an informed decision on the matter.

Anyway, extrapolating off the available data and drawing on the feedback from divers considerably more experienced than me, I now think I can safely extend the scrubber duration as follows:

• By using a more representative CO2 production rate of 1 litre/minute (based on my actual oxygen consumption rate measured over fifty dives), I can go up to 210 minutes;

• By using a Sofnolime CO2 uptake limit of 140 litres/kg, I can go to 180 minutes.

Taking these two together and discounting any further benefit from diving in warmer water (I haven’t been able to find any useful data at all in this regard), 180 minutes seems a reasonable limit for normal, run of the mill diving and that’s what I now work to.

I’d be interested in people’s views on all the above. Is there any way of getting a better handle on these variables in the interests of allowing me and others to manage the risks involved in CCR diving? And should APD be doing more?

Graham

nigelh

They tested it under heavy conditions and quote you a number that is safe. If you have the scrubber monitor you have a better handle on what is happening but I wouldn't just go longer on spec. CO2 is implicated in too many bad incidents.

Is 'lime that expensive where you are? I had one run in with CO2 break-through and never doing that again is much higher on my list of priorities than saving a keg a year.

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nigelh

CCR900

Even with CCR, divers do not run their gas tanks to zero. Why should it be any different with your scrubber? Yes I am sure you can squeeze more out of your scrubber but might I suggest that you keep that 'spare lime time' for when you need it. If the proverbiable dung hits the fan (just when you come to the end of your 2 hour time limit) and you get nervous and start breathing like a git and making plenty of CO2 then that extra time you never get to use might just come in handy.

Unless you are planning on doing very long duration single dives the Evolution scrubber duration is just fine.

Rgds

Fil

Mark Chase

My Classic scrubber is rated to three hours but its not often I actually do three hours on it. Dives of 120min's will result in a refill unless I am doing a shallow one the same day.

In the warm blue stuff I have pushed 4 hours out of it but thats over 3-4 shalowish dives. As soon as I contemplate depth fresh stuff goes in.

Often I have done something like 90mins on a dive and then not dived for two weeks, fresh stuff goes in again.

Its just not worth the risk for £8.00 and I like the feeling of having total confidence in the fill when I am on a dive.

The only time I am seriously contemplating pushing a scrubber is in warm water. In the UK for me a 180-200min dive is a big dive so I am not faced with the dilemma of pushing the stuff for four or five hours in cold water and relying on the fact most of the time your less than 20m.

ATB

Mark Chase

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dave t

Also dont assume that because there is 2kgs of sofnolime in the can it all gets used by the time breakthrough occurs, it doesnt there is a percentage of "wastage" thats why the molecular products quoted figure for amount of co2 absorbed per kilo cant be used in real time

best

Dave

divetheworld

All the advise above is excellent and I applaud every one of you.

To make further a point;

If your fill costs £8 for 180mins, and you push your scrubber to 240mins you will save a huge £2.00. You may also get breakthrough you may die. Bummer.

Now consider the £2.00 you are saving by pushing the scrubber that bit further. If you were panting on the sea bed with a huge CO2 hit and I said I could save you for the princely sum of £2.00 would you pay me?
Too late by then my friend, better waste the extra £2.00 now and call it insurance money.

Happy new year!

Brent.

One of the areas that you may find very interesting when making informed decisions relating to scrubber duration is "dwell time". Consider gas density, CO2 production rates and absorbtion capability under increased ambient pressure as variables effecting dwell time.
Anyone who says they have never contemplated this subject in the same way as you is probably lying!

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Andy M

Eh? I must have missed something fundamental, but why should oxygen consumption rate not be the same as CO2 production rate? How did you work this out please...

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Know your PO2 at all times

dave t

I think its because for every litre of o2 you consume you only produce 0.90 of a litre of co2

grahamsp

Thanks for all the comments. To pick up on some the points made.

Re cost. Believe me, after investing the best part of A$20 k so far in my Evo and associated bits and pieces, saving the odd dollar or two by pushing my scrubber past its use by date is not something I focus on. In any case, Sofnolime here works out at A$27 per fill so it's absolutely not about cost. My motto is when in doubt, change it out.

What it is about is knowing the safe limits of the equipment under a range of conditions. It seems pretty clear to me that, whilst we can extrapolate safely from the facts we are given by APD and others, it would be better all round to have good, trials based facts from which to work. Why APD restricts the in formation they provide to a dive profile that I doubt 1 diver in a 1000 would ever do is a matter for them. It just bugs me but hopefully one day they'll treat their customers with the respect they deserve and do the proper trials

Re variables. Yes dwell time is one of them; looking at MP's data sheet and doing some highly precise calculations on the back of a handy condom wrapper, 1 sec seemed OK to me. Can't think why!

Re CO2 production. Yes, apparently it is 90% of the O2 consumed but don't ask me why. Any chemists around who might know?

Graham

PS I should have said that the Tempstik is a great gadget and needless to say I pay it close attention!

divetheworld

I'm not a chemist, but its starting to make sense where farts come from now.
I reckon 10% of the oxygen I inspire comes out my arse, or so my wife says.

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