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Thread: CO2 in loop during pre-breathe - yes/no and how much?

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    Post Response from Dr Mike at MP...

    The following is an e-mail discussion on the above. My questions could have been better, but I think I got there in the end. Unfortunately, I didn't manage to get all I wanted, but I do know two things now;

    1) I know I want a set of rules regarding prebreathe, temperature and depth for bailing out to a second rebreather,

    and

    2) I don't think there is any official public data on this so I'm going to have to get my own.


    I hope this is of some interest. If anyone has any info from APD, I'd be very grateful...


    The e-mails:

    -----Original Message-----
    From: John [mailto:]
    Sent: 16 February 2005 13:11
    To: sales@molprod.com
    Subject: Sofnolime operating temperatures


    I wonder if you could help please?

    1) Is there a minimum temperature below which sofnolime will not absorb CO2?

    2) Does a temperature of 4 to 6 degrees centigrade delay the absorption of CO2?

    The application is diving. The minimum working temperature is generally 4 degrees centigrade. These questions have arisen over a discussion as to whether switching to a different rebreather underwater is dangerous. The theory being that the new rebreather hasn't been breathed and is therefore at the ambient (e.g. 4 degrees) temperature. Will the first few breaths have high CO2 as the sofnolime will be too cold to work?


    From your diagrams it would appear that as long as the CO2 is in contact with the scrubber for at least 0.5 seconds, the CO2 will be absorbed regardless of temperature (assuming 4 degrees and above). Is this the case please?


    Many thanks,


    John

    From: Jacqueline Raeside
    Sent: 16 February 2005 17:38
    To: Michael Clarke
    Cc: Sales
    Subject: FW: Sofnolime operating temperatures


    Mike

    Can you help?


    Jackie
    ………………………………………………………………………………………
    Hi John,

    You would find it would take some minutes to establish a proper low CO2 circuit level if you were to change to a new absorber that had been cooled to below about 10°C and at temperatures below 5°C the reaction can be hard to start..

    As Sofnolime (or any soda lime) cools towards zero the rate of CO2 absorbtion falls to zero as liquid water is required for the reaction to occur. At 5°C and 5 bar the reaction rate is about 10 - 20 times slower than at 20°C and 1 bar. Pressure does not effect the rate much but low temperatures and high pressures together do have a significant effect. The design of the absorber also has an effect. It is the rate of reaction that is effected not the capacity, which remains unchanged

    For cold water diving I'm sure you are aware it is normal to prebreath the absorber for several minutes before entering cold water. This significantly increases the temperature in the reaction zone of the absorber so that it is in effect running at higher temperatures than the water - This has been demonstrated to work even in water very close to 0°C in arctic waters. The combination of warm breath and the heat generated by the reaction of CO2 with Sofnolime is what maintains the reaction at acceptably high rates a pre cooled absorber at 5°C or less is unlikely to remove all the CO2 introduced for the initial few minutes - depending on the insulation and efficiency of the absorber - I would suggest a change over underwater could be very dangerous if the existing in use absorber had already exhausted as there could already be high levels of CO2 in the circuit. It is difficult to see how to achieve this safely because you would need to use a high level of CO2 to start the unit quickly but do not want to expose the diver to these high levels. If the water was very close to freezing the absorber may start at all.

    The 0.5 seconds is the residence time required in the unused part of the absorber for a unit that is being breathed into (37°C breath at 3-4 % CO2) and in which the reaction has been established. This is then not effected much by external water temperature as the circuit gas is the only effective way of transferring heat to or from the absorber bed.


    I hope this helps.


    Regards

    Dr Mike Clarke
    Senior Technical Manager
    Tel: +44 (0)1371 830676
    Fax +44 (0)1371 830998
    e-mail: mc@molprod.com
    web site.
    www.molecularproducts.co.uk


    From: John [mailto:mdemon@talk21.com]
    Sent: 22 February 2005 10:53
    To: Michael Clarke
    Subject: RE: Sofnolime operating temperatures


    Hi Mike,

    Many thanks for your response! I wonder if I could add some parameters? I'm trying to get a picture of what happens within normal diving environments...

    You mention 5 degrees C. In the sea we rarely encounters temperatures this low. However, inland, in quarries, I have encountered temps in the 5 to 6 degree range. So, I'll make the assumption that the min temp we will encounter will be 5 degrees (any lower and different rules will apply...). In the sea, the coldest I get is around 10 degrees, +-1 degree.

    I'll use the Inspiration rebreather as a yardstick.

    Within set parameters: (e.g. >=5 degrees, second rebreather has no CO2 in the loop at the start, 5 bar or less)

    Is a pre-breathe to start the reaction always necessary?

    How long would a pre-breathe last?

    What would be a minimum temp required to guarantee no CO2 passing through un-removed?

    The use of spare rebreathers may seem vastly over-complicated - please let me explain the thinking behind it. (And please forgive any egg-sucking - I don't know your diving knowledge which I suspect will be much more extensive than mine!)

    Rebreathers give us fantastic range and duration compared with usual (OC - Open Circuit) diving. However, when they go wrong we have traditionally relied on OC bailout. This means carrying lots of extra heavy cylinders of various mixes of gas. This also results in much longer decompression times.

    The definitive solution would appear to be to have a spare rebreather to bail out to. This gives us an equal range and duration to enable a safe return without affecting decompression plans etc. (There will always be some OC bailout to cover any transition phase from the original rebreather's own cylinders.) This has been used successfully by cave divers and some of us are looking for more confirmation before applying it to deep wreck/deep open water dives.

    The question of the danger of cold scrubbers was raised. I want to define a set of rules/conditions where the dangers are minimal.

    I would like to be able to say:

    1) If the scrubber temperature (because of the ambient water temperature) is 5 degrees or higher, you can switch to and breathe a spare, un-pre-breathed rebreather at 5 bar or less without dangerous levels of CO2 getting through the scrubber unabsorbed

    and/or

    2) If a spare rebreather is pre-breathed so that the scrubber is X degrees or higher at the time of switching, you can breathe this rebreather without dangerous levels of CO2 getting through the scrubber unabsorbed at 5 bar or less. (So, any ideas on X? 10 degrees maybe?)

    If we could work out how long a pre-breathed rebreather holds its temperature for, we could also work out, if we know the insulation properties of the scrubber containing the absorbent, the time between the end of the pre-breathe to the time the temperature falls below X degrees in statement 2). If this period of time is sufficiently long enough, we could then say that as long as the rebreather has been pre-breathed, you can switch to it at any time during a dive, or for the first Y minutes. Before Y minutes is up (if less than the duration of the dive), you would have to switch to "re-breathe" the spare rebreather to give yourself another Y minutes.

    Am I getting close please?!

    Many thanks in advance for your help. This is part of an interesting ongoing set of discussions, both on the Inspiration List and on www.RebreatherWorld.com. There is a lot of theoretical discussion and empirical data from actual dives but I wanted information from the experts (yourself) rather than just other divers... I suppose having two rebreathers on each dive wouldn't harm sales of sofnolime either!

    Thanks once again,

    Yours,

    John
    …………………………………………………………………………….
    From:

    "Michael Clarke" Add to Address Book

    To:

    John

    Subject:

    RE: Sofnolime operating temperatures

    Date:

    Thu, 24 Feb 2005 16:44:21 -0000

    Hi John,

    I can see where you are going with this and agree that in principle this seems like a good idea. I am familiar with the Inspiration dive set - we do some work with our friends at APD. I've not trained on one myself, although I do dive (BSAC Dive Leader using open circuit only).

    Our experience with cold water diving comes from sponsoring the Ice Island dive a few years ago which gave us feedback on Sofnolime in very cold water - suprecooled to below zero inside an iceberg!


    The problem you are going to have is starting up an already cold absorber bed. If the stand-by absorber has been immersed in cold water without a gas flow it can be assumed it will be at water temperature within an hour or so even if it was pre breathed at the start. You would need to prebreath it at the surface before the dive anyway, as part of your safety checks to ensure you have an operational unit - I would suggest you would be seriously disappointed to find it didn't work after running out on your main circuit!


    Ideally what you need to do is to develop a procedure that allows the diver to pass some of his expired air through the fresh absorber and monitor the outlet. You may be able to accomplish this if you can run the stand by unit semi open circuit to start it, i.e. allow the expired air to pass through the fresh absorber then venting it from the circuit so that the initial output from the fresh absorber is not recirculated and hence avoids the exposure to high CO2 levels. If this can be achieved and the CO2 monitored on the inspiration dive computer you could then change over to the stand by set knowing it was working at the start and also avoid having to scrub down the initial high CO2 level in the second circuit. This would make efficient use of the second absorber CO2 capacity but would sacrifice some off the second circuit gas supply, but this may be acceptable - it also lends itself to a procedure that could be safely practiced before it's needed for real.


    I don't think we have the data to say with any safety at what temperature and pressure the second set would start without the initial CO2 slippage being an issue for the diver, especially as the work rate and breathing rate would also effect the start up time. It might be worth contacting APV to see if they have any experience of start up times from cold in diving bell conditions, or real CO2 / time data from very cold ambient conditions which are likely to be the closest thing I can think of to what you are trying to achieve. Remember when ice diving the ambient air temperature is often significantly colder that the water conditions although the heat transfer characteristics of the dive set are also different in air than water.

    I hope this helps


    Regards

    Dr Mike Clarke
    Senior Technical Manager
    Tel: +44 (0)1371 830676
    Fax +44 (0)1371 830998
    e-mail:
    mc@molprod.com
    web site.
    www.molecularproducts.co.uk

    …………………………………………………………………………………………………………

    From: John [mailto:mdemon@talk21.com]
    Sent: 24 February 2005 17:57
    To: Michael Clarke
    Subject: RE: Sofnolime operating temperatures


    Hi Mike,

    Yes, that does help in several areas.

    Firstly, it seems that running the bailout (second) rebreather semi-closed as a start-up procedure removes any possibility of a CO2 hit and would seem a sensible safety precaution. This leads me to wonder how long that SCR phase should last but I guess the standard three minutes wouldn't be a bad start point.

    Secondly, it also helps to know that there is no data. I can stop looking and we can do some tests. I will ask AP but they may not be willing to share...

    Unfortunately, CO2 is not monitored by the Inspiration. Whilst CO2 monitors exist, they aren't generally suitable for diving. Therefore detecting the CO2 would have to be done by someone with the right equipment or just try it on a human guinea pig! I have had several people tell me that they have done it successfully. I suppose I could use just 100% O2 in the loop and analyse the %O2 having been through the scrubber - if less than 100% then we know we have a potential problem...

    Back to the drawing board!

    Would you mind if I post your thoughts on the Rebreather forum?

    Many thanks again,

    John
    …………………………………………………………………………………

    Hi John ,

    No problem with posting the message to your rebreather forum

    Hope it was some help.

    Regards

    Dr Mike Clarke
    Senior Technical Manager
    Tel: +44 (0)1371 830676
    Fax +44 (0)1371 830998
    e-mail:
    mc@molprod.com
    web site.
    www.molecularproducts.co.uk


  2. #2
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    (Note - this is the follow-on from the original thread. It's been split for ease of reading...)

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