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Old 18th August 2010, 09:16   #1 (permalink)
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Colorimetric end-tidal CO2 detectors

Being a med-student a just coming off an ITU attachment i came across a product i thought would interest rebreather world. It is a small plastic box that gets stuck on the end of a ET tube to test if it is located properly by changing colour quite dramatically in response to c02 (it goes from purple to bright yellow).

http://www.nellcor.com/_Catalog/PDF/..._Detection.pdf

However although they claim to last for 2 hours this is unlikely to be the case under pressure and it is far too sensitive to track changes in end tidal c02. The one thing i though it could be usefull for is to be brought into the loop toward the end of long dives (where 10 for extra security is more likely to be spent anyway) to check for scrubber breakthrough. So no where near ready to slap into you loop yet but I thought it was interesting nontheless.
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Old 18th August 2010, 11:58   #2 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

Quote: (Originally Posted by nobody) View Original Post
However although they claim to last for 2 hours this is unlikely to be the case under pressure and it is far too sensitive to track changes in end tidal c02. The one thing i though it could be usefull for is to be brought into the loop toward the end of long dives (where 10 for extra security is more likely to be spent anyway) to check for scrubber breakthrough. So no where near ready to slap into you loop yet but I thought it was interesting nontheless.
My understanding about CO2 detection in the loop is that the high humidity makes it not work. Saying that I know nothing about this subject :-(

Cheers
Matt.
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Old 18th August 2010, 13:18   #3 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

Quote: (Originally Posted by matthewoutram) View Original Post
My understanding about CO2 detection in the loop is that the high humidity makes it not work.
So how does it work in a Sentinel then?

I've seen this a bit on here, people talking about CO2 dectection as if it's the 'holy grail' of Rebreather manufacture, but ignoring the fact that it's available...

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Old 18th August 2010, 13:37   #4 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

Quote: (Originally Posted by matthewoutram) View Original Post
My understanding about CO2 detection in the loop is that the high humidity makes it not work. Saying that I know nothing about this subject :-(

Cheers
Matt.
well a quick flick through some of the manufacturers spec sheets and one or two claim this system works in 100% humidity
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Old 18th August 2010, 14:31   #5 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

Quote: (Originally Posted by nobody) View Original Post
well a quick flick through some of the manufacturers spec sheets and one or two claim this system works in 100% humidity
Reading the blurb about the Sentinel it seems that they are not very accurate, just as the OP said - it does tells you something is wrong.

The manual says it is checking the scrubber is fitted and sealed, and to warn about reduced efficiency, but not for determining duration.

So it seems to work for those cases, but does not work for determining duration.

I now know more than I did before.

Cheers
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Old 18th August 2010, 14:33   #6 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

Quote: (Originally Posted by jackdiver) View Original Post
So how does it work in a Sentinel then?

I've seen this a bit on here, people talking about CO2 dectection as if it's the 'holy grail' of Rebreather manufacture, but ignoring the fact that it's available...

I'm wondering if the holy-grail you speak of is not detection but measurement, meaning duration measurement rather than bail-out time detection?

Personally I have a stop-watch.

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Old 18th August 2010, 15:29   #7 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

Quote: (Originally Posted by matthewoutram) View Original Post
I'm wondering if the holy-grail you speak of is not detection but measurement, meaning duration measurement rather than bail-out time detection?

Personally I have a stop-watch.
I'm not looking for the grail...

I have a Sentinel, but haven't got the CO2 monitor...

My understanding is that it's been designed to detect over breathing the scrubber rather than break through, and should not be used to extend the scrubber duration.

Personally I don't see how a CO2 monitor could be used the predict scrubber duration, as surely by the time the CO2 has risen in the loop you're already screwed?


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Old 19th August 2010, 08:40   #8 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

Quote: (Originally Posted by jackdiver) View Original Post
I'm not looking for the grail...

Personally I don't see how a CO2 monitor could be used the predict scrubber duration, as surely by the time the CO2 has risen in the loop you're already screwed?

Good, because you're not going to find it

I don't see the point of the gas monitor either, for exactly the reason you say.

I don't see the point in this device as it is trying to uncover something that just doesn't happen. The only time I have heard of someone "out breathing a scrubber" is on second (or more) usage of the same scrubber. That's very easy to avoid.

I do hope the service you get for the Sentinel is better than that I get for my VR3.

Cheers
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Old 19th August 2010, 14:43   #9 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

A well designed, accurate CO2 monitor will alert a diver to any situation where inspired CO2 is above a range of acceptability. There are myriad reasons for CO2 buildup within a closed breathing loop, but the reasons do not really matter (except after the fact diagnostics) because CO2 breakthrough is the end result. If you do a bit of research on this site, you will find volumes of information on the subject and I would highly recommend that you become intimately familiar with those issues before dismissing them as something which can be avoided with a stopwatch.

As far as colorimetric detectors, they have been looked at by many people but can not track changes in CO2 levels and require a great deal of diver maintenance as to be practically useless.

I have not seen peer reviewed data on the Sentinal CO2 monitor to know if and, more importantly, how well it works. The only other rebreather CO2 monitor which has been talked about at length does not yet exist as far as I know.
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Old 19th August 2010, 14:49   #10 (permalink)
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Re: Colorimetric end-tidal CO2 detectors

Quote: (Originally Posted by saveourseas) View Original Post
A well designed, accurate CO2 monitor will alert a diver to any situation where inspired CO2 is above a range of acceptability. There are myriad reasons for CO2 buildup within a closed breathing loop, but the reasons do not really matter (except after the fact diagnostics) because CO2 breakthrough is the end result. If you do a bit of research on this site, you will find volumes of information on the subject and I would highly recommend that you become intimately familiar with those issues before dismissing them as something which can be avoided with a stopwatch.
You do not have to like, agree, or use any method that I may use to counteract CO2 issues. However the best practice I can see for avoidance of CO2 issues is to use fresh material every dive and to use it within the time constraints set by your units manufacturer. For this all I need is a stopwatch.

Thanks for your concern,
Matt.
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