How to tell how much of your scrubber you used after a dive

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LFMarm

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Question. Is there a way to tell how much of your scrubber you used when opening the canister at the end of a dive?

Context. I am not trying to find this out to reuse or save on scrubber but rather to have a feedback loop on the assumption made on scrubber duration. I would love to be able to check after a dive if the manufacturer rated scrubber duration is conservative enough for my specific diving (RMV, ambient temperature, etc.) by checking if at the end of a 2h dive in >70F the scrubber has still 45% of life left
 
On a revo and some inspo's they have temp sticks that can sense the frontier where CO2 binds with the sorb and give you an estimate of remaining scubber duration.

But to poor out the sorb an look at it NO.
 
You can get color indicating sorb. Carefully dump or scoop it from the scrubber (into the trash) and note how much or what depth the color change occured. Even with non indicating sorb the is a color/texture change to spent sorb. It gets less white as it is spent.

This may give some indication of what proportion of the sorb is still fresh after say 2 hours.

Also, in my experience if you push your scrubber past its fresh limits you can feel it as you breath. It gets a raspy taste or feel and it time to end the dive and start flushing the loop frequently.
 
As you dump the sorb there is a point where the loose sorb change to a solid clump. This is not a flat line. It gives some idea of what has been used but is NOT accurate. The front is not a sharp line that you see, it is a transition window that will very in thickness depending on depth.

It sounds like you are trying to figure out how conservative the manufacturer was when they rated the scrubber. From other posts it looks like you are trying to extend the bottom time. If you really want more bottom time, get a rebreather with a large enough scrubber to make the time. Not throw the conservative rating out the window and try to make your own numbers. Say the manufacturer is 50% reserve, but you get a bit of channeling and a spot goes 90%. Everything is fine. That is what the conservative factor is there for. As the reserve goes down, the potential goes up exponentially. Even with the manufacturer's conservative numbers there are still times where breakthroughs do happen.
 
As you dump the sorb there is a point where the loose sorb change to a solid clump. This is not a flat line. It gives some idea of what has been used but is NOT accurate. The front is not a sharp line that you see, it is a transition window that will very in thickness depending on depth.

It sounds like you are trying to figure out how conservative the manufacturer was when they rated the scrubber. From other posts it looks like you are trying to extend the bottom time. If you really want more bottom time, get a rebreather with a large enough scrubber to make the time. Not throw the conservative rating out the window and try to make your own numbers. Say the manufacturer is 50% reserve, but you get a bit of channeling and a spot goes 90%. Everything is fine. That is what the conservative factor is there for. As the reserve goes down, the potential goes up exponentially. Even with the manufacturer's conservative numbers there are still times where breakthroughs do happen.
Thanks for your comment. I am definitely trying to figure out what is a safe bottom time; without trying to cut any corners though. I understand that scrubber duration is very dependent on ambient temperature and the rating from the manufacturer only goes up to 70 F so I would like to complement that with a few observations based on my own diving to figure out if the 220 mins rated by the manufacturer could safely be pushed to 240 min in very warm waters (>80 F).
 
Thanks for your comment. I am definitely trying to figure out what is a safe bottom time; without trying to cut any corners though. I understand that scrubber duration is very dependent on ambient temperature and the rating from the manufacturer only goes up to 70 F so I would like to complement that with a few observations based on my own diving to figure out if the 220 mins rated by the manufacturer could safely be pushed to 240 min in very warm waters (>80 F).
Don't leave out the stress factor. If you have a dive when the workload is higher than normal, say swimming into a mild current, you will increase the burn rate on your scrubber. For example, I have done warm water dives in Socorro where there was a decent current flowing and had to spend extended time swimming into current or surge. I might have planned for 4 hours on the scrubber, but after 3 hours it started to be funky and breath rough.

For comparison, I usually dive in 50F (46-52F) in the PNW. On my Kiss Spirt I budget the scrubber at 3 dives or 4 hours, whichever comes first. That works great in light workloads. If I have a difficult dive I will cut it short.
 
A couple of points for using the color indicating sorb to tell actual usage:
1 - It must be checked immediately after ending the dive, as it will often begin to change back to a white color over a short amount of time.
2 - Just because you can see how much was used and now know exactly how much is left, you still MUST take into account that there needs be a layer deducted to be used as a reaction front.
3 - You will begin to get break through before you use all of the sorb, because the reaction front is diminishing.
 
The US Navy used a brand of Barium Hydroxide as an CO2 absorbent before the mid-1970s. It changed from a pink color to purple when it converted into Barium Carbonate. I'm not recommending it as a superior absorbent but I wonder if the feature is available on some modern absorbents.

It seems like it would as useful on an anesthesiology machine as a rebreather.

Ping @rsingler and @Compressor
 

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