How long can you expose a scrubber to air?

[Agro]

I had a discussion with a CCR diver, we were talking about a non closed scrubber. How long are you allowed to leave it like this?

Theory: air contains CO2, sofnolime absorbs CO2, sofnolime looses capacity. But how much is this effect? Can you measure it?

Is there a scientific approach?

 

[davehicks]

I had a discussion with a CCR diver, we were talking about a non closed scrubber. How long are you allowed to leave it like this?

Theory: air contains CO2, sofnolime absorbs CO2, sofnolime looses capacity. But how much is this effect? Can you measure it?

Is there a scientific approach?

You should keep it sealed but a few hours won't hurt it much if there is not significant air movement over or through the scrubber.

My evidence - 20 years of CCR diving.

 

[Agro]

You should keep it sealed but a few hours won't hurt it much if there is not significant air movement over or through the scrubber.

My evidence - 20 years of CCR diving.

That's what we always hear. But did anyone measure this?

 

[Snusmumrik]

That's what we always hear. But did anyone measure this?

Yes. https://www.researchgate.net/profil...eather-carbon-dioxide-absorbent-canisters.pdf

Long words short :

Methods: An Evolution Plus™ rebreather was mechanically ventilated in a benchtop circuit. Respiratory minute volume was 45 L·min-1 and CO2 was introduced to the expiratory limb at 2 L·min-1. The scrubber canister was packed with 2.64 kg of Sofnolime 797™. Scrubbers were run in this circuit for 90 minutes then removed from the rebreather and stored in packed form under one of three conditions: "open" (unsealed) for 28 days (n = 4); vacuum "sealed" in an airtight plastic bag for 28 days (n = 5); or open overnight (n = 5). Following storage the scrubber canisters were placed back in the rebreather and run as above until the PCO2 in the inspired gas exceeded 1 kPa. The total duration of operation to reach this end-point in each storage condition was compared.

Results: The mean run times to reach an inspired CO2 of 1 kPa were 188, 241, and 239 minutes in the open-28-day, the sealed-28-day and the open-overnight storage conditions, respectively.

So we see more than 22% reduced endurance time.

Conclusion: Rebreather divers should consider placing partially used soda lime scrubber canisters in vacuum-sealed plastic bags if storing them for longer periods than overnight. If a partially used scrubber canister is to be used again the next day then the storage modality is unlikely to influence scrubber efficacy.

Personally i've prefer to use blind pugs for canister to seal it between dives.
And when filling - not to leave scrubber un-sealed for more time than needed to set it into canister and blind-plug it.

 

[rjack321]

I had a discussion with a CCR diver, we were talking about a non closed scrubber. How long are you allowed to leave it like this?

Theory: air contains CO2, sofnolime absorbs CO2, sofnolime looses capacity. But how much is this effect? Can you measure it?

Is there a scientific approach?

No there's no scientific approach because there is no standardized air flow. Nor is there a standardized scrubber.

But as a general rule it's not a big deal in the short term (day). Exhaled breath has percent quantities of CO2 and the atmosphere as part per million. The degradation experienced by a scrubber accidentally left exposed to the atmosphere overnight is inconsequential.

 

[rjack321]

So we see more than 22% reduced endurance time.

This was over a month-long duration! And in that particular axial CCR scrubber which cannot really be extrapolated to other scrubbers designs like a radial, other than "open for a month is bad"

 

[Agro]

Yes. https://www.researchgate.net/profil...eather-carbon-dioxide-absorbent-canisters.pdf

Long words short :


So we see more than 22% reduced endurance time.



Personally i've prefer to use blind pugs for canister to seal it between dives.
And when filling - not to leave scrubber un-sealed for more time than needed to set it into canister and blind-plug it.

Great link, thank you very much. Facts and figures.

 

[Agro]

This was over a month-long duration! And in that particular axial CCR scrubber which cannot really be extrapolated to other scrubbers designs like a radial, other than "open for a month is bad"

I don't fully agree.

Open for a month is bad, which is a valuable output. Other scrubber cases will produce a slighty different percantage of loss of function. Here we have about 25%, others may have 10% or 50%, which does not matter. Fact is: don't leave it open for 1 Month.

Open for 1 night does not change scrubber's efficiency on this type of scrubber. I can hardly imagine any other scrubber housing will produce a different result. So output is: you may leave it open for 1 night. This also means you dont have to hurry while filling. Valuable output as well.

 

[Mobulai]

It’s a very cool study, and it does touch on some concerns, especially with the whole repacking thing

Scrubbers were run in this circuit for 90 minutes then removed from the rebreather and stored in packed form under one of three conditions: "open" (unsealed) for 28 days (n = 4); vacuum "sealed" in an airtight plastic bag for 28 days (n = 5); or open overnight (n = 5). Following storage the scrubber canisters were placed back in the rebreather and run as above until the PCO2 in the inspired gas exceeded 1 kPa

The overnight ones seem to outperform the vaccum sealed ones (wider deviations, and the plots also kinda show it)

Given that the testbench described did introduce humidity into the loop, I wonder if that’s bad test design (depends on your goal ofc ) as unpacking and sealing would cause some clumping (and maybe even some caustic reaction)
Wouldn’t it be better to airdry overnight and then seal

I wish they had one or 2 more variations, leaving the sorb in the can (overnight open + sealing) to see the diff against non-repacked sorb

 

[rjack321]

I don't fully agree.

Open for a month is bad, which is a valuable output. Other scrubber cases will produce a slighty different percantage of loss of function. Here we have about 25%, others may have 10% or 50%, which does not matter. Fact is: don't leave it open for 1 Month.

Open for 1 night does not change scrubber's efficiency on this type of scrubber. I can hardly imagine any other scrubber housing will produce a different result. So output is: you may leave it open for 1 night. This also means you dont have to hurry while filling. Valuable output as well.

Shrugs you asked for science. Got a study with 2 data points 28 days apart which was really testing if sealing was effective for a 28 day pause between dives (it is). Decided less than overnight was science enough