Published Scrubber Research

[Mark Chase]

Funny. The point was more that you can't assume 0.85 is anywhere near the actual ratio of CO2 to O2 because it vary significantly and fast. So when I see guys rip out calculations based on various assumptions that they've somehow decided mean they can run this sorb change for a 3l O2 cylinder however long that takes, I shudder at the wrongness of it all.

The problem is with the test datas showing 135min run time and me having at least 2-300 dives where I have run 180 -220mins and a few longer

Thers got to be some factor that kept me alive and I had always put thst down to my SAC / C02 production being massivly lower thant the figures used in the testing.

And I am a light weight

Thers divers doing 9-10 hours on an inspo scrubber

SO what IS the basius for this? ANd how do we estimate it?

I have always used V02 as a guide. I have a long history of data for my SAC so I am confident its going to be in the ball park and just so long as i understand increased SAC = decreased scrubber time I feel I have some level of control over the situation.


Its not as if I do the math calculate 7 hours and dive it for 6 hours 59 mins. I have done the math and work 30% inside the result but it does appear that my upper end estimate was wrong for Spherasorb and I was running much closer to the actual limit than I had calculated.

 

[Captain Starfish]

I'm eagerly awaiting Simon's report on temp stick utility so I can see just how hard I happen to be pushing sorb. At the moment I clip it to 3-4 hours max, when I get a temp stick I'll probably keep it there for the time being because knowing where the front is doesn't help much unless you know how deep the front is and how much reserve bed you need to leave to prevent breakthrough.

 

[apitkin]

The problem is with the test datas showing 135min run time and me having at least 2-300 dives where I have run 180 -220mins and a few longer

Thers got to be some factor that kept me alive and I had always put thst down to my SAC / C02 production being massivly lower thant the figures used in the testing.

And I am a light weight

Thers divers doing 9-10 hours on an inspo scrubber

SO what IS the basius for this? ANd how do we estimate it?

I have always used V02 as a guide. I have a long history of data for my SAC so I am confident its going to be in the ball park and just so long as i understand increased SAC = decreased scrubber time I feel I have some level of control over the situation.


Its not as if I do the math calculate 7 hours and dive it for 6 hours 59 mins. I have done the math and work 30% inside the result but it does appear that my upper end estimate was wrong for Spherasorb and I was running much closer to the actual limit than I had calculated.

You're quite correct, your CO2 production is much lower than used in the testing. The problem is, you have no way of knowing how what it is, and your method of estimating it is highly unreliable, in my opinion.

Many of us are doing dives much much longer than the rated duration of the scrubber and we are relying on that principle. I have done 12 hours on a 2.7kg Prism scrubber but 3/4 of that was deco. I have no idea how much longer it would have been before getting breakthrough.

 

[matt]

The problem is with the test datas showing 135min run time and me having at least 2-300 dives where I have run 180 -220mins and a few longer

The test in the paper isn't using characteristics we are familiar with - 6 METs, at the surface, 20c, and to 1kPa. That's like a good long surface swim in the Med, not a bimble off the south-coast at 50m. And the CE endurance limit is 5mbar (0.5kPa) not 1kPa per this test. The paper explains why these parameters are selected, but I would have prefered to see the values much closer to what you and I see as useful; ~4 METs, at depth (40m), 15c and to 0.5kPa. Or selecting the CE conditions.

Unless there is more to it it looks like co-incidence that the result for Sofnolime 797 is close the 3hrs (a value we are all familiar with).

Thers divers doing 9-10 hours on an inspo scrubber

Exactly. I have done those dives in warm water (28c) at more like 3 METs without issue. And with the Tempstick and the CO2 sensor (5mbar) telling me all is fine. I do not sense CO2 either.

http://www.futurefit.co.uk/uploads/documents/SOPT downloadable resources/MET values_RM.pdf

I think we'd all like to know how to correct these figures for warmer-water and sensible/realistic exercises rate and at different depths.

The argument is the same as the "50RMV is too high", it may be the best to use, but it's not necessarily that useful in the real-world. This test was done at 44RMV, BTW.

That said this is the best information I have ever seen on the topic and we must thank Simon for that.

I'm intrigued to know the technique and measure for tight-pack, and if that affects WOB, at all. 2.64kg of Sofnolime is 0.19kg more than needed for a normal-pack (7.75% more).

Cheers
Matt.

 

[matt]

I'm eagerly awaiting Simon's report on temp stick utility so I can see just how hard I happen to be pushing sorb. At the moment I clip it to 3-4 hours max, when I get a temp stick I'll probably keep it there for the time being because knowing where the front is doesn't help much unless you know how deep the front is and how much reserve bed you need to leave to prevent breakthrough.

Me too, as I have done 100's of in-water trials with it and I figure it works well, and has given me massive opportunity diving around the world.

I'm also keen to see the storage-of-used-lime-test. I've just completed 4 dives over 3 months with a scrubber packed 91 days before last use (7c, 240 mins) and the Tempstick is at 50%.

Matt.

 

[Mark Chase]

You're quite correct, your CO2 production is much lower than used in the testing. The problem is, you have no way of knowing how what it is, and your method of estimating it is highly unreliable, in my opinion.

Many of us are doing dives much much longer than the rated duration of the scrubber and we are relying on that principle. I have done 12 hours on a 2.7kg Prism scrubber but 3/4 of that was deco. I have no idea how much longer it would have been before getting breakthrough.

SO what system did you use befroe the 12 hour dive to decide wether or not the scrubber could manage the planned dive?


As i say I use the V02 system and its not perfict I am sure but then I always allowed a 30% margin of error.

Telling me V02 calculation alters when the SAC changes is stateing the obvious WHat i want to know is a normal diver on a normal dive what will cause a variation in C02 production on a 15 SAC with normal workload that would make my calculation dangerously inacurate?

I think we have ruled out drinking cola mid dive

ATB

 

[Captain Starfish]

For fixed exercise levels and assuming no food (or at least very high GI food) within an hour or two before the dive, I'm guessing but I think it would stay pretty constant unless the immersion itself triggers some response in metabolic function.

So then you're down to the fact that, as a function of diet/fitness/health, you could be steady anywhere in the range 0.6 - 1.2 and you're assuming you're at 0.85. So you could be guessing 42% higher than actual or 43% lower than actual.

 

[Simon Mitchell]

Hello Guys,

It is not really appropriate to try to interpolate scrubber durations in real world diving from our data. That was not the point of the study, and if it had been, we would have run the trials with rebreathers immersed in water, and perhaps tried a few CO2 exposure regimens that are more confluent with what we actually do (such as a period of simulated exercise at the start of the dive, followed by a period of comparative rest - corresponding to decompression).

The purpose of the experiment was to compare two sorbs under a set of very standardised conditions because there has been debate about the relative durations of these two products. I think the relative duration question has been answered. Strictly speaking, we should not extrapolate even that beyond the bounds of our experimental conditions, but I think the point has been made that spherasorb is not as good a CO2 absorbent as sofnolime 797 in a diving rebreather, and that probably applies across a wide range of conditions. As Paul Raymaekers has pointed out elsewhere the difference may be exaggerated in very cold water.

And the CE endurance limit is 5mbar (0.5kPa) not 1kPa per this test.

Hi Matt, if you go to figure 2 you can "slide" the end point line up or down to whatever endurance limit you want. As you will see, because both curves are steep it really makes little difference to the fundamental conclusions.

Or selecting the CE conditions.

We didn't select the CE conditions because we are not testing under pressure and I have never seen a physiological explanation for the other parameters. I can, at least, explain the physiological provenance of our test parameters because they were derived from human testing at a level of sustained exercise considered relevant to real world diving.

In relation to duration under more real world conditions you will probably be interested in the temp stick study because (using sofnolime) we conducted a number of experimental runs with simulated 6 met exercise for a period of "bottom time", followed by 2 met exercise (to simulate relative rest during decompression) until the scrubber broke through. As you might expect the scrubber lasts a lot longer under these conditions. The rebreather was immersed for these experiments. The data will be presented at Eurotek.

Simon M

 

[matt]

Hi Matt, if you go to figure 2 you can "slide" the end point line up or down to whatever endurance limit you want. As you will see, because both curves are steep it really makes little difference to the fundamental conclusions.

Yes, I see that. I guess that the slope is less for less work-load, but for this test it didn't matter.

In relation to duration under more real world conditions you will probably be interested in the temp stick study because (using sofnolime) we conducted a number of experimental runs with simulated 6 met exercise for a period of "bottom time", followed by 2 met exercise (to simulate relative rest during decompression) until the scrubber broke through. As you might expect the scrubber lasts a lot longer under these conditions. The rebreather was immersed for these experiments. The data will be presented at Eurotek.

Simon M

Yes, I am very interested in these results. I have done many manned trials with the TempStick in a very unscientific test (i.e. me on a dive). I've not done much in cold (10c) water; that would be really fascinating. I have done a fair bit in warm-water (>25c).

I did recently do a little test for storing a scrubber in my bedroom, uncovered. It's on no scientific value, but for me at least I didn't see any need to store the scrubber sealed. It was also cold water (image below, 4 x 1 hour dives, total time ~240 mins).

The test I would really like to see is one where we have 3 mets for period x followed by 8 mets for a few minutes. I suspect that towards the end of the scrubber life the 8 mets will breakthrough much more rapidly than the earlier part of the test. I won't be doing that test myself.

My checking with the TempStick is done against the CO2 sensor, so totally unscientific.

Matt.

 

[MadUKDiver]

I agree Matt, the results will be interesting and Simon's careful interpretation / messaging even more so.

My unscientific test tells me not to be paranoid about storing lime in the unit. These 2 dives are 180+ days apart with the scrubber cartridge just lying on my bench in the garage in between (unheated building, UK climate). Temp stick shown just before ascent.

​

Certainly no need to panic if a dive gets cancelled and it's a week or so before the next one.