Scrubber duration in warm water

[DA Aquamaster]

If you do the calculations of total CO2 removal capacity, then convert that into metabolized oxygen, then convert that into the capacity of the O2 cylinder, you should get a number.

How useful, depends. Poseidon stipulates that the 3L O2 be filled to 135 bar. Reason? thats the amount of O2-CO2-scrubber that their scrubber can handle. Ive noticed that in colder water when working hard, I get a little over 180 min. Warm water, relaxed dive I get closer to 220 min.

When I change the O2 its time to change the scrubber. FWIW the Poseidon is also rated CE for 180 min.

The theoretical numbers include a lot of variables that may or may not apply to your actual diving, and they involve some assumptions that may or may not be accurate - but it's a best case starting point.

I think your observation on the Poseidon scrubber versus O2 limit often works that way in other designs as well, probably by intentional design. There may be some merit to having a "balanced" design where O2 capacity doesn't encourage the diver to exceed the scrubber limit.

The math and chemistry more or less support this on our KISS Sidekicks, in warm water at least.

For example, Intersorb 812's spec sheet indicates it will absorb 245 liters per kilogram of CO2 at a scrubber temp of 90 degrees, 40 Lpm inspiration rates and 1.6 Lpm CO2 production rates. Our KISS Sidekick scrubbers hold 2.5 kg of Intersorb 812, and that gives a theoretical scrubber limit of 612 Liters of CO2 absorbed (assuming an unlikely 100% efficiency). However, at a conservative 1:1 ratio that would require 612 liters of O2 to be metabolized in order to max out the scrubber. If you back off to a more likely ratio of .8 Lpm CO2 produced per 1.0 L of O2 metabolized, you'd need 765 liters of metabolized O2 to max out the scrubber.

Given that we dive a 2L, 205 bar O2 bottle with 410 liters of O2, and use perhaps another 80-100 L of O2 from the diluent, (call it 500 L of O2 total metabolized on the dive) we should run out of O2 before we exceed the scrubber capacity, even at the more conservative 1:1 ratio, assuming warm water and light levels of exertion.

In terms of actual O2 use, we've found that we average 1.0 to 1.25 Lpm on multi-level cave dives with set points in the 1.0-1.2 range, and an O2 flush to check for current limited sensors. That gives an average O2 duration of about 5.4 hours (1.25 Lpm) with either an AL 13 or Faber 15 O2 bottle filled to 3000 psi.

In practice we've done 4 hour swimming dives using Intersorb 812 on our Sidekicks in 70 degree water, and with light to moderate work load. That's well short of the theoretical maximum, but that's also as far as we are inclined to push it. It's probably worth noting that the scrubber in the Sidekick is also surrounded by the counter lung and even the exterior of the scrubber tends to stay slightly warmer than the water temperature.

-----

Spherasorb 408's spec sheet indicates it will absorb 184 Liters per kg, and given that our Sidekick Scubbers hold 2.4 kg of the bulkier 408 and that gives a theoretical capacity of 441 Liters of CO2.

At a 1:1 ratio, and considering the maximum possible O2 metabolized in the diluent on a multi-level dive to (conservative) to 500 L total, the O2 available in a 2L bottle is now slightly exceeding the scrubber duration. At a more likely 0.8 : 1.0 ratio, the O2 should still exceed the scrubber capacity on a warm water, light to moderate effort dive, but 8-12 mesh sorb adds a greater safety margin.

Obviously, in either case if you used a 3L bottle filled to capacity with a Sidekick, you're now in a position where the scrubber capacity will be the limiting factor on the dive, even in warm water. And in cold water, the limiting factor would almost certainly be the scrubber, given decreased efficiency of the reactions involved and greater potential for breakthrough.

 

[DA Aquamaster]

.../

/....If you're already spending a bunch of coin on the CE certification and testing, and that gives you a scrubber duration number that will probably be quite defensible in a legal arena, why would you pay to do additional testing?

And finally, how many people really do dives that exceed the duration of CE scrubber rating? The number of people that do dives in excess of 3.5 in duration is very small.

Legally defensible is the key.

It doesn't really matter how many people are exceeding the CE rated scrubber duration. 100% of those that do are exceeding the unit's rated scrubber duration and the company's attorneys will use that as part of any legal defense in the event of an accident.

Would a scrubber duration based on test standards that reflect both warm and cold water temps and more reasonable ventilation and CO2 production rates be more useful to divers? Absolutely.

Would those more realistic numbers sell more units? Probably.

Would some moron exceed the ventilation and CO2 production rates of the standard and suffer a CO2 hit and die? Almost certainly.

And then his or her grieving spouse would sue and win.

Which is why we can't have nice things, like realistic scrubber duration estimates.

----

I agree with you Ken that the number or percentage of divers who are doing dives in excess of 3.5 hours is small. However, I suspect the number and percentage of CCR divers doing two dives on a single fill of sorb in the scrubber that exceed 3.5 hours in total is a lot larger. That practice also adds a number of new variables into scrubber efficiency, and is potentially even more problematic.

Even with realistic scrubber numbers, I suspect the single dive efficiency would be greater than the multiple dive efficiency, particularly if the diver left the scrubber packed over night between dives.

Testing would have to account for all of that as well as the other variables, and it's not likely that any company or cert agency is going to take that risk.

 

[doctormike]

Legally defensible is the key.

It doesn't really matter how many people are exceeding the CE rated scrubber duration. 100% of those that do are exceeding the unit's rated scrubber duration and the company's attorneys will use that as part of any legal defense in the event of an accident.

Would a scrubber duration based on test standards that reflect both warm and cold water temps and more reasonable ventilation and CO2 production rates be more useful to divers? Absolutely.

Would those more realistic numbers sell more units? Probably.

Would some moron exceed the ventilation and CO2 production rates of the standard and suffer a CO2 hit and die? Almost certainly.

And then his or her grieving spouse would sue and win.

Which is why we can't have nice things, like realistic scrubber duration estimates.

But why would the 4°C CE standards be a reliable defense for the manufacturer, but a 25° C duration calculated with the same (overly conservative) ventilation and CO2 production rates not be a reliable defense?

If you are saying that a CCR manufacturer has to provide extremely conservative scrubber estimates that don't reflect real world usage for many divers for legal reasons, why wouldn't the same apply to dive computer manufacturers and their algorithms?

I'm really interested in this question, not trying to be difficult. I realize that I am a CCR newbie, but it just seems that there is a lot of temperature related variability in scrubber duration, and it makes sense to determine that with real data.

 

[kensuf]

Right, but the same manufacturer can produce a non-CE version for international sales (like Meg and JJ do, and maybe others). That doesn't impair European sales, but it does give the manufacturer a sales advantage elsewhere. For example, if JJ didn't sell an international version, I would have to consider the downside of possibly not being able to rent tanks locally when I dive in the Caribbean (like I did last week), and maybe I would go with another unit. So you might pay to do additional testing if it would make your product more attractive to people who were buying non-CE rebreathers anyway (USA, Caribbean, Pacific, South America, middle east - not a small diving market). Whether or not the additional sales would pay for additional testing is a business question that I can't answer.

And for me, it's not necessarily about long dives, it's logistics and avoiding extra scrubber changes for multi dive days. Again, not looking for a safety shortcut, I'm looking for reliable data about how long the scrubber actually lasts in the relevant conditions.

But why would the 4°C CE standards be a reliable defense for the manufacturer, but a 25° C duration calculated with the same (overly conservative) ventilation and CO2 production rates not be a reliable defense?

If you are saying that a CCR manufacturer has to provide extremely conservative scrubber estimates that don't reflect real world usage for many divers for legal reasons, why wouldn't the same apply to dive computer manufacturers and their algorithms?

I'm really interested in this question, not trying to be difficult. I realize that I am a CCR newbie, but it just seems that there is a lot of temperature related variability in scrubber duration, and it makes sense to determine that with real data.

Mike,

I'm replying to a couple of things here in this one post..

As I said above, the manufacturers have to spend a fair amount of money for the CE rating. Those CE tests result in a fairly conservative scrubber duration, which will should keep most people safe. What advantage is there to the CCR manufacturer for them to pay for additional tests that will produce less conservative numbers?

Now, let's say manufacturer Y provided scrubber duration test data in accordance with the CE standards (4° C) and also provided scrubber duration test data for 25° C water. How long do you think it would be before someone decides to start using the 25° C duration data for all of their dives, including in water temps in-between 4° and 25°?

Regarding the logistics of scrubber changes and multi-day diving, my general rule of thumb is that if I'm diving deep, I always start with a fresh packed scrubber. If the scrubber has sat for a week, it gets tossed. If the time on the sorb plus the planned time of the next dive exceeds 80% of what my can is good for, it gets tossed and I start with a fresh packed scrubber.

At the end of the day, sorb is roughly $15-20 a fill. Even in very remote places, I'm only paying $30 a fill for sorb. The risks of breakthrough by playing silly games to extend the duration of my scrubber is not worth it to me.

 

[doctormike]

Now, let's say manufacturer Y provided scrubber duration test data in accordance with the CE standards (4° C) and also provided scrubber duration test data for 25° C water. How long do you think it would be before someone decides to start using the 25° C duration data for all of their dives, including in water temps in-between 4° and 25°?

Thanks for the reply, Ken - I appreciate your experience, and I'm very new at this! But I guess I would say that any diver is responsible for following parameters and standards (e.g. gas planning, gradient factors, ascent rates, setpoints, scrubber duration), and that deviating from those standards is always going to involve increased risks. I just don't see why providing accurate information would encourage that more than having only one data point. I guess your point is that using 4° C is to build in a buffer so that everybody will be changing sorb more frequently than may be required at warmer temperatures, but my point is that there is no end to that strategy. At some point you make the standards so conservative that they may actually promote pushing limits more than discouraging it.

Regarding the logistics of scrubber changes and multi-day diving, my general rule of thumb is that if I'm diving deep, I always start with a fresh packed scrubber. If the scrubber has sat for a week, it gets tossed. If the time on the sorb plus the planned time of the next dive exceeds 80% of what my can is good for, it gets tossed and I start with a fresh packed scrubber.

At the end of the day, sorb is roughly $15-20 a fill. Even in very remote places, I'm only paying $30 a fill for sorb. The risks of breakthrough by playing silly games to extend the duration of my scrubber is not worth it to me.

Right, and this is more relevant to me doing relatively warm, shallow dives. Which of course begs the question of why I am using a CCR to do warm, shallow dives, but I already did a thread about that question!

For example, I was diving in St. Croix last week, doing about 3-4 hours of diving a day. So I pack the scrubber in the morning, head out for some dives, and for the last dive of the day (the spectacular Fredericksted pier), do I stop 45 minutes into an awesome dive in 30 feet of 80 degree water, or do I let the scrubber go to 3:20? Tempting, but I'm a new diver, so I did the conservative thing. It would just be nice to have some actual data for that scenario.

Anyway, I do appreciate everyone's input here...

 

[kensuf]

Thanks for the reply, Ken - I appreciate your experience, and I'm very new at this! But I guess I would say that any diver is responsible for following parameters and standards (e.g. gas planning, gradient factors, ascent rates, setpoints, scrubber duration), and that deviating from those standards is always going to involve increased risks. I just don't see why providing accurate information would encourage that more than having only one data point. I guess your point is that using 4° C is to build in a buffer so that everybody will be changing sorb more frequently than may be required at warmer temperatures, but my point is that there is no end to that strategy. At some point you make the standards so conservative that they may actually promote pushing limits more than discouraging it.

Without having a method to measure my blood alcohol level, how many shots of whiskey can I have on Friday night and still drive home from the bar?

What about tequila?

How many double IPA's?

Sometimes it's best to just go with the most conservative answer and accept that anyone going beyond that conservative number is off the reservation.

BTW -- we've already seen Teledyne leave the CCR market after a lawsuit was brought against them. Teledyne decided the worldwide CCR market was too small, and the lawsuits were too big, that it was not worth their time to manufacture sensors. I completely understand why CCR manufacturers would be unwilling to provide additional datapoints that may encourage people to make risky decisions.

 

[MegDiver792]

My dive buddy and I dive 40-50d water in our megs. We easily get 4 hours, if we are running scooters, we'll go 4.5-5 although I get nervous past 4.5.
Heading for truk end of the year, and figure will get close to 6 hours.

So theres at least one real world example, and these are all deep water tec dives(200ft+)

 

[Tassi Devil Diver]

Well I am glad when I push my scrubber on my rEvo beyond the manufacturers recommendations I am doing it with empirical scrubber performance data, ie rMs temp probes, if they fail, which they do from time to time it’s back to the manufacturers recommendations for scrubber duration.

 

[wedivebc]

I believe the answer is cost of testing for what return? I know rEvo owns their own breathing machine and tests their scrubber duration and WOB extensively in house. I know ISC spends massive amounts of money on independent testing (HSE in UK) to get the data you quoted above. For other manufacturers to get that level of testing they need to spend lots of money. I doubt the rewards justify the investment in most cases.

 

[Dr Simon Mitchell]

Hello,

This is a very tricky subject.

Testing at different temperatures is only part of the issue as others have alluded to. An even bigger influence on scrubber duration is the diver's CO2 production; that is, their workload during the dive. To provide more pragmatic estimates of duration, manufacturers would have to test over a range of temperatures, depths, and workloads because all of these affect scrubber duration. And even then it is debatable how useful these would be in actually titrating dive time against anticipated scrubber duration. How, for example, can a diver accurately track their workload. Maybe by oxygen consumption, but this presupposes that no gas is lost from the loop other than by metabolism. It all gets very difficult, and as others have said, I can completely understand why manufacturers test against a single "demanding" standard (moderate exercise, moderate depth, cold) and leave it at that.

Having said that, I will give you a sense of how duration can vary with workload. However, what I am about to say has to be placed firmly in context. We have been performing various experiments with scrubbers to answer certain questions, usually relating to comparisons of things like different scrubber materials, or different ways of storing scrubbers. Some of these data are published....

eg http://bit.do/sorb

.....and some are still being written up but NONE OF THE WORK was undertaken in an attempt to define acceptable guidelines for duration of scrubbers. All of the experiments were undertaken at 1 ATA (scrubbers do not last as long at depth) and the coldest water temperature was 18 degrees Celsius.

With that said, and in relation to the question being asked here, we have (for example) run inspo scrubbers with ~2.6kg of sofnolime 797 under two protocols, both with the rebreather immersed in water at 18 degrees Celsius. In the first, the protocol simulates 6 MET exercise (respiratory minute volume 44 litres per minute and CO2 addition 2 litres per minute) until the scrubber breaks through to 1 kPa (10 millibars, 7.5 mmHg) inspired CO2. In the second, designed to more realistically emulate a real dive with high activity at the start and then rest during deco, the protocol simulates 6 MET exercise for 90 minutes, and then 2 MET exercise (respiratory minute volume 17 litres per minute and CO2 addition 0.67 litres per minute) until the same break through threshold. In the 6 MET continuous exercise protocol the typical duration was about 3 hours, and in the 6 MET / 2 MET protocol the typical duration was about 7.5 hours. I repeat, these are NOT recommendations for duration of scrubbers in practical use, but the data do serve to illustrate the potential variation that can occur just by changing one of the relevant parameters (in this case exercise level).

I know you will be interested, but I cannot discuss the reasons we were doing these experiments at this stage. Once the papers are published we will organise early release (as we did with the one I have linked to above) and make them available for everyone quickly.

Simon M