Canister Insulation

[doctormike]

I suppose that the bonding has something to do with the efficiency of the system's heat retention, etc... but it seems to me that it would be pretty easy to fabricate some sort of slightly stretchy insulating cylinder that would slip over the JJ canister, with slots to let the stand slide through, or even just a wrap with velcro straps.

Sure, it wouldn't be as good as high tech, bonded syntactic foam, and it might add a little positive buoyancy if it wasn't "syntactic enough", but since the scrubber is generating heat it might be an easy way to keep some of that from being lost to the surrounding cold water. Easy enough to test the temperature difference in a lab.

Way beyond my DIY abilities, but since many CCR divers love tinkering enough to dive homebrew rigs, this might be a pretty minor modification project for someone.

 

[KevinNM]

Well, apparently the scrubber duration is also affected by pressure, so it gets more complex when you also need ~10 bar to simulate a dive.

 

[KevinNM]

I think the article is somewhere in John Clark’s web site. Here an article as a start

Diving a Rebreather in Frigid Water: Canister Concerns

 

[doctormike]

Well, apparently the scrubber duration is also affected by pressure, so it gets more complex when you also need ~10 bar to simulate a dive.

You would need 10 ATA to stimulate a diver to that depth, but that doesn't mean that you couldn't compare insulated and non-insulated scrubbers at shallower depths. The more data points you had, the more you could say about the linearity of the effect of insulation over a pressure range, but it would still be interesting to see the difference closer to the surface.

 

[wedivebc]

The absorbent efficiency at different temperatures is available. I wonder if there are any small temperature recording devices that you could put in the canister? It would reduce absorbent capacity a little but would give you real numbers without adding a penetrator to the canister for a thermistor.

Yeah I get that but without continuously measuring the stack temp and adjusting the sorb time accordingly I don't see how putting insulation on the can would provide a safe reliable method of extending scrubber life. They do have temp sticks in a couple of units and even then the results seem less than predictable. It's like the old cell argument, those who store their cells in the refrigerator of in bags of helium still should be changing them annually. No net gain from that action either.

 

[reefrat]

There are many factors affecting scrubber performance, not the least of which is the human element in packing and assembly. It seems strange to me that there appears to be a declining interest in the industry to develop more practical CO2 sensors when CO2 breakthrough (for whatever reason) is probably the potential situation that scares RB divers the most!
I would personally like to see a CO2 readout on my handset just like the PPO2 so I could monitor the performance of the scrubber, maybe one day “know your PPO2” could be “know your PPO2 and CO2”!

 

[doctormike]

There are many factors affecting scrubber performance, not the least of which is the human element in packing and assembly.

Right, but a manufacturer can optimize performance by design choices that will improve duration in all cases (things like scrubber design and possibly canister insulation), while there isn't much that they can do to make sure that the diver does a good job packing sorb. That's one appealing thing about the ExtendAir cartridges, they eliminate a potential human failure point.

It seems strange to me that there appears to be a declining interest in the industry to develop more practical CO2 sensors when CO2 breakthrough (for whatever reason) is probably the potential situation that scares RB divers the most!

I don't know if there is a declining interest, or if manufacturers are just bumping up against the current technical limits of installing a CO2 monitor in a rebreather. Presumably, research and development is ongoing...

 

[DA Aquamaster]

There are many factors affecting scrubber performance, not the least of which is the human element in packing and assembly. It seems strange to me that there appears to be a declining interest in the industry to develop more practical CO2 sensors when CO2 breakthrough (for whatever reason) is probably the potential situation that scares RB divers the most!.../

It's not all upside.

As it is now, prudent CCR divers put a great deal of effort into preventing breakthrough by:
- buying quality sorb;
- keeping it properly stored;
- using it by it's expiration date;
- doing a very thorough and carful job packing the scrubber in order to prevent channeling; and
- diving the scrubber conservatively in terms of time and work load, or at a minimum, extending the scrubber duration incrementally with due regard to variables like work load, water temp, respiration rate, etc.

Now, let's imagine that we get a CO2 Sensor that works reasonably well inside a CCR. What happens next? The odds are that divers will start relying on the sensor to tell them when the CO2 level starts to get higher. That will lead to divers pushing the scrubber life farther, and perhaps some divers will even start paying less attention to carefully packing the scrubber, since they don't get higher CO2 indications with their quicker, looser packed scrubber.

In the end that technological advancement may degrade the current multi layer defense against breakthrough and replace it with a technology that works fine - right up until the point it fails. That will lead to redundant CO2 sensors, which may be less than perfect if they both fail under similar conditions.

In the end, what exactly are we going to gain?

Personally, I don't see any significant gain at all. I focus on the approaches outlined above and Co2 breakthrough is low on my list of worries during a dive. And yes, all I have for scrubber duration data, are progressively longer dives under certain known limits and parameters, but it's at least real world data and not something I got off a chart or read on the internet that may or may not apply to my personal diving conditions.

A CO2 sensor might serve to provide actual CO2 readings to validate or quantify the "no observed symptoms" observation at the end of a six hour dive, but it would still not be something I'd want to use to actually drive scrubber duration decision making. All it knows is the CO2 level. It would not know depth, water temperature, peak workload at various points in the dive, respiration rate (dwell time in the scrubber), average work load during the dive, how much of the dive was spent on deco, etc.

In short a CO2 sensor will never (and should never) replace a knowledgeable and thinking CCR diver who is keeping track of and considering all of the factors that effect scrubber duration.

 

[doctormike]

It's not all upside.

As it is now, prudent CCR divers put a great deal of effort into preventing breakthrough by:
- buying quality sorb;
- keeping it properly stored;
- using it by it's expiration date;
- doing a very thorough and carful job packing the scrubber in order to prevent channeling; and
- diving the scrubber conservatively in terms of time and work load, or at a minimum, extending the scrubber duration incrementally with due regard to variables like work load, water temp, respiration rate, etc.

Now, let's imagine that we get a CO2 Sensor that works reasonably well inside a CCR. What happens next? The odds are that divers will start relying on the sensor to tell them when the CO2 level starts to get higher.

That's a really good point.

It's probably hard to find data on this, but I wonder what the incidence of breakthrough / scrubber failure / hypercapnea is for divers who pack their scrubbers carefully and don't exceed the manufacturer's duration limits. My suspicion is that it's probably pretty low.

I guess a significant undetected flood or a flapper valve failure (after passing the pre-dive stereo check) could result in an "undeserved" CO2 hit, but those are rare scenarios.

 

[Akimbo]

@wedivebc

All good points. I also question if insulating just the canister would make a meaningful difference, especially on Trimix. You really need to insulate the bags too as a minimum. Don't forget the advantage of limiting respiratory heat loss is also part of the overall equation -- not the least of which is reducing CO2 production and improved decompression efficiency.

What we really need is a better absorbent. Let's face it, absorbent is made for the medical industry who doesn't have packing problems or even that big a dust issue. Does anyone remember the old Barium Hydroxide pellets the Navy used on their O2 rebreathers in the 50s and 60s? It was horrible stuff when it got wet but the plusses include the shape and the color change.

As I recall, pellets looked like a rigid extruded tube with an OD about 1/4", ID about 1/8", and cut about 1/4" long. There was very little dust and flow was much more consistent, especially with elevated moisture from the diver's breath. Instead of coming in 5 gallon buckets, it came in 1-quart waxed paper milk cartons.

​

Somebody really needs to come up with a solid-state hemoglobin-like material the loves CO2 instead of O2 and releases it with a low voltage recharge. Oh, and sort out that carbon nanotube "thing" for HP cylinders while they're at it.

I'd take either of them under my Christmas Tree!