Measure scrubber runtime?

[KevinNM]

This is an old and complex problem. I was only tangentially exposed to US Navy studies on different pure O2, semi-closed, and closed circuit HeO2 rebreathers. They spent a LOT of money to develop these charts for each rig with temperature and activity curves. For example, EOD (Explosive Ordinance Disposal) divers typically are not working as hard as combat swimmers.

In many ways, the military's effort is simpler because the age and physical condition for each activity group was much more consistent than CCR users in the recreational market. They have breathing machines for inhalation and exhalation resistance that can be programmed across huge volume variations but I have never heard of one that can meter CO2 into the system for scrubber studies.

A welding flowmeter used for shielding gas should be able to do this, though I don’t want to claim it would work at 10 atmosphere on the low pressure side without testing.

 

[Akimbo]

A welding flowmeter used for shielding gas should be able to do this, though I don’t want to claim it would work at 10 atmosphere on the low pressure side without testing.

That would work at a fixed depth but it is simple enough to meter CO2 electronically via controller by a CO2 sensor and follow real world dive profiles for varying exertion. Industrial controllers have gotten really versatile and programmer-friendly. Now that I think about it, DiveLab probably has the capability.

 

[uwxplorer]

On extremely rare occasions, I dive without the Ali80, reverting to the 3 litre onboard bailout. This is on a shallow dive, 15m or shallower, and where my buddy has full independent bailout. It is rare luxury to not be carrying the ali80.

I know a rebreather instructor (not mine, I just happened to be his "buddy" that time) who pulled this prank on me (i.e. he dove with on-board "bailout"...to >130 ft, something we had not planned). Needless to say, that was my last dive with him, and I certainly had no intention to bail him out if he had run into trouble.
15 m wouldn't have changed my attitude: you dive without a bailout cylinder "with" me, you are on your own, as I have no way to predict what your next move is.
If your buddy AND you ever are in trouble, it will be too late to regret not having brought a bailout cylinder.
It always amazes me when someone seems to believe that planning for a good day (anyone remembers TrueDive?) is actually planning...

 

[uwxplorer]

Urrrr no, continue on the dive as normal, if you are coming to the end of the recommended scrubber duration when a failure occurs, I would start to think about rounding out the dive, complete any necessary decompression safety stops etc. I have never aborted a dive due to the rMS “crapping out”, rMS failure has happened to me plenty of times early on, I still have an old probe in one of my 5 canisters that throws up lost probe errors from time to time. No cause for alarm.

Yes. You have now built in your experience of the working rMS to trust that a fresh scrubber will actually last twice as long as the recommended duration (to oversimplify, I assume a factor 2).
But the original conservative numbers were so that if you had one bad canister (channeling, say) out of 2, you still would have been fine.
Now, if your rMS dies in the middle of a dive which is longer than the original recommended duration before rotation (or worse, for dumping both canisters), you have to assume that no canister will channel. If you are cautious, you should tell yourself: OK, now that I have no way to monitor the health of my scrubbers, I have to bet that they are both healthy otherwise, if one channels, I am already past the safety margin on a single canister.
It is your decision to reason differently.
It is a dangerous one.

 

[Gareth J]

I know a rebreather instructor (not mine, I just happened to be his "buddy" that time) who pulled this prank on me (i.e. he dove with on-board "bailout"...to >130 ft, something we had not planned). Needless to say, that was my last dive with him, and I certainly had no intention to bail him out if he had run into trouble.
15 m wouldn't have changed my attitude: you dive without a bailout cylinder "with" me, you are on your own, as I have no way to predict what your next move is.
If your buddy AND you ever are in trouble, it will be too late to regret not having brought a bailout cylinder.
It always amazes me when someone seems to believe that planning for a good day (anyone remembers TrueDive?) is actually planning...

I am shocked that you would do a dive without a buddy briefing, especially to 40m, with an unfamiliar buddy.
I would also suggest a 40m+ dive is significantly different to a 15m dive.
I don't know about the last dive, I wouldn't have done the dive.

All the agencies have the same recommendation for my unit. Dives beyond 20m, recommend an independent bailout.

The first dives on my MOD1 course were all without independent bailout. Bailouts where the first exercise we did on the course - after all - "if in doubt bailout" is the mantra at MOD1. Independent bailout cylinders were introduced on preparation for the 20m dives on the course (2nd or third day). Which was quite amusing, both myself and the other student where mixed gas divers. With the unfamiliar harness, we both faffed a bit with the unclipping and clipping of the stages. The instructor was amused to note we both continued to remove and replace the stages until we could do it with our eyes closed in mid water. She, expected it, but it did amuse her. Mind you, I think we clocked twice the normal in water time for the course. We took ever opportunity, to spend as much time in the water as we could.

Out of interest, how big are the Revo on board cylinders? I'm not familiar with Revo.

Gareth

 

[Tassi Devil Diver]

But I feel it does add redundancy and another level of safety, if you are close to time limit to cycle (RCT), and thus obviously getting closer to the limit of remaining scrubber time (RST), your much more a tune to the symptoms of hypercapnia, and I would think it's less likely to creep up on you unexpectedly.

Yes it does and the rMS is very conservative, so should you ever hit RST, no need to panic no need to bail out (as others have once suggested) remain calm and finish your dive completing any deco or safety stops that is needed, but be vigilant not alarmed.

Yes. You have now built in your experience of the working rMS to trust that a fresh scrubber will actually last twice as long as the recommended duration (to oversimplify, I assume a factor 2).
But the original conservative numbers were so that if you had one bad canister (channeling, say) out of 2, you still would have been fine.
Now, if your rMS dies in the middle of a dive which is longer than the original recommended duration before rotation (or worse, for dumping both canisters), you have to assume that no canister will channel. If you are cautious, you should tell yourself: OK, now that I have no way to monitor the health of my scrubbers, I have to bet that they are both healthy otherwise, if one channels, I am already past the safety margin on a single canister.
It is your decision to reason differently.
It is a dangerous one.

Don't agree, channeling is so unlikely, the small rEvo canisters are very easy to pack tightly, sounds like you are trying to justify in your own mind your reasons for abandoning the rMS system, which I don't blame anyone for doing given the hardware issues, but I am very glad I have persisted with it.

On extremely rare occasions, I dive without the Ali80, reverting to the 3 litre onboard bailout.

I too enjoy the occasional shallow rebreather dive just using the on board for bailout with the BOV

 

[Brad_Horn]

This is an old and complex problem. I was only tangentially exposed to US Navy studies on different pure O2, semi-closed, and closed circuit HeO2 rebreathers. They spent a LOT of money to develop these charts for each rig with temperature and activity curves. For example, EOD (Explosive Ordinance Disposal) divers typically are not working as hard as combat swimmers.

In many ways, the military's effort is simpler because the age and physical condition for each activity group was much more consistent than CCR users in the recreational market. They have breathing machines for inhalation and exhalation resistance that can be programmed across huge volume variations but I have never heard of one that can meter CO2 into the system for scrubber studies.

See Page 8 https://www.opensafetyglobal.com/Safety_files/DV_OR_ScrubberEndurance_Retest_SRB_101215.pdf

The breathing machines for the testing are COTS Open Safety | Respiratory Test and Validation
and have significantly decreased in cost to the end user Open Safety Equipment Ltd

 

[uwxplorer]

I am shocked that you would do a dive without a buddy briefing, especially to 40m, with an unfamiliar buddy.

He presented himself as my "buddy". I had planned a solo dive. When I saw what kind of wazoo he was (and the absence of any bailout), this is effectively what my dive was. And I only kept a bewildered eye on him when he darted to some shape in the sand, depth unknown.

I'd be curious to know what agency teaches no off-board bailout (except for SSA).

 

[broncobowsher]

I have only done no off-board bail out twice. When I knew nothing and the sales reps were doing demos in a pool. Poseidon and Hollis. Once I learned a little more, never did it again.

But there are times where just taking an AL40 of air for a recreational dive is a nice break from a couple big bottles on a deco dive.

That has me thinking, I have an AL30, I wonder if I could mount that where I keep the drysuit bottle? Just for the recreational stuff.

 

[Dusty123]

Well technically you could do a 3 hour dive at 60ft running a 1.3 and bail with a spare air. A 2l would be a ton of gas. Makes me wonder about rigging up a 2l/3l diluent with an h valve for some sort of redundancy for a shallow reef drift with no overhead/entanglement hazards