Depth effect on sorb duration

[LionFi2s]

Hi all,

can someone explain in simple terms the reason behind sorb duration reduction relative to the increase of depth?

Is it related to gas density?

TIA!

 

[rjack]

The number of Co2 molecules is basically constant for a given O2 consumption, but the proportion of CO2 molecules in the loop is lower which makes it slightly harder for a given CO2 molecule to find a site on the sorbent to stick to (in theory). In practice its a bit more nebulous than that. http://johnclarkeonline.com/2013/11/11/how-does-your-rebreather-scrubber-handle-the-deep/

 

[divingabe]

The depth of the active working field in the sorb increases with pressure. The density of the gas increases which crowds every cubic centimeters and causes interference for the coupling of the co2 molecules to the sorb.
Actual sorb duration does not change but its efficiency to eliminate all co2 decreases with higher density.
I remember a nice pictorial in UNDERSTANDING REBREATHERS by Jeff Bozanic which explained it better than I can with words.
Gabe

 

[inverted_bear]

Is the effect balanced out by the higher proportion of time spent shallow for decompression on deep dives or does the lost scrubber efficiency on the bottom effectively cut your max runtime?

 

[Capt Morgan]

This is a good article by Gordon Henderson from 2003
that still rings true today.

 

[rjack]

Is the effect balanced out by the higher proportion of time spent shallow for decompression on deep dives or does the lost scrubber efficiency on the bottom effectively cut your max runtime?

Compared to what? A test run at a constant depth? Nobody is publishing sorb results for variable profile dives so I would say "who knows..."

I used the 30m 4C test as my max usage in cold water even though my water is not 4C, my max depth is often deeper than 30m, yet my average depth (accounting for deco) is shallower than 30m.

 

[inverted_bear]

From a theoretical standpoint, does less efficient scrubbing at extreme depth (70+m) have more of a negative effect on scrubber life than the relatively lengthy shallow decompression helps? My real question is: If the sorb gets used faster then obviously not. However, if its just an efficiency issue, wouldn't the sorb grains that didn't fully react at depth react a little more once shallow and thus the efficiency loss wouldn't matter for scrubber life planning?

Every scrubber test I've come across is essentially useless. The water is always way colder than what I'm diving, the CO2 output is way higher than I could possibly sustain, and the profiles are not realistic compared to most scenarios that I would encounter in the caves. It's nice to know how long my scrubber will last for sure. However, when the test is saying 1.5-2.5hrs @4C @100m @3x my sustainable CO2 output it's not very useful for planning a cave dive in 22C water @30-50m. It's probably safe to quadruple the runtimes from those results in many instances...

 

[Mark Chase]

Depth is relevant, water temperature is relevant SAC/C02 production is IMPORTANT

Most deep dives will be short duration at depth and a lot of time shallow.

I have done dives of over 5 hours with the deep bit being over 100m and its not bothered me
Id be more bothered on the third dive doing three 50m dives totalling 5+ hours

If you do he SAC math and ignore temperature an inspo sized scrubber on a 15 SAC will do 10 hours but that's not realy the point. The point is every min past the 3hours recommended is cutting your margin of error down and down.

For a single dive exposure I max at 6 hours but I think my longest was about 5 1/2 and its unlikely id ever do this again

For two deepish dives (max 50m) id max 6 hours

For multiple shallow dives 30-40m id max 5 hours

I am sure the scruber can do more but that's my personal limit of risk

But I stress the word LIMIT.

90% of my diving I push no where near the limit. 90% of my diving is on a fresh fill even if the last dive was only 2 hours.

The limit is like driving 90MPH when your late. Sooner or later it will get you and if your lucky its just a big fine.

ATB

 

[inverted_bear]

it would be nice to know the exact safety margin though. If you knew the single dive exposure limit under one set of circumstances, and you had a rough idea of how the depth affects the reaction efficiency for dives with large chunks of shallow deco, then you could figure out your safety margin almost exactly.

 

[JukkaO]

If you do not work too hard you may survive with a scrubber that has been used for many many hours.
If you need to work hard or you get exited and breath more, you may get into trouble with any rebreather even with quite fresh scrubber.
In shallow you do not recognize the difference but in deep it is very clear. Some individuals just seem not to feel anything before it is too late.
If you stay within the limits in the manuals you probably do not die because of the scrubber capacity. Many divers are going beyond the limits and think they manage to bail-out if needed. Some of those just die as they fail to bail-out. There is no exact limit.

Jukka

 

[rjack]

it would be nice to know the exact safety margin though. If you knew the single dive exposure limit under one set of circumstances, and you had a rough idea of how the depth affects the reaction efficiency for dives with large chunks of shallow deco, then you could figure out your safety margin almost exactly.

Won't ever happen.
The relationship with depth, water temperature, thermal conductivity of the gas in the loop, and O2 metabolism (because the reaction produces heat itself) aren't linear.

 

[inverted_bear]

Won't ever happen.
The relationship with depth, water temperature, thermal conductivity of the gas in the loop, and O2 metabolism (because the reaction produces heat itself) aren't linear.

We'll never get an exact equation, but we could get close enough for it to be useful. All it would take is running the breakthrough tests on the scrubbers at temps that actually match your dive site with profiles that actually match a normal decompression dive.

jukkaO, the real issue is that the tests fail to match conditions or depth profiles for 99% of dives. It's virtually useless to know the breakthrough time for 1.5L/min co2 production at 4c @ 100m. It's going to give you insanely short scrubber durations. If your scrubber could only take you an 90 minutes, you'd be better off in doubles.

 

[Garspeed]

The real issue here is not getting a scrubber test to match a realistic dive profile. Instead it is understanding that any perceived "safety factor" in your scrubber duration is fully dependent on your actual dive conditions on the day. For example, I have been involved in running scrubber duration testing where we achieved over 6 hours on a 3 hour scrubber at hypoxic depths using low breathing rates. You may think that this would give a high safety factor i.e. the scrubber can last twice as long as predicted but the crux is that the additional 3 hours are only available if the breathing rate remains at the same low RMV as used in the test. As soon as the breathing rate is elevated the safety factor is out of the window and you are playing Russian roulette with your life. CO2 break through is then likely to happen at any time, the higher the RMV, the quicker the breakthrough will come. All it takes is a little stress, lost buddy, line entanglement, loss of exit from overhead environment etc. I have seen the above first hand in the lab and it has made me seriously question decisions I have made in the past about re-using lime for gas dives

Do yourself and your family a favour and don't play roulette with your scrubber on deep dives to save a few $'s

 

[plazma]

I made this post at Scubaboard last year.
https://www.scubaboard.com/communit...ility-to-scrub-co2.566099/page-2#post-8378997

The post may be wrong and left out something important. It feels like most just stuck with the “Here on Zord…” explanation. But why does not the extra molecules then bounce O2 from the O2 sensor? O2 cells show the right reading regardless of other gas partial pressure.

Some chemist or physicist should chime in

 

[Bobjr]

I made this post at Scubaboard last year.
https://www.scubaboard.com/communit...ility-to-scrub-co2.566099/page-2#post-8378997

The post may be wrong and left out something important. It feels like most just stuck with the “Here on Zord…” explanation. But why does not the extra molecules then bounce O2 from the O2 sensor? O2 cells show the right reading regardless of other gas partial pressure.

Some chemist or physicist should chime in

The O2 cells don't filter O2. Deeper you go the more gas there is but the same amount of C02. A filter will struggle more in these conditions.

 

[plazma]

The O2 cells don't filter O2. Deeper you go the more gas there is but the same amount of C02. A filter will struggle more in these conditions.

The O2 cell does not filter but requires contact with O2 molecules for the electrochemical reaction. The deeper you go the more gas there is but the same amount of O2 (the same PO2). Still the other gas molecules don't block the O2 cell as per the old explanation why scrubber works worse.

The scrubber struggles at depth. That is sure. The reason just is not caused by other gas molecules bounsing CO2 molecules away from sofnolime. At depth the gas is denser with higher specific heat capacity.

Also the denser gas is more turbulent (density is part of Reynolds number equation). This affects the WOB but I'm not sure how it affects the chemical reaction.

 

[Bobjr]

The O2 cell does not filter but requires contact with O2 molecules for the electrochemical reaction. The deeper you go the more gas there is but the same amount of O2 (the same PO2). Still the other gas molecules don't block the O2 cell as per the old explanation why scrubber works worse.

The scrubber struggles at depth. That is sure. The reason just is not caused by other gas molecules bounsing CO2 molecules away from sofnolime. At depth the gas is denser with higher specific heat capacity.

Also the denser gas is more turbulent (density is part of Reynolds number equation). This affects the WOB but I'm not sure how it affects the chemical reaction.

O2 cells are working by diffusion into an electrolyte - The C02 reaction is happening at the surface area of the sorb - it is not the same process? I suspect gas molecules getting in the way do have an effect.

 

[plazma]

O2 cells are working by diffusion into an electrolyte - The C02 reaction is happening at the surface area of the sorb - it is not the same process? I suspect gas molecules getting in the way do have an effect.

In both cases CO2 and O2 need to get to the material of sorb or cell electrolyte. Inert gas do not bounce them away. Gas molecule collisions do happen but they have statistically no effect. Inert gas can bounce a CO2 molecule traveling past sorb towards sorb. Or a CO2 molecule traveling towards sorb is bouced away by inert gas. It is random and cancels itself out. Collisions do slow down diffusion in mixing gases but we are dealing with a mixed gas already.

 

[plazma]

 

[JukkaO]

Sorry to mix the discussion but how close the truth you get by measuring and calculating O2 consumption? It is not that depth related but it tells something about how much work you have done.