Why depth has an effect on sorb capability to scrub CO2

[Superlyte27]

I apologise if you feel attacked. That's certainly not my intention. I was in fact just typing about the nice video collection.

My point all along has been simple, if not blunt...
I'm not familiar with your rebreather specs, but I am pretty familiar with sorb. 1lb per hour works pretty well even at depth in warm water. Myself and dozens of others in cave country florida who have collectively thousands of hours on ccr at those times and depth can prove the efficacy of that statement.

I do apologise for the playful jab about "real dives". Some can't hear my humor through text.

 

[DA Aquamaster]

I'll play devils advocate a bit and make a couple observations that support both points of view:

1) Does anyone here drive a turbocharged care or fly a turbo charged aircraft? The easiest way to add power in an engine design is to increase the mass flow through the engine. You can do that three ways. You can increase the displacement by increasing the bore and/or stroke, you can increase the mass flow per minute by increasing the maximum rpm, or you can turbo charge or super charge (or turbo-supercharge) the engine to increase the pressure and mass of the air entering the engine and thus get a lot more mass flow with the same bore and stroke (displacement) at the original comparatively low maximum rpm.

Turbo charging is essentially what is happening at depth in the CCR where you'd got about 135 psi of additional pressure in the loop compared to the surface.

In terms of mass flow through a scrubber, if my tidal lung volume is 0.5 L of gas at 1 ATA, I'm moving 2.0 L at 100 ft (4 ATA), 3.5 L of gas at 200" (7 ATA) and 5 L of gas at 300' (10 ATA). At 300' that is a 10 fold increase in gas flow through the scrubber in the same amount of time, since I am still making the same number of breaths per minute. However that same respiration rate and tidal volume at 10 times the pressure with 10 times the mass flow, means the gas and the CO2 molecules in it have to pass through the scrubber in 1/10th the time it would at the surface, and the consequently the scrubber has a only 10% of the original dwell time to remove the CO2 from the gas.

2) Off the top of my head a square profile dive to 300' with a 10 hour run time will involve an actual bottom time of about 60 minutes with 10/70. That will mean you'll have this limited dwell time issue for the first 60 minutes of the dive. The first deco stop will probably be around 220-230 ft where you're back down to 3.5 L per breath with 142% of the dwell time you had at 300' (and that's not taking into consideration the reduced work load during deco). By about 150 minutes of run time, you'll be at 100' with 2 L per breath and 250% of the dwell time you had at 300'. The remaining 450 or so minutes on the back end of the scrubber life are all at 100' or less - averaging maybe 30 ft, with the the dwell time increasing with every deco stop until you're spending 100 or so minutes at a 10' stop with a tidal volume of 0.65 L and about 770% more dwell time than you had at 300'.

---

So yes, there is a very real effect of increased mass flow at depth on dwell time in the scrubber, and describing it more in terms of mass flow than relative numbers of CO2, O2 and He molecules might be more understandable.

However, no, it isn't going to have a profound effect on a 300', 10 hour dive as the scrubber only suffers from the limited dwell time early in the dive when the gas is moving through a majority of fresh sorb in the scrubber, and the scrubber gains dwell time throughout the dive. Now...that's based on the assumption that the scrubber has adequate capacity to manage that gas flow and limited dwell time at depth, but that also tends to go hand in had with a scrubber with enough sorb for a 10 hour duration near the surface any way.

Now...if you were diving out of a habitat on a 300' saturation dive where you are at 300' the entire time, then no, you probably would not get anywhere close to the same 10 hour scrubber duration, due to the increased mass flow and decreased dwell time at that depth.

 

[fsardone]

I apologise if you feel attacked. That's certainly not my intention. I was in fact just typing about the nice video collection.

My point all along has been simple, if not blunt...
I'm not familiar with your rebreather specs, but I am pretty familiar with sorb. 1lb per hour works pretty well even at depth in warm water. Myself and dozens of others in cave country florida who have collectively thousands of hours on ccr at those times and depth can prove the efficacy of that statement.

I do apologise for the playful jab about "real dives". Some can't hear my humor through text.

Ok,
no problem then.
My rebreather scrubbe limitation are in post #2.
I only bring camera up to 60 meters and my usual dive is 30-40 minutes at 60 which means a runtime of 100-110.
This is because I can be self sufficient with bailout.
Below 70 and up to 100 I tend not to bring camera and add. 4th stage and stay within 180 minutes.
The last dive is Haven wreck and it is pretty rapresentative of my usual diving and my confort zone.

What I am saying, and trying to promote is the concept that depth will cause early breakthrough even if the scrubber is not exhausted because you need a deeper reaction front which could reach the end of the scrubber bead early.

People need to be aware of this especially when re-using the scrubber for the second dive. Ie after doing a dive like the one I did (video haven) I would not dare doing another one like that with the same scrubber even if both of them are 90 minutes and the sum would be 180. This is because the deep patt of the second dive would be at risk of breakthrough.

On the same partially used scrubber I would do easily a 40 mt dive for 50 minutes and then additional 30-40’ above 20 mt.

What am saying is supported by testing done by APD it is in the Sherwater research page and it is in the 1985 Navy paper. Hope that speaking of this makes dives safer for others. Me I am good. Maybe not extracting all I can from my sofnolime, but it is cheap.

Cheers

 

[Superlyte27]

I get what your saying. DA helped with his analogy. And I certainly agree with not starting a second big dive on partially used sorb.

I need to look at the AP. I have no idea what amount of sorb and type of sorb it uses. I know that I have an axial CCR and a radial CCR, both with roughly 6lb scrubbers. I have quite a bit of time on them and know what THEY are capable of, despite what the machine that breathes like a gorilla says.

 

[Superlyte27]

Fsardone:

You know that water tastes funny wreck diving right? It's like salty and stuff. You should come to Florida and do some cave diving.

 

[fsardone]

Fsardone:

You know that water tastes funny wreck diving right? It's like salty and stuff. You should come to Florida and do some cave diving.

I have been living in VA Beach for a few years (2007-2010) but young kids brought me to the parks when I flew in FL by a turbocharged 350 HP full fadec Cirrus SR22 ... [referring to @DA Aquamaster post] I was not diving while my kids were young.

Now I am back in Italy and FL is a bit out of the way ... But, if I get posted to Tampa for another tour or to Norfolk again, cave country is on my to do list, in the mean time I need to make do with Italian Caves ... not so many traverses as in cave country but still fun!
There is the video made by one of my instructors during my cave training. And I also do dry caving to get to the water!

Cheers

 

[Schwob]

Just saying: To someone just getting his feet wet in this topic, this is a very interesting discussion! Even so the part of discussing what's presented based on facts and experience and determining if there maybe is a common valid explanation (with which @DA Aquamaster may have a point) ... that explains the on the surface differing opinions on what happens or ought to happen in a scrubber... that part seems to at times just be killed by just opinion... and it is indeed hard to distinguish what's meant as a playful jab and what is meant seriously.... anyway.... even so, this is very interesting. Thanks to all!

Now remember, non experts to subject matter idiots (like me) are reading this too... In the absence of evidence to the contrary we view you all as great expert divers, certainly in comparison to us mere mortals. From that point of view on a subject matter like this, it's neither necessary to validate or da-validate arguments based on who makes the deeper or longer dives... or has more theoretical or emperical (hands on) know how. Nor does it ever validate all that much. But the part where the arguments are on the facts or on explaining your experiences and why you think the facts might not line up in light of those... and why they yet may... All that is just great... and good food for thought... and understanding....
Hope more might be coming. Won't chime in again and go back to fly on the wall status.

Thanks.

 

[Superlyte27]

I'm afraid of bats and spiders and dirt. No dry caving for me.

They do look beautiful though.

 

[Akimbo]

Ok, but this is a huge difference because in a rebreather the gas goes through the absorbant bead exactly onle for each breath the diver takes. The flow in a recirculating system can be many times higher than the sum of the sum of tidal volume of all occupants per minute.

Cheers

True, the usefulness of this comparison only relates absorbent function.

The circulation rate is much higher along with the relative huge system volumes (slow CO2 change rates), consistency of absorbent packing (thanks to gravity), and minimal movement. The point is the absorbent life or efficacy is not the variable -- canister design and other constraints imposed by CCR portability are. These sat systems don't see significant variability of absorbent life per diver because they are not constrained by lung power, minimizing weight and displacement, or channeling induced by attitude. We also don't have to worry about flooding the canister(s).

 

[stuartv]

In terms of mass flow through a scrubber, if my tidal lung volume is 0.5 L of gas at 1 ATA, I'm moving 2.0 L at 100 ft (4 ATA), 3.5 L of gas at 200" (7 ATA) and 5 L of gas at 300' (10 ATA). At 300' that is a 10 fold increase in gas flow through the scrubber in the same amount of time, since I am still making the same number of breaths per minute. However that same respiration rate and tidal volume at 10 times the pressure with 10 times the mass flow, means the gas and the CO2 molecules in it have to pass through the scrubber in 1/10th the time it would at the surface, and the consequently the scrubber has a only 10% of the original dwell time to remove the CO2 from the gas.

I don't have much to contribute here. It's a very interesting topic.

But, this particular statement seems to be wrong, to me.

Yes, there is 10 X mass going through the scrubber, but it is 10 X as dense, so the dwell time is the same. I.e. A CO2 molecule going through the loop is going to spend just as much time in the scrubber (unless shortened by reacting and being "scrubbed"), whether you're at 100m or 10m. Right?