Overbreathing the scrubber

[scubanimal]

Very nice discussion.

On the original topic I have a couple things to add I did not notice posted. Since the basic principle of the scrubber is that a CO2 molecule must impact the scrubber material to get a reaction (bonding), The density of the gas passing through the scrubber can/will affect the opportunity of the CO2 molecules actually contacting. This is because the other molecules of N2, O2 etc insulate the CO2 to some extent. Hence one of the reasons dwell time is important, aka time and opportunity. This is why deep dives are done on fresh scrubbers to maximize the dwell time. When confronted with heavy work loads there are times I may bail out temporarily just to be on the safe side. And I'm very careful (now) to get off the loop on the surface if I have nearly used up my scrubber or if I am having to swim hard, or am using a hypoxic Dil.

Another point is that the capacity of the divers lungs and the rate of flow relative to the total loop volume can affect the dwell time as the rate of flow and increased pressure from breathing hard can further reduce the dwell time. I've twice over breathed the EAC cartridge in my Optima resulting in hypercapnia hits; fortunately I was luckly, but a trip to the doctors office confirmed I have very large lungs. So now I choose which RB I use for the dive is in part looking at what my work load may be.

 

[jimdiverman]

Another point is that the capacity of the divers lungs and the rate of flow relative to the total loop volume can affect the dwell time as the rate of flow and increased pressure from breathing hard can further reduce the dwell time. I've twice over breathed the EAC cartridge in my Optima resulting in hypercapnia hits; fortunately I was luckly, but a trip to the doctors office confirmed I have very large lungs. So now I choose which RB I use for the dive is in part looking at what my work load may be.

Good points.

Should the loop be sized to account for the volume of the diver's lungs? That is, if you have larger longs, then the loop piping should be longer to account for that.

 

[PerroneFord]

I thought that's what the varying sizes of counterlungs were for?

 

[Tienuts]

Good points.

Should the loop be sized to account for the volume of the diver's lungs? That is, if you have larger longs, then the loop piping should be longer to account for that.

I thought that's what the varying sizes of counterlungs were for?

Exactly right. The Optima has CL's available in 3 liter (standard) and 2 liter. With the 3L lungs on my unit, I can take 2 and 1/2 full inhales from a full loop to deplete it completely. By comparison the standard Meg CL's are 4.7 liters a peice - you'd have to have lungs like a horse to deplete it.

 

[PerroneFord]

Exactly right. The Optima has CL's available in 3 liter (standard) and 2 liter. With the 3L lungs on my unit, I can take 2 and 1/2 full inhales from a full loop to deplete it completely. By comparison the standard Meg CL's are 4.7 liters a peice - you'd have to have lungs like a horse to deplete it.

The neoprene CLs that ISC sells are quite a bit smaller, but I don't know their capacity offhand.

 

[scubanimal]

One other consideration is the diameter of the hoses, and the effort required to exhale a full breath; on some units a fair amount of the WOB is just getting the air through the DSV. When you consider the fairly small opening for the mouthpiece, the right angle turn required, and getting past the mushroom valves that adds up in resistance. Add to that a small hose diameter, further constricted by the attachment points.
One of the tests i do now when I try out a new RB in a pool is to put my hands up agains the pool side and start kicking like mad for a few mins until I'm really huffing and puffing. Then I assess how hard I am working just to breathe. Am I fulling deflating the inhale lung or getting puffy cheeks trying to exhale, do I trigger the ADV on inhale. As someone with large lung capacity (and not in the best shape) this is one of the areas I look at carefully now.

 

[PerroneFord]

One of the tests i do now when I try out a new RB in a pool is to put my hands up agains the pool side and start kicking like mad for a few mins until I'm really huffing and puffing. Then I assess how hard I am working just to breathe. Am I fulling deflating the inhale lung or getting puffy cheeks trying to exhale, do I trigger the ADV on inhale. As someone with large lung capacity (and not in the best shape) this is one of the areas I look at carefully now.

I was shocked at the low WOB on the Meg. I think that is one of it's strong points. I haven't tried the Boros yet, but many claim it's even better.

 

[Tienuts]

This thread is getting a bit boring, so let me stir it up a bit by making the claim:

WOB is overrated BS.

Work of breathing might be detectable by a machine in some laboratory - measured in mmHG, however I challenge any human being to tell me the difference underwater. If there were some way to make a "blind comparison," I don't think anyone could tell the difference. I have breathed from all the units we sell - Sentinal, Meg, Evo and Optima. They all felt the same to me - like nothing. Breathing from the loop felt like I was breathing on the surface.

Now positioning underwater has a difference, along with the volume of gas you have in the loop. Sometimes I get in a position underwater, where the inhale or exhale will have resistance, but this is from water pressure, not hoses, fittings, or scrubber material.

 

[PerroneFord]

Sometimes I get in a position underwater, where the inhale or exhale will have resistance, but this is from water pressure, not hoses, fittings, or scrubber material.

And this is of paramount importance to cave divers...

 

[Tienuts]

And this is of paramount importance to cave divers...

I'm not sure what you mean by that, but in a good horizontal position, the WOB on any unit is fine. And you will not find me in anything but that while in a cave.

WOB is even OK in a vertical position (for the Jersey guys )

The positions where the WOB can sometimes become an issue, is when you're upside down in a head down attitude. This I discovered goofing off on some reef dives.