Loop Volume Question

[jgttrey]

The reaction of the scrubber is also producing H2O some of which will be water vapor in the loop. So there is additional gas entering the loop with the scrubber reaction. The solid sorb granules themselves contain about 20% water before reaction and this gets released by the reaction.

Soda lime - Wikipedia

The reaction mechanism of carbon dioxide with soda lime can be decomposed in three elementary steps:
1) CO2(g) → CO2(aq) (CO2 dissolves in water – slow and rate-determining)
2) CO2(aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)
3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH ((NaOH recycled to step 2)

If that were the explanation, it would be present at all phases of the dive, not just on deco. I can hang out at a constant depth when not offgassing all day and maintain PO2 simply by adding a little O2. My scrubber is not adding inert gas to the loop in any meaningful quantity.

Edit: Plus, water vapor does not increase indefinitely. It condenses and is flushed out, or you accumulate a little in the counterlung in liquid form. A breathing loop is probably already pretty much 100% humidity. This isn't the explanation for the PO2 drop.

I think it is the solenoid action in an eccr which leads to buoyancy effect and thus noticing the volume increase here.

Not for me. I'm running manual on deco. Plus, the solenoid is only injecting O2 and if inert gas were not being added to the loop through offgassing, loop volume would remain the same, only slightly oscillating as O2 is metabolized then replaced to maintain a constant PO2.

 

[wedivebc]

The reaction of the scrubber is also producing H2O some of which will be water vapor in the loop. So there is additional gas entering the loop with the scrubber reaction. The solid sorb granules themselves contain about 20% water before reaction and this gets released by the reaction.

Soda lime - Wikipedia

The reaction mechanism of carbon dioxide with soda lime can be decomposed in three elementary steps:
1) CO2(g) → CO2(aq) (CO2 dissolves in water – slow and rate-determining)
2) CO2(aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)
3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH ((NaOH recycled to step 2)

Since the RH in a rebreather loop is 100% pretty much from the beginning of the dive I don't see how H2O vapor will affect volume. If anything H2O absorbed by the scrubber will increase the weight of the scrubber material.

 

[rddvet]

This is an excellent discussion. I had noticed since adding helium to the mix, keeping a higher p02 at deco was a little annoying. I thought maybe it was just me. The last few dives I solved it by venting well and refilling with oxygen, but I assumed it was just gas I hadn't cleared out of the lung diluting my oxygen. Never realized it was from the rapid offgassing of helium. Very cool. We talked about it some in my class, but it was a lot of info to take in being a new cc diver and so I suspect I missed it somewhere.

 

[taimen]

Not for me. I'm running manual on deco. Plus, the solenoid is only injecting O2 and if inert gas were not being added to the loop through offgassing, loop volume would remain the same, only slightly oscillating as O2 is metabolized then replaced to maintain a constant PO2.

Fully agree. I notice the off-gassing in my loop. What I tried to say is that the actual volume effect is quite small and easily anticipated when running manual. If running eccr and not exactly knowing what is going on, it can be confusing like OP described.

 

[silent running]

I don’t know about anybody else, but even if I have an anchor chain or deco station bar to hold or clip off to, unless the conditions are very calm or the stop is very deep, I might have up to a 1.5m depth change up and down while “holding” a stop. If the OP is speaking of holding a stop closer to the surface, the pressure change from slight depth variations will be more severe than if at a deeper stop. Even mild surge can cause significant depth changes while holding a stop in the last 10m, then your PO2 drops and all the loop gas expands. Add off gassing, computer and gage checking, and most of us will be struggling to actually “hold” a shallow stop without constantly adjusting loop volume, especially those on ECCR...

 

[kensuf]

I don’t know about anybody else, but even if I have an anchor chain or deco station bar to hold or clip off to, unless the conditions are very calm or the stop is very deep, I might have up to a 1.5m depth change up and down while “holding” a stop. If the OP is speaking of holding a stop closer to the surface, the pressure change from slight depth variations will be more severe than if at a deeper stop. Even mild surge can cause significant depth changes while holding a stop in the last 10m, then your PO2 drops and all the loop gas expands. Add off gassing, computer and gage checking, and most of us will be struggling to actually “hold” a shallow stop without constantly adjusting loop volume, especially those on ECCR...

Have you tried either using a jon line or drifting under lift-bag? I find that if I'm holding on to a fixed point on the bottom (like an anchor line) I'll have those variations in depth, but if I'm able to move up and down as the water column rises, like under a lift bag or while attached to a jon line, my depths are much more stable.

Here's a bunch of people finishing their 10' stop while drifting in a 2kt current with 3'-4' seas. None of us were on a fixed anchor point and we all found it easy to hold our stops.

 

[broncobowsher]

DIL IP adjusting itself. With DIL 1st stage depth compensating like any regular regulator the absolute pressure rises as you descend. On ascent it has to bleed. Typically there is an OPV valve on the 1st stage. But it is also possible (depending on the rebreather design) for that excess pressure to simply bleed past the ADV into the loop.

When you reach your stop, put a little DIL in the loop before flushing it down with O2. This should correct the high IP condition if that is the cause.

 

[silent running]

Have you tried either using a jon line or drifting under lift-bag? I find that if I'm holding on to a fixed point on the bottom (like an anchor line) I'll have those variations in depth, but if I'm able to move up and down as the water column rises, like under a lift bag or while attached to a jon line, my depths are much more stable.

Here's a bunch of people finishing their 10' stop while drifting in a 2kt current with 3'-4' seas. None of us were on a fixed anchor point and we all found it easy to hold our stops.

Thanks for the video, yes, I have used a jon line and done plenty of stops hanging on a lift bag. But as the OP was not specific about his particular stop conditions, I thought it was worth mentioning the depth change issues when holding onto a fixed object like an anchor chain or deco bar under a boat.

But even hanging on a lift bag in moderate seas, I’ve certainly seen depth changes on my computer and felt them in my ears. Like anything that floats, lift bags go up and down with the waves, and unless you are very relaxed and totally focused on allowing your arm to move up and down with the bag, your depth and PO2 will change, perhaps enough to get a solenoid to fire and buoyancy changes...

 

[Pandora's Box]

We can go up with the bag but not back down. What we need is solid string

 

[amtaher]

I have noticed that when I am holding a stop at a fixed depth, the loop volume steadily slowly increases until I have to vent some gas out. I cannot understand why this is the case - the electronic system is adding in enough oxygen such that it replaces the oxygen that i have metabolised. So why is the volume increasing? When at depth, this becomes a non issue, the shallower I get the more noticeable it becomes. I have tried to close the flow stop just to make sure the ADV was not putting in any gas without me noticing - no difference.

Check your mavs, most probably your dil mav is leaking