Saturation volumes?

MeRodent

MeRodent

New Member
In another thread Chris made a comment regarding the effect of off-gassing in buoyancy. He indicated that this could cause a noticeable change.

I'm a bit sceptical about this as I'm not sure that the volume of gas would be sufficient to cause a buoyancy change.

However I'm always happy to be proven wrong so would love some input as to the actual volume of gas we would be off-gassing from a dive as well as the volumetric rate of change (which should allow us to calculate change in buoyancy). Does anyone have any ideas on how to calculate these?
 
Practically?

Get to the end of a longish dive and when you're on a 5m safety stop, kill your ADV and flush the loop a few times to get your 1.5bar PPO2. Then drop to minimum loop.

At this point the loop is full of oxygen, your lungs (as part of the loop and flushing) are full of oxygen. Oxygen comes into the circuit when you hit the MAV, goes out through your metabolism, CO2 comes into the loop from same but gets yoinked by the sorb before it hits the sensor - assuming your sensors are downstream of the sorb.

Watch your PPO2 for five minutes. It will steadily drop and the only place the diluent gas to do that can come from is you.

Not a lot, maybe, but plenty enough for me to watch a steady drop after a 90 minute to 18m dive. And if you're finely tuned to hover at 5m it's enough to get you floaty, enough that it needs to be managed.
 
Captain Starfish said:
Not a lot, maybe, but plenty enough for me to watch a steady drop after a 90 minute to 18m dive. And if you're finely tuned to hover at 5m it's enough to get you floaty, enough that it needs to be managed.
Click to expand...

+1 to this and it's a nifty way of telling when your deco is likely done as you start to feel loop crush due to the loss of O2 rather than a drop in PPO2. It's a feel good on top of what ever your deco algorithm is telling you.
 
Captain Starfish said:
Practically?

Get to the end of a longish dive and when you're on a 5m safety stop, kill your ADV and flush the loop a few times to get your 1.5bar PPO2. Then drop to minimum loop.

At this point the loop is full of oxygen, your lungs (as part of the loop and flushing) are full of oxygen. Oxygen comes into the circuit when you hit the MAV, goes out through your metabolism, CO2 comes into the loop from same but gets yoinked by the sorb before it hits the sensor - assuming your sensors are downstream of the sorb.

Watch your PPO2 for five minutes. It will steadily drop and the only place the diluent gas to do that can come from is you.

Not a lot, maybe, but plenty enough for me to watch a steady drop after a 90 minute to 18m dive. And if you're finely tuned to hover at 5m it's enough to get you floaty, enough that it needs to be managed.
Click to expand...

I'm not sure if I see it exactly that way with regard to 'diluent coming from you'.

You are consuming O2 and replacing most of it with CO2 which is then absorbed. So there is less gas, the loop volume is reduced, and for this reason the PO2 drops.

Ok so you might well be off gassing some N2 and He as well which would further reduce the PO2 but I don't think this is the main reason, I thought this had been shown to be negligible on previous threads.

Of course I am quite likely to be wrong here.
 
Baron015 said:
I'm not sure if I see it exactly that way with regard to 'diluent coming from you'.

You are consuming O2 and replacing most of it with CO2 which is then absorbed. So there is less gas, the loop volume is reduced, and for this reason the PO2 drops.

Ok so you might well be off gassing some N2 and He as well which would further reduce the PO2 but I don't think this is the main reason, I thought this had been shown to be negligible on previous threads.

Of course I am quite likely to be wrong here.
Click to expand...

I think you might have missed it, in this case. The O2 disappears, so the loop volume drops because it stays at ambient pressure. The only reason the pressure of O2 will drop in this circumstance is if the fraction of O2 drops, and the only reason that's going to happen in a pure O2 circuit is if some other gas is added.
 
Baron015 said:
I'm not sure if I see it exactly that way with regard to 'diluent coming from you'.

You are consuming O2 and replacing most of it with CO2 which is then absorbed. So there is less gas, the loop volume is reduced, and for this reason the PO2 drops.

Ok so you might well be off gassing some N2 and He as well which would further reduce the PO2 but I don't think this is the main reason, I thought this had been shown to be negligible on previous threads.

Of course I am quite likely to be wrong here.
Click to expand...

If you have a loop that contains pure o2 and you consume some of it, the loop volume will decrease but it will still contain pure o2 so the po2 will not change. Assuming the depth remains the same the only way to change the po2 would be to dilute the o2.

Sent from my GT-I9300 using Tapatalk
 
Ok fair enough.

But in the real world my loop is at 1.4-1.5 when at 6m stop, no not pure O2. Therefore there is existing diluent gas which decreases the fraction of O2 as you consume it. Therefore ppO2 drops. I would have thought this was a bigger effect than off gassing.

However I am keen for someone to do the calc and actually tell us how many litres of gas come out of supersaturation and are vented through the lungs during stops.
 
MeRodent said:
In another thread Chris made a comment regarding the effect of off-gassing in buoyancy. He indicated that this could cause a noticeable change.

I'm a bit sceptical about this as I'm not sure that the volume of gas would be sufficient to cause a buoyancy change.
Click to expand...

Do a normal CCR ascent with no deco obligation. Repeat x 1000

Do a 100m dive with a ton of deco and see what happens when you get to 6m. EVEN WITH PO2 ABOVE SET POINT AND NO SOLENOID. Watch the counter lungs inflate over time.

Then see what happens when you have 'off-gassed'

It all stops! Magic! :)

Also - see what happens to your p02 when all that inert gas comes out. Weird!
 
have to say i cant tell/dont notice , when im at 6m i often vent the unit and stick in more o2 ,
can say the loop gets more Stable towards the end of my deco .

so id go with what chris is saying ,

i also remember someone working on a gizmo the would look at off gasses in the loop and tell you when the deco was done,
 
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