Liberty Deco Question

[v101]

Wanted to ask a question to all Liberty divers out there, when switching modes on your liberty from say your high set point to your deco set point, does the computer control your PP02 level to your deco set point or do you have to manually control?

My experience is say i am at 20' or even 25' i set the computer to deco mode, and my PP02 just sits there. i have to manually add oxygen. I did see this in the users manual;

"The evaluation of the ability to reach the setpoint is influenced by the maximum FO2. For instance, a setpoint of 1.3 in 3 meters can be reached, but only if there is 100% of oxygen in the loop, but if the FO2 is set to 80% then the setpoint will be displayed in yellow, and the solenoid will not add oxygen for as long as the value of ppO2 does not drop below 1.04bar [1.3×0.8]. The setpoint value will be displayed in red only when the setpoint is changed."

This statement makes sense, having the computer in deco mode at the surface, the higher set point say 1.4 or 1.6 isn't achievable.

but...

Achieving a PP02 of say 1.4 at 20' is definitely able to be reached. I did a test at a deeper depth (80') I entered the deco mode, and sure enough I could see my PP02 rising, I switched back to my HIGH set point, and added some DIL and PP02 returned to my high set point.

wanted to get any input, I am still re-reading the manual, and Im going to reach out to my instructor on this question.

 

[R80]

My experience is say i am at 20' or even 25' i set the computer to deco mode, and my PP02 just sits there. i have to manually add oxygen.

What are the High/Deco values, what's the FO2 limit set on your computer, and what's the PO2 you're having to drive up from? FO2 limit is under menu - setup - setpoints - FO2 limit. Mine's set for 90%, which acts as the upper limit that the solenoid should drive the unit to. So if the setpoint calls for 95% O2 at the current depth but the limit is 90% then that will act as the upper limit that the unit will attempt.

 

[Alekseolsen]

As a curios No-ccr certified diver, what is the FO2 limit there for? Preventing the unit from bursting in O2 to the point were you have to vent the loop?

 

[R80]

As a curios No-ccr certified diver, what is the FO2 limit there for? Preventing the unit from bursting in O2 to the point were you have to vent the loop?

Naively, let's say you set a Deco setpoint of 1.5 on a computer that's programmed to inject O2 via solenoid when PO2 falls below 1.5. At 20 feet everything is fine and reaches a steady state, but now you ascend to 11' (chosen for easier math). First, the solenoid can only add O2, it can't pull N2 so it can increase the ratio but it can't ever get you to 100% FO2. Second, FO2 100% yields PO2 1.3 ((11/33+1)*1=1.33). So, there's no amount of oxygen injection in the universe that can get you back up to PO2 of 1.5 at that depth. The second reason means there should be a limit to sought PO2 no higher that 100% FO2 at depth, and for the first reason it makes sense to back the FO2 limit down from 100% to a value that is allowed to contain some N2.

In short, the limit is there to prevent the solenoid from chasing conditions it can never achieve.

 

[caju barbosa]

Great topic