Inspiration PPO2 Setpoints

[mempilot]

I've been reading up on the Inspiration, as I'm thinking of transitioning to CCR next year. As I understand it, the low setpoint is .7 bar for understandable reasons. I have a question about the high setpoint of 1.3 bar. When I OC technical dive with a decompression obligation, I often plan my mix for 1.5 PPO2 at max depth. Is there a reason why this would not be advisable on CCR?

 

[padiscubapro]

I've been reading up on the Inspiration, as I'm thinking of transitioning to CCR next year. As I understand it, the low setpoint is .7 bar for understandable reasons. I have a question about the high setpoint of 1.3 bar. When I OC technical dive with a decompression obligation, I often plan my mix for 1.5 PPO2 at max depth. Is there a reason why this would not be advisable on CCR?

first of all you are most likely doing longer dives so your oxygen clock will be running.... on oc you are only at the high po2 at depth, it immediately begins to drop on a CCR you have it all the time...
The Inspiration scrubber is rated at 3 hours, a po2 of 1.3 has 180 minutes ssingle exposure limit (the 24 hour limit is a bit higher)

ANothe point most ccr controllers overshoot their setpoint.. on normal inspiration electronics with a sp of 1.3 on deep dives its not uncommon for it to spike .1 (or more) higher..

lastly on a CCR, a difference between using a 1.5 and a 1.3 is not that great, for a majority of profiles its just a few minutes... You are already offgassing very effifciently because you are maintainging a high po2 all the way up.

 

[mempilot]

Thanks for the explanation. It makes perfect sense to me now. Sometimes I can't see the forest through the trees!

first of all you are most likely doing longer dives so your oxygen clock will be running.... on oc you are only at the high po2 at depth, it immediately begins to drop on a CCR you have it all the time...

The Inspiration scrubber is rated at 3 hours, a po2 of 1.3 has 180 minutes ssingle exposure limit (the 24 hour limit is a bit higher)

ANothe point most ccr controllers overshoot their setpoint.. on normal inspiration electronics with a sp of 1.3 on deep dives its not uncommon for it to spike .1 (or more) higher..

lastly on a CCR, a difference between using a 1.5 and a 1.3 is not that great, for a majority of profiles its just a few minutes... You are already offgassing very effifciently because you are maintainging a high po2 all the way up.

 

[Scubagrunt]

You are off gassing more efficiently all the way up using a constant PPO2 like 1.3, but there is nothing wrong with increasing the set point to 1.5 at the 20 fsw stop, so long as you have planed for the extra O2 time and do not violate the clock. But as mentioned it will only save you a bit of time depending on the profile, and may not be worth the extra effort.
How are the other RB divers doing the deco at 1.3 or 1.5? I would like to see what everyone else is doing.

 

[DrMike]

You are off gassing more efficiently all the way up using a constant PPO2 like 1.3, but there is nothing wrong with increasing the set point to 1.5 at the 20 fsw stop, so long as you have planed for the extra O2 time and do not violate the clock. But as mentioned it will only save you a bit of time depending on the profile, and may not be worth the extra effort.
How are the other RB divers doing the deco at 1.3 or 1.5? I would like to see what everyone else is doing.

Full O2 flush at 6m (several) to remove all inert gas then min loop volume O2 injection (using ADV plumbed into offboard O2 ) to keep ppo2 around 1.5ish.

 

[padiscubapro]

You are off gassing more efficiently all the way up using a constant PPO2 like 1.3, but there is nothing wrong with increasing the set point to 1.5 at the 20 fsw stop, so long as you have planed for the extra O2 time and do not violate the clock. But as mentioned it will only save you a bit of time depending on the profile, and may not be worth the extra effort.
How are the other RB divers doing the deco at 1.3 or 1.5? I would like to see what everyone else is doing.

1) Leave setpoints as is DO not raise
2) disable ADV (I prefer to leave it connect to onboard dil)
3) evacuate and flush loop several times
4) add o2 manually as necessary...
5) When Po2 reaches about 1.4-1.45 do another oxygen flush

if you raise setpoint to "maintain" the high setpoint your buoyancy will be all over the place since you are offgassing nert gas into the loop so more oxygen will be needed to raise PO2.. If the loop can expand you will become more positive, if your loop volume is fixed you waste more oxygen than a flush..

Personally after a deep dive, I actually switch to a lower setpoint and manually control the po2 (usually around 12m/40) so I am in full control of the loop PO2 ( and my buoyancy) rather than at the mercy of the controller on ascent.. I have seen a few too many experienced RB divers lose control of their buoyancy because they got pre occupied with other stuff and were complacent that the controller will keep the loop PO2 correct