Setpoint during fast decent, what do you use?

[mr_v]

Im curious, what is the thinking for manually bumping up to 1.2 vs just letting the solenoid do it?

That never occurred to me but I’m going to have to give it a shot. I switch when I hit the bottom and I always do a brief pause after the switch to make sure the solenoid is behaving.

Better buoyancy control / fewer variables outside of your control to worry about.

If the delta between the current and desired PPO2s is big, the solenoid may fire a lot and increase loop volume substantially.

However, if you add O2 manually, you can fine tune the PPO2 and loop volume better. Once you reach your target depth, add O2 and take a deep breath right away to push fresh gas over the cells so that they adjust rapidly. Sit tight for a few seconds, then adjust as necessary.

 

[lostsheep]

Better buoyancy control / fewer variables outside of your control to worry about.

If the delta between the current and desired PPO2s is big, the solenoid may fire a lot and increase loop volume substantially.

However, if you add O2 manually, you can fine tune the PPO2 and loop volume better. Once you reach your target depth, add O2 and take a deep breath right away to push fresh gas over the cells so that they adjust rapidly. Sit tight for a few seconds, then adjust as necessary.

My solenoid does a good job of minimizing hysteresis, I've never noticed an undesirable volume increase. I thought I read somewhere it's a PID controller (I dive a Chop FWIW)

 

[mr_v]

My solenoid does a good job of minimizing hysteresis, I've never noticed an undesirable volume increase. I thought I read somewhere it's a PID controller (I dive a Chop FWIW)

What is your bottom PPO2 and what is your high set point?

You can use the guideline mentioned above - manual PPO2 control on descend then set the controller to high set point once you reach your target depth - in any virtually any situation.

 

[lostsheep]

What is your bottom PPO2 and what is your high set point?

You can use the guideline mentioned above - manual PPO2 control on descend then set the controller to high set point once you reach your target depth - in any virtually any situation.

MAX Dil PO2 of 1.2 on the bottom which is my high setpoint also.

 

[tbone1004]

Im curious, what is the thinking for manually bumping up to 1.2 vs just letting the solenoid do it?

That never occurred to me but I’m going to have to give it a shot. I switch when I hit the bottom and I always do a brief pause after the switch to make sure the solenoid is behaving.

If the ppO2 is lower than the setpoint you are switching to then you risk some janky buoyancy situation as it adds gas to the loop. Far better for you to stabilize the ppO2 without compromising buoyancy then switch over to let the unit maintain that ppO2 IMO.

 

[lostsheep]

If the ppO2 is lower than the setpoint you are switching to then you risk some janky buoyancy situation as it adds gas to the loop. Far better for you to stabilize the ppO2 without compromising buoyancy then switch over to let the unit maintain that ppO2 IMO.

Explain to me why the solenoid would be adding more O2 than I would using the MAV to bring the po2 up to the high setpoint.

That the solenoid would be worse, assuming no overshoot which I thought I mentioned with the minimal hysteresis , makes no sense.

 

[stretchthepenn]

Explain to me why the solenoid would be adding more O2 than I would using the MAV to bring the po2 up to the high setpoint.

That the solenoid would be worse, assuming no overshoot which I thought I mentioned with the minimal hysteresis , makes no sense.

I don't think the total added volume is the problem; as you said, the volume is equal.

I think the problem with buoyancy comes from the O2 injections' timing and the diver's lack of control over them.

If the solenoid takes over, then you're probably going to get a handful of injections rather than just one long-ish blast. *Fffft. Fffft. Fffft.* You'll definitely need to adjust your buoyancy after the series of injections is done, but when that series finishes is unclear. A stream of short injections can leave you playing catch-up.

Compare that situation to injecting O2 manually. You might overshoot your high setpoint, but you're also in direct control of the unit, meaning you'll have a better feel for the O2 injection's effect on your buoyancy.

 

[GF99/99]

Why not just match your dil with your bottom SP.

You arrive at depth and your PO2 will be really close to your SP. Flip over to high SP and have a nice dive

Keep it simple.

 

[nickbutcher]

Why not just match your dil with your bottom SP.

You arrive at depth and your PO2 will be really close to your SP. Flip over to high SP and have a nice dive

Keep it simple.

Makes dil-flushing more difficult

 

[mr_v]

Explain to me why the solenoid would be adding more O2 than I would using the MAV to bring the po2 up to the high setpoint.

That the solenoid would be worse, assuming no overshoot which I thought I mentioned with the minimal hysteresis , makes no sense.

It is not the volume but rapid solenoid firing (i.e., volume increase) that may screw you up.

Suppose your bottom dil PPO2 is 1.0 but your high set point is 1.2. You arrive to your target depth, PPO close to 1.0. If the solenoid is set to fire automatically, the solenoid will try to rapidly make up the difference between 1.0 and 1.2. That's a lot of gas added in a short time, depending on your context, it may screw you up.

Here is an analogy. You're driving at 80mph on cruise control. You have to slow down to 60mph. Then, instead of accelerating slowly and at your pack to the previous speed, you resume cruise control and is rapidly trying to take you to 80mph regardless of what is on the road, your lap, etc. It is not the acceleration that will screw you up, it is the context in which acceleration happens.

While it is setup dependent, O2 sensors may not react quickly to O2 content changes in the loop. Therefore, the solenoid will fire many times in sequence w/o necessarily waiting for the gas to blend and propagate. That's another reason why manual addition on the bottom may help. Add O2, pull it through the loop by inhaling, adjust. You can experiment with this by having a lower PPO2 than your high set point, then let solenoid dump O2 - notice how quickly it happens and how much it affects your unit.

You can argue that you bottom DIL PPO2 is very close to your high set point and that during the descent you'll manage to get PPO2 close to the high set point anyway... That's ok, no one is arguing against that possibility. This thread, however, outlines a generic solution that will likely simplify things in most cases, including diving with hypoxic mixes which may yield 1.0 PPO2 at your target depth.