Setpoint during fast decent, what do you use?

[mr_v]

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.

As @nickbutcher said - dil flushing becomes more efficient.

Operations is another reason. Suppose you dove with 15/55 to 75m; dil PPO was 1.27 on the bottom and your high set point was 1.3.

You still some of that gas left, but your next dive is 60m. What do you do? Well, if you don't have extra CCR tanks, you probably want to use 15/55 again. However, now your bottom dil PPO2 is 1.05.

 

[mderrick]

As another poster mentioned, for the most part it doesn't really matter because on rapid descents to significant depths I am adding diluent almost continuously. I routinely dive two different rebreathers: an O2ptima which does an instantaneous switch from surface to bottom setpoint and that I have configured to switch at 20 fsw from 0.5 to 1.2 , or a Poseidon which gradually ramps up the setpoint from the surface to the bottom setpoint and I have configured to switch during descent from 0.7 to 1.2 po2. With the Poseidon if I don't bump the O2 MAV once or twice during the descent I will typically arrive at the target depth before the loop PO2 catches up to the setpoint.

Since nearly all my diving is hot drop I routinely descent at measured speeds well exceeding 100 feet per minute. I prefer to jump with my ADV off, so with those ultra rapid descents my hand is pretty much flying back and forth between the MAV to inject dil and my mask nose pocket to clear during the entire descent. I don't recall ever spiking my PO2 during descent on these two units unless I've manually injected O2 during the descent or started the descent with near 100% in the loop.

 

[tbone1004]

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.

What was said above. Basically it is going to add it all in one go so you're going to be venting from your nose to keep min-loop volume while trying to breathe to keep the gas mixing to make sure the sensors see the right mix, etc. If you bring it up manually you can take a breath in, see what the ppO2 is, add some while exhaling to min-loop, then repeat as necessary. Stability is nice with CCR's, rapid injection of a gas is annoying.

 

[Wibble]

(Wanted to go through the numbers on this one as an exercise so have shared it here).

Normaly I switch from 0.7 to 1.3 during descent at about 20m. I had a discussion: Dil 21/35, 20m, switch to 1.3, stay for 1 min, then very fast descent to 60m. A lot of dil is added, which brings the mix close to 21/35. But you still have the mix which was in the loop at 20m, this is a hotter mix.

Firstly STOP. Your 21% diluent is way too hot!
The MOD of the diluent should be less than 1.1 at the target depth of 60m/7ATA.
The PPO2 for 21% at 60m = pressure (7ATA) x mix (0.21) = 1.47 way way too hot for a rebreather (and too hot even for OC where 21% has a mod of 57m/190ft).

The optimum mix for 60m = PPO2 (1.1) / pressure (7) = 0.157, i.e. 15%.
Therefore, the correct diluent would contain 15% oxygen. Am only considering the oxygen here and not interested in the helium/nitrogen mixes as they are both inert gases.

Comparing the pressures: surface=1ATA, 20m/60ft=3ATA, 60m/200ft=7ATA.

Assume a total 5 litre minimum loop volume -- meaning all the fixed spaces: scrubber, hoses, etc. You normally run minimum loop, certainly during a descent.

Jump in with a setpoint of 0.7 x 5 litres = 3.5 litres of oxygen + 1.5 litres of inert diluent gases

As you're descending, the loop collapses and you add ONLY diluent (15%), probably through the ADV (auto diluent valve) or manual diluent injection.

Descend to 20m where the loop volume is now 3ATA x 5 litres = 15 litres as in you've added 10 litres of diluent to be able to breathe. This gives you the original 3.5 litres of oxygen from the surface, plus the rest which is your diluent, 15 - 3.5 = 11.5 litres of 15% diluent (11.5 litres x 0.15% = 1.725 litres of oxygen), total oxygen in the loop is now the original 3.5 litres of oxygen plus the 1.725 litres from the diluent which is about 5.2 litres.
5.2 litres of oxygen in a loop containing 15 litres of gas is about = 35% oxygen and 65% inert gas. For the PPO2, this is multiplied by the absolute pressure; 35% x 3 = 1.05, with 65% x 3 = 1.95 (i.e. 1.05 bar of oxygen, 1.95 bar of inert gas, total = 3 bar)

Continuing the descent through to 60m/7ATA, only adding diluent containing 15% oxygen as the loop collapses...
The loop now contains 5 litres (the surface volume) x 7ATA water pressure = 5 x 7 = 35 litres. This consists of the original 3.5 litres oxygen injected on the surface, plus 35 - 3.5 = 31.5 litres of diluent containing 15% oxygen = 31.5 * 0.15 = 4.725 litres of oxygen from the diluent, total of 3.5 + 4.7 = 8.2 litres of oxygen.
The inert gas is the rest, so 35 litres - 8.2 (oxygen) = 26.8 litres of inert gas, i.e. 0.23% of oxygen and 76% inert gas.

For the PPO2, the percentage (23%) is multiplied by the absolute pressure (7ATA) =
0.23 x 7 = PPO2=1.61, with 76% * 7 = 5.32 (i.e. 1.61 bar of oxygen, 5.32 bar of inert gas, total = 7 bar)

Hang on... that's a lot of oxygen in the loop as we've not considerd the human who's breathing and metabolising the oxygen.

Jumping in and descending isn't the most relaxing phase of any dive; a bit of stress and exertion. Assume 1.5 litres of oxygen would be consumed per minute and assume it takes 3 mins to descend to the target depth of 60m/200ft (which is pretty quick -- check out your dive logs, most of my 60+m dives take longer to reach the bottom).

This means we can subtract 1.5 litres/min consumed x 3 minutes = 4.5 litres of oxygen from the loop.

Re-work the numbers:
3.5 litres of oxygen on the surface plus 4.7 litres of oxygen from the 15% diluent minus the 4.5 litres of oxygen metabolised plus the (4.5 litres of diluent at 15% = 0.675 litres) which replaces the metabolised oxygen = 4.375 litres of oxygen in the loop. 4.375/35 = 0.125 = 12.5%.
PPO2 is 0.125 percent x 7 ATA pressure = PPO2=0.875.

Once on the bottom you can sort out the setpoint high (1.3 in your case) as part of your arrival checks. Probably you'd have injected some additional oxygen as you descended to bring the PPO2 up a bit -- experience, etc.

Need to restate that the actual bottom PPO2 would depend on a load of factors and this exercise has assumed a fixed 5 litre loop which may be on the low side as the lungs need gas too. Your metabolic rate will vary, as will the time it took to descend. Most of my deeper dives have ended up with around PPO2 of 1.0 to 1.1 at the target depth as my Revo has a "leaky valve" / orifice. Tend to manually inject oxygen and run the unit in parachute mode (e.g. setpoint of 1.2 and run it manually at 1.3; the hiss of the solenoid is a kick in the backside meaning that I've not monitored/predicted the PPO2).

Edit:

(Wibble, this differs from your final result because you neglected to refill the loop after metabolism.)

Many thanks for spotting that. Have updated the lower PPO2.

 

[formernuke]

I keep my Low SP (which, for me, is 0.5) until I get to the bottom and manually add Dil & O2 during the descent (O2 until 5-6m and then generally just Dil). I then switch it to High SP at the bottom of the descent.

I also switch back to my Low SP when I start my ascent and manually manage my PPO2 during ascent & deco.

Basically, the only time my solenoid is firing is on the bottom portion of the dive.

- brett

Same.

I hit 1.68 during a decent from 50 to 100. Fast to correct but still...

 

[lostsheep]

I don’t know what to tell you guys but I don’t have problems with a solenoid screwing up my buoyancy as I hit the the bottom and switch setpoint. Maybe it’s because I am negative on descent and as I approach the bottom I’m adding suit gas , wing gas, and switching SP almost all at once to arrest the descent. Maybe it’s because I’m just used to my units behavior. Maybe it’s because my PO2 is already close to high SP that there’s just not much being added. Maybe I’m just diving so deep it only needs a tiny bit of o2 to bring the po2 up

I’ll go try manually bumping it up before switching and see if I’m just FOS on this one, but I am skeptical.

 

[Agro]

Firstly STOP. Your 21% diluent is way too hot!
The MOD of the diluent should be less than 1.1 at the target depth of 60m/7ATA.

I sometimes hear so. 20 years ago instructor said: if you dive pO2=1.3 then MOD of Dil should be about target depth at 1.3.

It seems this has changed. I was told it should be less than 1.1 for a better dil flush. I must admit I do not understand this. If I do a dil flush (at any depth) I must know what is the expected, resulting pO2. If Dil flush produces this pO2, sensors are fine. So why should MOD be less then 1.1? I am sure there is a reason but which one?

 

[tbone1004]

I don’t know what to tell you guys but I don’t have problems with a solenoid screwing up my buoyancy as I hit the the bottom and switch setpoint. Maybe it’s because I am negative on descent and as I approach the bottom I’m adding suit gas , wing gas, and switching SP almost all at once to arrest the descent. Maybe it’s because I’m just used to my units behavior. Maybe it’s because my PO2 is already close to high SP that there’s just not much being added. Maybe I’m just diving so deep it only needs a tiny bit of o2 to bring the po2 up

I’ll go try manually bumping it up before switching and see if I’m just FOS on this one, but I am skeptical.

It's all about a balance and when you switch and also about how fast you're rocketing down. If you have a hot dil mix it's less bad, if you have the unit on high setpoint the whole descent it's less bad, but if you're diving a lean mix and switch late then it can add a bucket of O2 to the mix. It's something that after a few dives of letting the unit do the work that you probably would forget about and just automatically adjust without thinking about it but the most efficient way to do it in terms of gas usage in this case is to actually do it manually if you have a big discrepancy. Since I run the same mixes that I would on OC I'm usually above my setpoint after a rapid descent so I'm breathing the gas down anyway so switching setpoints doesn't change anything but the discussion of which mix you should be on is hotly debated....

 

[formernuke]

I was told it was so dil would bring ppo2 down if you needed to.

 

[tbone1004]

I sometimes hear so. 20 years ago instructor said: if you dive pO2=1.3 then MOD of Dil should be about target depth at 1.3.

It seems this has changed. I was told it should be less than 1.1 for a better dil flush. I must admit I do not understand this. If I do a dil flush (at any depth) I must know what is the expected, resulting pO2. If Dil flush produces this pO2, sensors are fine. So why should MOD be less then 1.1? I am sure there is a reason but which one?

the usual argument is faster recovery of a high ppO2 spike since it will crash the ppO2 faster. Ironically the Meg has O2 addition in the inhale counterlung to counteract really low ppO2's so it's all about your personal priorities. Hell when I dil flush on some dives it brings my ppO2 up since on my Meg I run a 1.1 because of the HUD but it will still bring a 1.6+ ppO2 down quite quickly. You get to choose your priorities