A question for homebuilders

[Brian Garner]

Hi

Has anyone ever tried an automatic setpoint adjuster, not depth triggered but the setpoint changes according to the loop PO2.

I am thinking about setting the lower setpoint at say 0.7. If a manual injection was done to push the setpoint to 1.1 then the controller would automatically set the sp to 1.3 and hold it. To lower the SP just flush and dump till the Loop is under 0.9 and the SP is switched to 0.7 again. This may be a bit fiddly on the ascent but it also means you dont need a separate switch for setpoint adjustment.

I have thought it through and I cant see a problem but I am sure someone out there will tell me why it wont work. There must be a point in the dive where it could cause problems but I cant see it at the moment.

Brian Garner

[johnv]

Originally Posted by Brian Garner

Hi

Has anyone ever tried an automatic setpoint adjuster, not depth triggered but the setpoint changes according to the loop PO2.

I am thinking about setting the lower setpoint at say 0.7. If a manual injection was done to push the setpoint to 1.1 then the controller would automatically set the sp to 1.3 and hold it. To lower the SP just flush and dump till the Loop is under 0.9 and the SP is switched to 0.7 again. This may be a bit fiddly on the ascent but it also means you dont need a separate switch for setpoint adjustment.
Brian Garner

I can see several issues, You really want to be able to change the "high" setpoint possibly underwater, maybe to say 1.0 for long dives. Also, you may want to change the setpoint to a low value before ascent, but wil be unable to do so if your dil PPo2 at your bottom depth is too high, this wastes oxygen on ascent. There is also the obvious big problem of accidental changes if you need to flush your loop anytime. interesting idea though.

John

John

[uri]

Don't forget that the pp02 spikes high on the way down. If that triggers your setpoint adjuster then it will inject 02 to try to get to and keep the high setpoint. This will spike the setpoint even higher and your continued descent will spike it even more. Sounds dangerous to me...

U.

[Brian Garner]

Hi John
> You really want to be able to change the "high" setpoint possibly >underwater, maybe to say 1.0 for long dives.

With my idea you would have to swap to the lower SP and then manually keep it at 1.0, not letting it get high enough to swap to 1.3,

>Also, you may want to change the setpoint to a low value before ascent,
This is a new idea on me. The Po2 drops on ascent so setting it to a low value may push it too close to a hypoxic situation.

>but wil be unable to do so if your dil PPo2 at your bottom depth is too high,
If I was diving air, say 40 m max then botton po2 = 1.05 I could just get away with. If a 10/50 mix then at 90m still max po2 of 1 bar.

On a deeper dive I assume there would be several deep stops and one of these would be ideal to swap the SP over. I dont see the logic of dropping to a lower Sp on ascent though. Surely you want the best Po2 to help off gassing.

>this wastes oxygen on ascent. There is also the obvious big problem of >accidental changes if you need to flush your loop anytime. interesting idea >though.

Yes flushes would change the po2 but you would be aware of this and compensate. I did think of a heads up led to show the SP anyway. say green for low and yellow for high.

Brian Garner

[Brian Garner]

Hi Uri

>Don't forget that the pp02 spikes high on the way down. If that triggers >your setpoint adjuster then it will inject 02 to try to get to and keep the >high setpoint. This will spike the setpoint even higher and your continued >descent will spike it even more. Sounds dangerous to me...


Good point but the Sp would only swap at say 1.1 or 1.15. If you were dropping fast enough you would soon hit the 1.3 setpoint anyway. In the time between 1.1 and 1.3 there would be O2 injection. A soon as 1.3 was reached there would be no more O2 injected and the Po2 still rising if you were imitating a brick on descent. I admit it would speed up the spike but not by as much as you seem to think. Not that much more than a descent fixed at 0.7.

Whats that about the Scamahorn slide anyway. Hope I spelled that properly

Thats the reason I picked 1.1 and 0.9 as switch points, They are chosen so the accidental changes are unlikely.
Dont forget that you would know these risks as you dived the rig and compensate for them. Thats the same with every rebreather. On a kiss the sp suffers the same problems. Still injecting on descent and dropping on ascent. Wether the continual injection on descent makes any difference I dont know but it is constant.

PS can you tell I dont know how to quote properly.

Brian

[wreckweasel]

Originally Posted by Brian Garner

On a kiss the sp suffers the same problems. Still injecting on descent and dropping on ascent. Wether the continual injection on descent makes any difference I dont know but it is constant.

Does it?

It is as John says an interesting idea, but surely it would interfere with your ability to do a dil flush? I guess you'd have to incorporate a delay unit to measure/control the injection.

[Brian Garner]

Hi wreckweasel

Surely you have the same problem on any eccr. When you do a dil flush the po2 drops and the controller tries to compensate.

I am not sure how much the continual injection on the kiss makes a difference during descent, I suppose it depends on how fast you go down. The Kiss only injects a small amount of o2 but it is constant. Dil will be added as the depth increases, The O2 will be extra. What I should have said perhaps that the rise of PO2 on descent and drop on ascent is inherent in all rebreathers. Also OC as well if you want to be picky. On rebreathers it has the chance to become a dangerous problem.

At the moment this is only an idea and I just want to kick it around with a few other Homebuilders and any other knowledgable person who wants to offer an opinion.

[wreckweasel]

Buggered if I know mate. Im fick

I tend to find that with the KISS I blow the loop to pure O2 on the surface and then as I descend, the ADV blows in dil and generally brings me pretty damn close to 1.2 at target depth.


/Z

[nigelH]

Originally Posted by wreckweasel

I tend to find that with the KISS I blow the loop to pure O2 on the surface and then as I descend, the ADV blows in dil and generally brings me pretty damn close to 1.2 at target depth.

Pretty much the same on the YBOD. Leave it on 0.7 until I hit target depth and then switch it and the VR3 up to 1.3 as we look round the bottom of the shot and plan the route. Switching up earlier on a "slack won't last this is a spring tide descent" is just looking for everything beeping at you. I've had 1.4bar at 50m while still set to 0.7 and that's with an 18% DIL. I'm sure a lot of that comes from doing more than glance up = bubble check at 6m.

[johnv]

Originally Posted by Brian Garner

"Also, you may want to change the setpoint to a low value before ascent"
This is a new idea on me. The Po2 drops on ascent so setting it to a low value may push it too close to a hypoxic situation.

Maybe this is an unconventional idea, but you waste a lot of oxygen if you keep a high setpoint when you come up. You (or the ccr) inject o2, then you dump loop gas, then you go up, then you add O2, ....

on a square profile, this isn't a problem, but on a long sawtooth profile (eg following certain cave passages), the volume of oxygen injected then exhaled on ascents can rapidly eat through your O2 (as well as your dil)

clearly though you need to maintain a life supporting value (that works for your decompression calcs), but for an inspiration specifically, unless you do a bouyant ascent the solenoid will keep up. on a CMF ccr, you have manual control and so don't waste O2 as much.

John