Depth Sensors for CCR Home Electronics

[Andy_Corkill]

Hi Guys

I'm looking into building an Electronic O2 injection system for use with my KISS rebreather. Thinking around plumbing in an electronic 'Depth Sensor' to stop the unit trying to reach an unobtainable setpoint. Has anyone considered this before ? Do you know of a source for a 'Depth Sensor' like one used for example in a dive computer (VR3).

Thanks in Advance

Andy

[narked at 90]

hello Andy why would you want to use a pressure transducer? would'nt it have to be corilated against you fio2 from you cells to be of any use in an injection unit but the cells give you that already as a ppo2..depth x fio2
all you have to do it is tell it when to stop/start injecting against your ppo2 not you deepth.
imho and i may be wrong!!
best regards john routley

[apitkin]

Originally Posted by narked at 90

hello Andy why would you want to use a pressure transducer? would'nt it have to be corilated against you fio2 from you cells to be of any use in an injection unit but the cells give you that already as a ppo2..depth x fio2
all you have to do it is tell it when to stop/start injecting against your ppo2 not you deepth.
imho and i may be wrong!!
best regards john routley

I think he wants the setpoint to switch automatically to something <1.0 close to the surface. The Prism does exactly this, and it's good design. It switches from a setpoint of 0.7 at depths of less than 5m to the user-selected setpoint (e.g. 1.3) at depth greater than 5m.

No messing around switching setpoints when you're busy with other things on the descent (e.g. shooting video, running a reel, equalising ears/drysuit/wing), and in fact removes the need for any means of switching setpoint. If you want something different, you can always switch off the electronics and fly it manually.

Andy

[nigelH]

Originally Posted by apitkin

I think he wants the setpoint to switch automatically to something <1.0 close to the surface.

ARGHHHHH!!!!
One of the things I do is hold switching the setpoint up until I am already a long way down the shot or you get spikes on the oxygen. If I switched up at 10m that's got the system trying for 65%. I've seen the system spike to 1.65 at 50m and had to flush it just because I switched up at 20m rather than waiting until I was deeper. A good flush at 50m is a lot of dil.
Similarly on an ascent I switch down while doing the long runs or it is chucking O2 in as the system is dumping gas. However if you pull this trick you must be very careful to switch back up on stops or you blow your schedule away.

As to a sensor. I had some in once from a Swiss firm that were accurate and cost effective but I can't find the address now. Sorry.

nigelH

[Drmike]

Originally Posted by nigelH

Similarly on an ascent I switch down while doing the long runs or it is chucking O2 in as the system is dumping gas. However if you pull this trick you must be very careful to switch back up on stops or you blow your schedule away.

Hmm, does that potentially have an impact on your deco? Your PO2 will be dropping like a stone on those long runs and you are offgassing not only at the stops but also during the ascents so I would imagine your deco will be adversely effected.

You then have to pump up the PO2 at the stops either manually or wait for the controller - either way hardly the constant setpoint your deco plan is based on - or are you using a HH?

I find with practice a slow ascent is possible with very good control on setpoint provided you maintain a min loop as you ascend. I can normally hold it between 1.2 and 1.3 during all parts of the ascent, but then again I do do very slow ascents.


Added: Ive just run a simulation and it doesnt seem to make any real difference allowing a drop from 1.3 down to 0.7 during the early parts of ascent - which I guess makes sense as you are still below offgassing depth.

I imagine above the off-gassing depth there might be some effect but am not able to model that so easily,

[nigelH]

Switching down the setpoint for an ascent...

Originally Posted by Drmike

Hmm, but doesn't that potentially have an impact on your deco? Your PO2 will be dropping like a stone on those long runs and you are offgassing not only at the stops but also during the ascents so your deco will be adversely effected.

It's not a significant factor. Most of the off gassing on an ascent and the first part of a stop is because the overall pp has dropped. By the time the mix is the driving factor the loop is back to 1.3

Originally Posted by Drmike

I find with practice a slow ascent is possible with very good control on setpoint provided you maintain a min loop as you ascend. I can normally hold it between 1.2 and 1.3 during all parts of the ascent.

I don't think this actually gives any benefit and it just means you have the controllers desperately banging in oxygen and changing the buoyancy. I suppose this isn't really a problem unless you are doing a free water ascent with no line or blob but I've made a complete yo-yo out of some of them until I learnt to switch down. I'm probably just compensating for personal inadequacies that other people don't have.

nigelH

[Drmike]

Originally Posted by nigelH

I don't think this actually gives any benefit and it just means you have the controllers desperately banging in oxygen and changing the buoyancy. I suppose this isn't really a problem unless you are doing a free water ascent with no line or blob but I've made a complete yo-yo out of some of them until I learnt to switch down. I'm probably just compensating for personal inadequacies that other people don't have.

I know that feeling!

Actually I used to have problems on ascent as you describe. If you have a full CL and you ascend the gas expands and the PO2 drops. The solenoid opens but because the loop volume is so big now it has to add much more O2 to try to raise the PO2 than it would if the loop was min volume. Then you end up in a runaway or a yoyo situation.

If you maintain min loop during ascent between injections the solenoid will be firing into a non-full CL and so will need to inject less gas. I found the trick is to breathe out just the right amount of gas to compensate for the increase in loop volume that will occur before the next breath is taken during the ascent.

So in effect you blow out the gas and have an almost empty CL for a moment as you ascend.


Well it works Ok so ar for me.

I used to hate doing the 6m/4m stop before I started doing loop O2 flushes for the same reason - damn yoyo!

[nigelH]

Originally Posted by Drmike

Actually I used to have problems on ascent as you describe. If you have a full CL and you ascend the gas expands and the PO2 drops. The solenoid opens but because the loop volume is so big now it has to add much more O2 to try to raise the PO2 than it would if the loop was min volume. Then you end up in a runaway or a yoyo situation.

Amen brother.

I'll try for what is virtually negative volume next time I get to play.
I admit my loop volume is a lot more minimum these days than when I first learnt the trick but I'll probably stick to the switch down trick. It smooths out a lot of the bumps and it's no effort as I have the master controller in my hand free ascending...

I assume you don't have a trick to stop spikes on the descent. Leaving the switch up seems to work well for that and I'm often showing 1.1 when I hit target depth and select high setpoint.

nigelH

[narked at 90]

Originally Posted by apitkin

I think he wants the setpoint to switch automatically to something <1.0 close to the surface. The Prism does exactly this, and it's good design. It switches from a setpoint of 0.7 at depths of less than 5m to the user-selected setpoint (e.g. 1.3) at depth greater than 5m.

No messing around switching setpoints when you're busy with other things on the descent (e.g. shooting video, running a reel, equalising ears/drysuit/wing), and in fact removes the need for any means of switching setpoint. If you want something different, you can always switch off the electronics and fly it manually.

Andy

hi andy,you may be right but its of little use if you have a little look around at say 8m when it decides for you you need a set piont of 1.3 then you decide to go deeper and you spike on decent and have to vent the loop and start again so any advantage would have been long lost
but i guess as you say you can turn it off.....if you remember!
john routley

[apitkin]

you may be right but its of little use if you have a little look around at say 8m when it decides for you you need a set piont of 1.3 then you decide to go deeper and you spike on decent and have to vent the loop and start again so any advantage would have been long lost
but i guess as you say you can turn it off.....if you remember!

I've never had to purge the loop on descent. OK, I haven't done some of the kamikazi descents I used to do on OC, and sometimes the loop PO2 gets up to around 1.5 on a 1.3 setpoint if I'm dropping down quickly, but that doesn't bother me. It soon drifts down again. You rarely need to descend so fast that the PO2 spikes to dangerous levels. On the Prism if you need to do a fast descent you just switch the electronics off for the descent and then back on again at the bottom. Difficult to forget, as there should be a green light in front of your face, the absence of which is a pretty strong reminder. If you're going to be bimbling around at 8m or so, then what's the need to tear down to 50m or so? In which case leave the electronics switched on. This just isn't a problem for me in normal diving.

Or maybe the Inspiration's dynamic PO2 control isn't as good as it could be?