PPO2 displays for dummies

[johnmaneely]

Hi,
I'm still studying at college and I'm thinking of making a ppo2 display or to for a project and hopefully dive with to. I know little of electronics at the moment so its a learning exercise for me as well. I know I probably need a 10 bit a/d converter, I think things pretty much all come with the DPM I choose to use, but needs modifying to get it reading sensibly with resistors and trim pots somewhere.

I've been reading this

Construction, Operation, and Maintenance of an O2 Meter

and talking to my lecturer and thats it. Can anyone one help me out with recommendations on parts and hopefully circuits diagrams. Any more explaination you feel I need would also be much apprieciated.

I know I'm asking alot and some of this info has been covered already, I'd just like a back to basics guide for complete idiots like me..

[johnmaneely]

Im thinking of using this in a bodge up rebreather, probably all manual control for fun in shallow water, not a o2 analyzer as the article, presume it makes no difference apart from the decimal point position.

[ardhill]

Don't suppose a calculator in a plastic bag would do

Sorry, that's the extent of my electronic engineering abilities

[nigelH]

Originally Posted by johnmaneely

I'm thinking of using this in a bodge up rebreather, probably all manual control for fun in shallow water, not a o2 analyzer as the article, presume it makes no difference apart from the decimal point position.

Well one kills you and the other doesn't but aside from that...

You are normally dealing with a cell that gives about 12mV in air, 60mV in pure O2 at 1 bar and you can factor from there.
You just read a voltage, apply a calibration factor that varies from day to day and read it out as either a percentage or bar.
You are talking about a device that is roughly linear- for the one I use that's 2% fsd a constant temperature and 5% fsd over it's quoted temperature range. To get enough volts on most A2Ds you may find you have to amplify it or your quantisation will show. If you don't amplify beware of the input load requirements on D2As - some want a very low source impedance.

Most home builders use a ready made display but that wouldn't make much of a project. If you're diving it think about oil filling the thing. Watch the display type. It's a lot easier than watertight boxes that always sound easy, sound like you can just buy one but somehow it never works out like that.

Good luck with it.

[zepp]

Originally Posted by johnmaneely

Im thinking of using this in a bodge up rebreather, probably all manual control for fun in shallow water, .

I can lend you either a Dyson or one made from a piece of bog-pipe...

[zepp]

Or a Russian one, come to think of it, but you will need a new o2 cylinder as the other went BANG !!!!

[Beardy]

Originally Posted by nigelH

Well one kills you and the other doesn't but aside from that...

You are normally dealing with a cell that gives about 12mV in air, 60mV in pure O2 at 1 bar and you can factor from there.
You just read a voltage, apply a calibration factor that varies from day to day and read it out as either a percentage or bar.
You are talking about a device that is roughly linear- for the one I use that's 2% fsd a constant temperature and 5% fsd over it's quoted temperature range. To get enough volts on most A2Ds you may find you have to amplify it or your quantisation will show. If you don't amplify beware of the input load requirements on D2As - some want a very low source impedance.

Blimey Nigel, the boy asked for an idiots guide for an electronics numpty
I only just followed what you've written,and I'd struggle a bit^H^H^H lot to put it into practise. And I've had some education in the ways of the electron.

[Nick Kay]

I got stuck after:

Originally Posted by nigelH

You are normally dealing with a cell that gives about 12mV in air

WTF is "air"???

I've heard of this substance called "swampgas" - is it some form of derivative of that?

And, of what use is a prison cell once you go underwater? Hang on, there's a prison ship down at Portland so is it something to do with that?

[nigelH]

Originally Posted by Beardy

Blimey Nigel, the boy asked for an idiots guide for an electronics numpty

That was numpty.
Try this.

[johnmaneely]

Sorry Nigel,(liked your page) didn't understand some of what you were talking of. I think a ready made display might be for me to start, the completion of the project is months away. So I could get to grips with this then maybe progress to something more complex. Recommended displays and circuit layouts would be great thanks. I am a complete novice as I said.

What about recommended cells and a data sheet, for them. Give me and my lecturer somewhere to start.
Cheers
John