Op Amp choice

[risototh]

I was today in my office and the oxymeter was still alive. Nothing strange on that, except, that it showed 72% of oxygen in the air. There were around 13 mV out of the cell. As you can see on the image below, it was calibrated on around 4 mV (maybe less as there was constant decrease in the time)... The cell is out of date and out of order, of course. But i'm surprised with the voltage! And it is quasi imune to oxygen change. Do you know something about this type of oxygen cell failure? (Yes, i checked the output with voltmeter),

[Igor P]

Never saw something similar...

Sent from my PAP4500DUO using Tapatalk 2

[Captain Starfish]

I had an AP12 cell fail reading over 200mV in air, two days after i opened the packet. Replaced under warranty but I have no idea how it did that: didn't even realise the chemistry would let it go that high!

[risototh]

It is old APD12...

I had an AP12 cell fail reading over 200mV in air, two days after i opened the packet. Replaced under warranty but I have no idea how it did that: didn't even realise the chemistry would let it go that high!

[testuser]

I know it'a a little late, but here's an alternative idea to try. AVR microcontrollers are 10-bits across the 0-AREF range, which is usually either teh supply rail(Vcc) or the internal 1.1V bandgap. I believe you can also supply your own voltage reference externally. putting a 200mV reference (e.g. a voltage divider from some precise LDO) would get you all 10 bits across that tiny range, and might produce a simpler design overall.

just a thought,
-James

[testuser]

nevermind, further down the datasheet i see that the minimum ARef voltage is 1V, so you'd only really get 1mV resolution ever. bummer.

[Snusmumrik]

I had an AP12 cell fail reading over 200mV in air, two days after i opened the packet. Replaced under warranty but I have no idea how it did that: didn't even realise the chemistry would let it go that high!

That's not the chemistry, but defective resistive voltage divider on the PCB attached to the cell.

[Captain Starfish]

That's not the chemistry, but defective resistive voltage divider on the PCB attached to the cell.

Thanks, kinda figured later on that it would be something like that. Meh, they replaced it.

Turtle Dude - got yours finished yet?

[turtle dude]

Ha ha, just been reading your new project Captain - nice.
Funny you asked about mine, I took my housing diving the other day and it flooded (was empty so no bigie), I was more concerned with my 2.2 bar ppo2 at 70m at the time didn't even notice my torch implode either lol.
Hoping to make up a new lid this week and test again soon.
Baz

[Captain Starfish]

Yeah, not looking forward to "debugging" that part of mine :-/