Homebuild Calibration

[Andy_Corkill]

Hi

Still playing with my homebrew electroinics facing the question 'How to calibrate my O2 sensors ?'

Showing my inexperience with electronics but.....

I'm currently using individual pot's on my Op amp to calibrate my sensors, which has it's advantages & disadvantages.

I realise there must be a different approach I can take because you don't calibrate using pots on the Inspiration. If I was to take this approach do I have to calibrate using the software only (using an error factor perhaps ?), or is there some groovy way of having a variable programmed resistor or Op amp.

Thanks Again

Andy

[nigelH]

Originally Posted by Andy_Corkill

Still playing with my homebrew electroinics facing the question 'How to calibrate my O2 sensors ?' I'm currently using individual pot's on my Op amp to calibrate my sensors, which has it's advantages & disadvantages.

The real problem is that as they age they drift so you will be forever going in with the screwdriver.

I realise there must be a different approach I can take because you don't calibrate using pots on the Inspiration. If I was to take this approach do I have to calibrate using the software only (using an error factor perhaps ?)

The YBOD just reads the raw voltages and during its set up phase blows pure oxygen in and when the cells stabalise says 'so that is what 100% looks like today' and generates a factor to multiply volts by to get bar. It also asks for the current air pressure as that will show in the second decimal place but few of us bother with that as I doubt my body is calibrated to be within a few percent of tables standard. Calibrating on 100% is much better as any errors at 21% are multiplied by 5 at one bar ppO2.

[wreckweasel]

Originally Posted by Andy_Corkill

Hi

Still playing with my homebrew electroinics facing the question 'How to calibrate my O2 sensors ?'

Showing my inexperience with electronics but.....

I'm currently using individual pot's on my Op amp to calibrate my sensors, which has it's advantages & disadvantages.

I realise there must be a different approach I can take because you don't calibrate using pots on the Inspiration. If I was to take this approach do I have to calibrate using the software only (using an error factor perhaps ?), or is there some groovy way of having a variable programmed resistor or Op amp.

Thanks Again

Andy

To do this you'd need something like a micro-pic. I've had an idea of a different way of doing this using 6 op-amps which should work, but the overall package would end up bigger than a micro pic. Arse.

[NotDeadYet]

I'd thought about adding a PIC into mine. Anyone know of a PICs for Numpties kit for someone who is an electronics incompetent?

[Brian Garner]

Hi

If you look on the sparkfun site there is some good info there.
Also if you need a basic compiler try the oshonsoft site. It is a nice intro and doesnt mean learning assembler. It is also very cheap. It does have its limitations but for the price you cant complain.

You will have to google for these but you will easily find them.

I have just done a similar thing myself, a software calibration for my homebuild.

Brian

[Andy_Corkill]

Hi,

I’m currently working with a PIC at the moment. Have got it set up read sensors, display output, light up a HUD & control my solenoid.

The PIC I’m using is a PICAXE-18X which has three analogue inputs – one for each of the cells. But I shall be upgrading shortly to a PICAXE-28X which has a forth analogue input which I intend using for a Pressure/Depth Sensor.

I’ve found the programming relatively easy (but there again I am a computer programmer), but have started as a complete novice with the electronics. The PICAXE chips are marketed towards educational uses & simple projects. The basics are reasonably easy and are well documented, but have now started the task of trying to make it a little more accurate. The big problem with the PICAXE chips is that the lines of code is limited, but it should be enough to get a basic rebreather electronics set-up off the ground. The idea to use these chips isn’t mine, take a look at

http://www.therebreathersite.nl/diver_dave_eccr.htm

Don’t take his example code as gospel (it seems to be working by the skin of it’s teeth), there’s plenty of room for improvement. There again it’s always easier to improve someone else’s idea than to start your own totally from scratch.

A word of warning from the PICAXE people ‘The PICAXE may never be used in a Safety Critical System.’

I guess that means rebreathers !!!!

Andy

[Andy_Corkill]

I should also add

I haven't dived it yet, its still stuck on a breadboard on my kitchen table !

Just a few wrinkles to iron out before taking it in the water.

Andy

[NotDeadYet]

Originally Posted by Andy_Corkill

its still stuck on a breadboard on my kitchen table !

All the best RBs start life in the kitchen

[Andy_Corkill]

& Fuelled by Red Wine I see ;-)

[NotDeadYet]

Ooooooh yes... Tesco's best!