DIY ppO2 meter

GKAM

Hi all,

Having experienced enough hassle with trimpots and simple voltmeters used on most KISS rigs, a friend and I have decided to build a DIY ppO2 meter for my IDA-KISS and a Ray SCR.

Here is how the unit works at the moment:

Cell calibration is done on air. Once you turn on the unit, it prompts you to expose the cells to air and counts down from 30 to 1 to allow for them to settle. The unit then proceeds to automatically calibrate both cells assuming they are exposed to air. If the mV values are too high or too low, then it will refuse to calibrate. This way, it is impossible to calibrate with the cells still on the loop or if the cells are fading/failing.

During dive mode, the unit checks ppO2 values and gives *STATUS OK* for acceptable ppO2 ranges or warns you in case of hypoxic or hyperoxic mixes. The warnings are given via i] flashing messages on the LCD screen, ii] flashing LED and iii] buzzer. The unit reads 2 cells, since on my IDA the third cell is separate feeding a HS Explorer Model-O.

You can use any sensor for the unit. Power source is a simple alkaline 9V battery, the type you can get anywhere in the world at a very reasonable cost abd we are also working on adding a low battery indication.

The display is 128x64 pixels [the same as on the HS Explorer]. We have decided not to cramp many details, adhering to the Keep It Simple, Stupid concept. The user gets only plain, big messages with few numbers to avoid confusion. Plus a flashing LED when there is a warning, and a loud buzzer. Even older divers will not have any trouble reading the large display.

Now, after the long intro, here comes the big question:

Air or O2 calibration???

At the moment, the unit is set to calibrate the Cells on air. I believe it is safer to calibrate on air and it is also more practical and more economical. However, I do acknowledge the issues faced with the not-so-linear linearity (wow, this is an oxymoron ) of the curve towards high O2%.

How significant is the error? I have not done tests yet but I am curious to see what others think. It would be great if you had some methodology to suggest for the tests or if anyone could contribute info&facts rather than axioms like "my instructor told me to always calibrate on O2..."

divetheworld

GKAM,

You appear to have had some experience on Rebreather's now so I would assume that this very important principle would be second nature to you.

Two reasons;

1. Accuracy. The closer you calibrate (on single point calibration) to the setpoint you will be diving at, the more accurate the control will be. The device you use to measure po2 is essentially a multimeter with a multiplier. An error of 3% is expected in good cells over this span. Cells with problems looming will not be linear and thats a recipe for disaster.
2. Current limiting. Cells are like batteries, when they die off, they have a reduced output which will inhibit their ability to show high PO2's. If you calibrate in air, you dont know if they can work at 1bar po2.

Think about it for a moment. You will be diving at upto 1.6 po2, you calibrate at 0.21. Thats a lot of error which can be produced by the time your instrument gets to FSD. Calibrate at 1.0 and you are much closer to the working point and less compound error to be introduced.

Additionally there is the problem of humidity. If you cal in dry air, the breathed loop at near setpoint will be somewhat differing from the measured value.

I could continue writing all about this subject but these threads will give you almost all the insight you will need.
http://www.rebreatherworld.com/techn...ibration.html?

http://www.rebreatherworld.com/techn...rocedures.html

Brent.

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denzel

True, but wouldn't it be workable if you'd cal on air before the dive, and do a full O2 flush at 6m/20ft?

When PPO2 reading comes up to 1.6 (or near enough) continue dive, if reading stays behind, end the dive. Ofcourse it would be nice to know the high end of your sensors before you get wet, but honestly, how often would you have to abort?

denz.

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Stephane

So why not calbrating it into pure O2 before diving? It would save you an aborted dive.

PCDiver

Why don't you make it user selectable?

denzel

Cal on O2 at the surface still "only" takes you up to 1. While doing the 6m/20ft O2 flush gives you the really high end, and is a sensible thing to do either way, even if calibrated on O2 on the surface, so why not make it a habit?

Unless it's technically no more difficult/ expensive to be able to cal on air and O2 on the surface. But, not hindered by too much experience, I'd probably be happy to skip the O2 call on the surface in favour of air cal on surface and O2 at 6m/20ft.

Denz.

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divetheworld

Its a bit late to find you need to replace cells when your at 6m with the USS Sarratoga looking up at you dont you think?
Giving the best cal, and the best chance of detecting a bad cell/s is bad because of what?
I have found cells directly out of the package that calbrate at 1.0 but read 1.6 at po2 of 2.0. Linearity upto 1.6 should be checked regularly by anyone diving rebreathers with setpoints greater than 1.0.

I'll leave you guys to it, this has been talked to death on other threads and it seems no-one is listening.

Brent.

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Stephane

You're absolutly right.

GKAM, what type of micro-controler are you using? Are you planning to give the schematics and programm of your PpO2 meter to the community?

GKAM

Hi Brent,

I am fully aware of these problems as you rightly said, however I remain to be convinced of the size of the actual error / offset. I mean, the cell health status is easy to establish since my ppO2 meter displays the mV at all times. Also, it is impossible to callibrate weak/failing cells because of the logic used.

Does anyone have measurements of this offset? I intend to run my own tests but it is not that easy to setup such a test environment. Can you provide any data for the actual error introduced when doing air cal?

What I am arguing is that it is safer to cal on air since it is the only true reference gas (provided the cal tradeoff is acceptable).Doing an O2 cal with the cells in the breathing loop leaves room for other types of mistakes and problems.

GKAM

jaap

Hi

I just built a small pressure pot that I test my cells in. I just built it a few weeks ago so I haven't tested it much yet.

I used instrument grade O2 (99,99+ % O2) purged the pot 4 times 1bar -> 2bar -> 1 bar. And used a good (?) manometer reading from 1-3 bar.

Cal at 100% O2 at 1atm to pO2 = 1

At 1,5 bar, reading 1,501
At 1,8 bar, reading 1,800
At 1.99 bar, reading 1,999 (max values of display)

The last 1,99 bar value is really 2,0 but its on the verge of maxing the display.

The interesting bit is that when I take the cells out and let them sit in air for 5 minutes they read: 0.231

So an air cal would give a reading that is a little low for high PO2.


The cells (R22) are about 6 months old and from Analytical Industries and give 10mV in air.

I also tested some of my old cells that I don't use anymore. Sure an mV test would indicate that they are old. With them its possible to calibrate to air (0,21 Po2) at 1 bar. And they give around 0.96 PO2 exposed to 1 bar 100% O2. But at 1.5 bar 100% O2 they are really off reading a 1.2 PO2.

So for me I stick to calibrating the cells with 100% O2 at 1 atm before the dive. And testing the cells in the pot now and then.

I feel that its to much hassel to do O2 purging to check cells on the way down.

If I get around to it I will test my cells in the pot having it in an ice bath and in a heated bath to check for temperature effects. And to have 100% humidity in the pot to see if there is an effect form that.