A(nother) DIY o2 analyser

[JMBL]

Myself, I find the design most impressive : good hand old, and no cell hanging from a wire. Seems easy to use. I tend to pay more and more heed to such details : old age is slowly but surely creeping in and my hands, which were never deft, are getting worse and worse... Sell the design to a manufacturer and I'll buy it.

 

[stiebs]

Any plans to add any other sensors? I'm really interested in a DIY O2 + CO analyzer.

Yes, definitely! I was looking at various CO sensors, but to find one that is readily available, sensitive enough for breathing air analysis, factory calibrated and reasonably priced is not easy.

Most CO sensors are geared toward HVAC type testing and have a detection range of 0-1000ppm +/- up to 50ppm. For scuba tanks, I really want 0-100ppm with +/- <1ppm.

The best I've found is this:

https://au.mouser.com/ProductDetail/Amphenol-SGX-Sensortech/PS1-CO-10-MOD

It has a 0-10ppm CO sensor on a module that takes care of all the tedious calibration and should be easy to integrate, but it does comes with a heft price tag and a long lead time. But given that I consider my analyser a completed product right now, it might be time to bite the bullet for Gasalyser O2CO.

 

[rmssetc]

Yes, definitely! I was looking at various CO sensors, but to find one that is readily available, sensitive enough for breathing air analysis, factory calibrated and reasonably priced is not easy.

Most CO sensors are geared toward HVAC type testing and have a detection range of 0-1000ppm +/- up to 50ppm. For scuba tanks, I really want 0-100ppm with +/- <1ppm.

Yep.

The best I've found is this:

https://au.mouser.com/ProductDetail/Amphenol-SGX-Sensortech/PS1-CO-10-MOD

Like the form factor, the estimated lifespan of 3~5 years, the resolution & range. The price...could be OK in larger quantities.

Digikey shows an 8-week lead time from the manufacturer, but 16 units in stock.

It has a 0-10ppm CO sensor on a module that takes care of all the tedious calibration and should be easy to integrate, but it does comes with a heft price tag and a long lead time. But given that I consider my analyser a completed product right now, it might be time to bite the bullet for Gasalyser O2CO.

If I'm reading the datasheet correctly, that requires a 20-minute warm up time in clean air before it will give reliable measurements. Even if that's a typically conservative estimate, a 10 minute warm up would be a problem in many situations (ie., on a boat, trying to check tanks before leaving the dock).

I've got an old Cootwo sensor, and I'd be glad to open it up and send you photos of the part numbers, or the device (please contact me directly).

Similarly, I wonder what Forensic Detectors uses in their CO analyzer.

 

[stiebs]

If I'm reading the datasheet correctly, that requires a 20-minute warm up time in clean air before it will give reliable measurements. Even if that's a typically conservative estimate, a 10 minute warm up would be a problem in many situations (ie., on a boat, trying to check tanks before leaving the dock).

The datasheet is a little unclear. It does mention a 20 minute warm up time in the user guide, but on the spec page it states "The first power-on under storage in clean air <120 seconds"

 

[bobziuchkovski]

You might be interested in some of the recent discussion in this thread as well. @VladimirG has been testing one of the Winsen sensors. I'm keenly interested in the topic but haven't purchased any sensors to test yet.

 

[VladimirG]

You might be interested in some of the recent discussion in this thread as well. @VladimirG has been testing one of the Winsen sensors. I'm keenly interested in the topic but haven't purchased any sensors to test yet.

The sensor I use is Winsen ZE07-CO. It measures 0-500ppm, comes factory calibrated and with temperature correction, and is cheap (10$). I have tried it with a calibrated gas and a few non-scientific methods (candle ) and it seems accurate.

It has a digital UART and an analog output. In my analyzer I read the analog one, because I have the ADC anyway.
I have written a bit of info here, but it is still a work in progress. The sensors work well, just the casing is not done. GitHub - vldgeorgiev/scuba-gas-analyzer: A DIY Trimix and CO scuba gas analyzer

 

[ScubaBunga]

Looks great - I'll have to look into the custom boards, for that price, it's great. I added a restrictor on mine so it simply connects to the BC hose - no need to work out picky valves to get flow right.

 

[OTF]

After being shelved a couple of times, earlier this year I finally got enough enthusiasm and motivation to complete the design and build of my own O2 analyser. In my usual style, it is was more over-engineered than it needs to be, but it certainly got the interest of some of the dive crew on my last liveaboard!

For the electronics/uC boffins:
It's Arduino based (well, Teensy) with an ADS1115 analog-digital convertor and an LCD display in a 3D printed case. My original plan was just to use breakout boards around the Teensy, but that was getting too difficult/messy so I designed and printed my own circuit board and used surface mount components to keep the size to a minimum. One of the challenges that I threw myself was that I wanted a soft-on (ie, press a momentary button, not a slide/toggle switch) so that it could auto-shutoff, but also draw zero power in this shutdown state so the battery will last indefinitely.


Calibration screen. This comes up on power-up. Shows the mV reading of the sensor, and the colour bar graph gives an indication of reading stability. When all the bars a green, the reading is stable. pressing the middle button toggles calibration between air and 100%
View attachment 855821

Analyse mode. O2 reading is yellow, turns green when it is stable.
Pressing the bottom button toggles MOD display between m/ft
Pressing the middle button toggles between 1.2/1.4/1.6 Po2 to calculate MOD.
If the O2 goes above 45%, Po2 switches to 1.6 if it drops below 42% is goes back to 1.4
View attachment 855822

Double-sided surface mount circuit board to keep the size to a minimum. Designed in Kicad and printed using a PCB manufacturer in China. US$5 to print the minimum run of 10! Could not believe how cheap that was!
View attachment 855823

LCD screen on its mounting board. Showing the very basic "best mix" screen - select the depth, and it will show the richest mix for the chosen PO2
View attachment 855825

Completed build, ready to insert the circuit. The
View attachment 855824

awesome job, looks great.

 

[stiebs]

The sensor I use is Winsen ZE07-CO. It measures 0-500ppm, comes factory calibrated and with temperature correction, and is cheap (10$). I have tried it with a calibrated gas and a few non-scientific methods (candle ) and it seems accurate.

It has a digital UART and an analog output. In my analyzer I read the analog one, because I have the ADC anyway.
I have written a bit of info here, but it is still a work in progress. The sensors work well, just the casing is not done. GitHub - vldgeorgiev/scuba-gas-analyzer: A DIY Trimix and CO scuba gas analyzer

Great find! I think I'll need to add that to my project!

 

[stiebs]

Looks great - I'll have to look into the custom boards, for that price, it's great. I added a restrictor on mine so it simply connects to the BC hose - no need to work out picky valves to get flow right.

Personally not a big fan. I'd rather measure straight off the valve especially when analysing multiple tanks. I don't really fancy having to attach/remove a first stage every time I want to analyse a tank.