DIY Touch Screen Trimix Analyser

[stevetto]

To compare linearity, a setup would be required where one would start with 100% He and then constantly lowering the He content by replacing it with another gas (eg. N2 to eliminate the thermal conductivity of O2).

MD61/62 are not linear, I am sure.
More or less I did the same what you suggested and detected the sensor's sensitivity curve.
The detected curve gives a pretty similar result how you make the correction with fixed values in your code.

Regarding to CO, I have never measured CO and do not know anybody who do it.
At dive centers where I dive they use good filters and replace filters and oil in the compressor regularly. Or even earlier than required.
So it is a bit strange for me why people here are so interested in CO analysis.

 

[Jarek Andrzejewski]

A/D converter ADS1115 has 4 channels, so it's possible to add measured values without adding new hardware (https://www.ti.com/lit/ds/symlink/ads1115.pdf).

 

[stepfen]

A/D converter ADS1115 has 4 channels, so it's possible to add measured values without adding new hardware (https://www.ti.com/lit/ds/symlink/ads1115.pdf).

From the data sheet: "Either four single-ended or two differential signals can be measured."
In this application the signals to be measured are differential hence only 2 signals per ADS1115 can be measured

 

[stepfen]

At dive centers where I dive they use good filters and replace filters and oil in the compressor regularly. Or even earlier than required.
So it is a bit strange for me why people here are so interested in CO analysis.

CO has not much to do with how old filters and/or oil are, but I'd suggest we don't discuss these here.
If interested there are plenty (and I mean PLENTY) of other threads discussing the need (or no need) of measuring CO.

 

[Jarek Andrzejewski]

In this application the signals to be measured are differential hence only 2 signals per ADS1115 can be measured

Not quite. At least oxygen sensor output is DC (0 to 50-65mV) and can be measured against GND. No need for differential input.

 

[The Chairman]

So it is a bit strange for me why people here are so interested in CO analysis.

Divers have died from it. I have personally gotten sick from it. It can be generated by a contaminated tank during partial pressure fills as well as a litany of compressor issues.

 

[Miyaru]

A/D converter ADS1115 has 4 channels, so it's possible to add measured values without adding new hardware (https://www.ti.com/lit/ds/symlink/ads1115.pdf).

From the data sheet: "Either four single-ended or two differential signals can be measured."
In this application the signals to be measured are differential hence only 2 signals per ADS1115 can be measured

Not quite. At least oxygen sensor output is DC (0 to 50-65mV) and can be measured against GND. No need for differential input.

Theoretically, 4 signals can be measured with a single ADS1115. The gain is general for the entire ADS but could be set differently before each action.
My experience with combining the oxygen sensor (0-50mV) with the carbon monoxide sensor (0.4-2V) is that they influence each other too much for an independent reliable measurement. But I'd be curious to know how to solve that issue!

 

[kelemvor]

Regarding to CO, I have never measured CO and do not know anybody who do it.
At dive centers where I dive they use good filters and replace filters and oil in the compressor regularly. Or even earlier than required.
So it is a bit strange for me why people here are so interested in CO analysis.

That's okay, the first step to correcting a problem is recognizing that there is a problem in the first place. Many a diver has gotten sick and some have died from tanks contaminated with carbon monoxide. It's a topic that's been thoroughly discussed on this site, even at a bit of a technical level with some of the doctors who contribute here. The short story is that CO is very bad to breathe at depth. Even small concentrations can make you sick or kill you. Therefore, it's best to avoid breathing CO during SCUBA diving. To that end (and to borrow a phrase) you can test the tanks you dive, or you can dive on hope. Testing is safer. Just like you, most people aren't even aware that CO is something that should be worried about. I've personally found CO in my tank once when it was filled from a local shop that I trust. I know for sure that their compressor is well maintained. It can be as simple as the exhaust from a vehicle engine happened to be too close to the intake for their compressor. In this case, the shop is in a "strip mall." We suspect that there was a large vehicle idling in the alley behind the building while he was filling is bank. Shop dismantled his setup and cleaned everything when I reported the 1ppm CO to him.

I won't derail the thread further, because this one is about a gas analyzer project rather than the need to test for a particular gas. Use the scubaboard search feature for posts about "CO" or "Carbon Monoxide" if you want to know more.

In other news, I'm going to switch my build to a seeduino xiao instead of Arduino Nano. The seeduino has an integrated 12 bit ADC. Hopefully 12 bit resolution will be sufficient. I tried using an artemis nano but ran into problems with their proprietary i2c plug and poor doccumentation. The XIAO offers a smaller size, faster processor, and 12 bit ADC over the arduino nano. I should be able to eliminate the separate ADC board. I got the ZE15-CO in the mail yesterday. I'm going to us an old oxygen sensor that came from a cootwo, and I'm going to skip helium testing as I don't need it.

@Miyaru have you started documenting the project online anywhere other than what's in this thread? If not, I was debating about writing a little bit up for an instructable once I finish mine.

 

[Miyaru]

@Miyaru have you started documenting the project online anywhere other than what's in this thread? If not, I was debating about writing a little bit up for an instructable once I finish mine.

I'm working on changing the analyser to ESP32, which solves the memory limitations of an Arduino. When I've finished that, it's a matter of building the simplified board and downloading the (upgradeable) firmware. No more programming required.

 

[kelemvor]

Well this thread got me interested in ESP32, and I ordered some of the new ESP32-C3 boards.

They finally arrived today - it only took about 6 months . My analyzer project is done, but I've got four C3's ready to rock on some new and more demanding project. Digikey now has 75 more in stock. ESP32-C3-DEVKITM-1 Espressif Systems | RF/IF and RFID | DigiKey