DIY Nitrox Analyzer (Arduino based)

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Miyaru

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Screen: ST7789
Resolution: 240x240
Connection: SPI interface

Screenshot_20200714-210840_Gallery.jpg
 

stevetto

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Hello,

I am also working on a nitrox/trimix analyser.
At the moment I make my resarch and just collecting info about different solutions, investigate the codes, components and try to find my solution.
At the moment the selection of the O2 sensor cause me some headache.
There are different sensors available, some of them has 2 but others have 3 pins.
As I see for the diy projects everybody use only 2 pins out of 3 on the O2 sensor.

So I made some research to find the purpose of the 3rd electrode and found these articles:

Voltammetry - Wikipedia

Amperometric Sensor - an overview | ScienceDirect Topics

I also found an interesting datasheet here, but the electrical part is too complicated for me at the moment:
https://www.sgxsensortech.com/conte...2-Design-of-Electronics-for-EC-Sensors-V4.pdf

I also found a diy analyser setup where a resistor was connected between the sensor pins.
Some O2 sensor datasheets also recomment to use a 10k load resistor.
I do not see any external load resistors in the diy projects here.
Why nobody use it?

This is a quite good description how o2 cells work and the purpose of the external load resistor and some failure analytics.
Understanding Oxygen Sensors • ADVANCED DIVER MAGAZINE • byPaul Raymaekers

I think for more precise measurement a 3 electrode system is necessary. And it requires an additional electric circuit.
But I do not know if we really need it. I have not seen any projects where the 3rd pin with a compensation circuit was used.

Do we need it or we can skip it?
 

NightShade00013

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An O2 sensor actually creates voltage when exposed to Oxygen,
Actually I was wondering about the screen too. Those quoted seem too small to convey all this info. Which screen are you using above @Miyaru ?? I can't find it. For prototyping I will be using an "old" 2 line LCD, but this colorful one is so much nicer.
Cheers


Looks like a 128X160 1.5" screen. Have one on order but to be honest a single color screen will convey the info just fine for an O2 meter. His build is geared more towards tec diving. The .96 screen really isn't too bad when you actually look at it though but the 128X128 1.5 will be better if you have eyesight issues. The OLED screens seem to work quite well in direct light also compared to the full color LCD's which will have some colors end up washed out.
 
OP
Pao

Pao

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Hello,

I am also working on a nitrox/trimix analyser.
At the moment I make my resarch and just collecting info about different solutions, investigate the codes, components and try to find my solution.
At the moment the selection of the O2 sensor cause me some headache.
There are different sensors available, some of them has 2 but others have 3 pins.
As I see for the diy projects everybody use only 2 pins out of 3 on the O2 sensor.

So I made some research to find the purpose of the 3rd electrode and found these articles:

Voltammetry - Wikipedia

Amperometric Sensor - an overview | ScienceDirect Topics

I also found an interesting datasheet here, but the electrical part is too complicated for me at the moment:
https://www.sgxsensortech.com/conte...2-Design-of-Electronics-for-EC-Sensors-V4.pdf

I also found a diy analyser setup where a resistor was connected between the sensor pins.
Some O2 sensor datasheets also recomment to use a 10k load resistor.
I do not see any external load resistors in the diy projects here.
Why nobody use it?

This is a quite good description how o2 cells work and the purpose of the external load resistor and some failure analytics.
Understanding Oxygen Sensors • ADVANCED DIVER MAGAZINE • byPaul Raymaekers

I think for more precise measurement a 3 electrode system is necessary. And it requires an additional electric circuit.
But I do not know if we really need it. I have not seen any projects where the 3rd pin with a compensation circuit was used.

Do we need it or we can skip it?
Look at the sensor data sheets. Most have internal compensation circuits for temperature.
 

Miyaru

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I mentioned the screen in post 71:
ST7789 with 2k2 and 3k3 resistors as voltage dividers, connected to the SPI bus.

There's an excellent topic about oxygen cells: Oxygen Sensor Fundamentals
The answers about current-driven and voltage-driven cells are discussed and explained very well by several CCR pilots.
 

zebu14

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Hello Miyaru,

I came across this thread looking for a DIY oxygen analyzer.
I have some friends with old/spare CCR oxygen cells and I was planning to give a try to the oxy analyzer.
Then I found your actual project and it looks awesome.
Putting together oxy/helium/CO cells is a must .
Really impressed of the results so far.

Do you plan to share your code in the future or is it a more "personal" project you want to keep ?

Keep up the good job !
 

stevetto

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I have started to test my own code and at the moment it seems to working well. I have finished more or less the 2 point calibration part of it.
I have an SGX VOX oxygen sensor which seems to be very sensitive. If I hit it's housing very slightly or jut put it down on my desk the measured value in my code changes rapidly between very low and high values. At first it goes to a high value and then decreasing close to zero (~ 0.3 mV).
If I do it for a few times the reading stabilizing around 11.4 mV which seems to be reasonable.

Can it have an internal contact problem?

The sensor is new.
 

Miyaru

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Hello Miyaru,

I came across this thread looking for a DIY oxygen analyzer.
I have some friends with old/spare CCR oxygen cells and I was planning to give a try to the oxy analyzer.
Then I found your actual project and it looks awesome.
Putting together oxy/helium/CO cells is a must .
Really impressed of the results so far.

Do you plan to share your code in the future or is it a more "personal" project you want to keep ?

Keep up the good job !
Once I'm confident that my code works in varying circumstances, I'll publish the code that is required for the main operation.
The part that's responsible for displaying everything on screen, and the menu structure, won't be open-source since it'll have some brand logos in it.

Right now I'm waiting for an MD61 to arrive (really curious about the difference with an MD62), and I am changing the power supply from a 9V battery block to an 18650 rechargeable LiPo with a protective charging circuit and the battery life displayed on screen.
 

stepfen

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Right now I'm waiting for an MD61 to arrive (really curious about the difference with an MD62),
Perfect - I've ordered my MD61 last week too :)
Although I've seen related projects and the code looks straight forward having another option is always good. Looking forward for your code
Cheers
 
https://www.shearwater.com/products/teric/

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