Carbon Monoxide Oxygen Analyser

A low-cost DIY analyser that measures oxygen and carbon monoxide in diving gas at the same time, built around an ESP32-C3.

View the code on GitHub Read the original write-up

The finished analyser: a 128 by 128 OLED showing O2 percent, cell millivolts, CO ppm and temperature, above illuminated power and calibration buttons.

Warning: this is a cheap, poorly tested DIY build with rough code, not a commercial product. The CO sensor's maker states it must not be used for life support. Think about what your life is worth before you rely on one.

Why

CO analysers sold for diving cost far more than comparable industrial safety analysers. So I built a dual analyser that tests a gas for O2 and CO in one go. The O2 side reuses a standard R22D oxygen cell that came out of my CCR after 12 months, which still has plenty of life left.

What it does

O2 and CO together

One sample, both readings: O2 % and CO ppm in large type, with the O2 cell's millivolts and the CO sensor's temperature.

Steady O2 reading

The O2 cell is read through a 16-bit ADC and averaged over 40 samples.

One-button calibration

Calibrate on air or pure oxygen; the analyser works out which from the cell's output and remembers it.

Factory-calibrated CO

Electrochemical CO sensor: 0–1000 ppm at 1 ppm resolution, expected life over 2 years.

Rechargeable

Runs on an 18650 cell, charged through a USB-C socket on the case.

Simple wiring

The display, ADC and CO sensor all share one I2C bus.

Calibration

Press CAL while the O2 cell is exposed to the calibration gas. The firmware decides which gas it is from the cell's output:

Cell output	Treated as	Calibration value
7–13 mV	Air	20.7% O2
Above 44 mV	Oxygen	98% O2
Anything else	Error	Shows `CAL ERROR` and stops until restarted

The calibration is saved to flash and loaded at start-up. On the very first start, the analyser calibrates straight away.

Photos

The analyser inside its open Pelican-style case, with coiled sample tubing beside the panel. — Inside the Pelican-style case.

The panel with the OLED readout above the POWER and CAL buttons. — The OLED readout with the POWER and CAL buttons.

The start-up screen listing serial, display, CO cell and O2 cell as OK, then Setup complete. — Start-up check of each part.

Sample tubing stowed in the case next to the panel and its USB-C charging socket. — Sample tubing stows in the lid; USB-C socket for charging.

The closed grey Pelican-style case. — Packed away.

Parts

Part	Notes
ESP32-C3 Super Mini	Microcontroller
DFRobot CO sensor (SEN0466)	I2C, address 0x74. Factory calibrated, 0–1000 ppm, 1 ppm resolution, expected life over 2 years
R22D oxygen cell	Reclaimed from a CCR; connected with a Molex connector
ADS1115 16-bit ADC	I2C; reads the O2 cell differentially on A0–A1
1.5" 128×128 OLED	SH1107 driver, I2C
18650 battery holder / UPS module	With an 18650 cell
USB-C socket	For charging
M16 toggle switch	Power
M16 momentary switch	Calibration
Pelican-style case	Cheap, but surprisingly decent
3D-printed internal assembly	Holds everything in the case; M4 threaded inserts
Stainless barbs, 4 × M6	Gas sample connections

Wiring

ESP32-C3 pin	Connects to
GPIO8 (SDA)	SDA on the OLED, ADS1115 and CO sensor
GPIO9 (SCL)	SCL on the OLED, ADS1115 and CO sensor
GPIO2	Calibration button to GND (internal pull-up)
ADS1115 A0 / A1	O2 cell + / −

Build and flash

The firmware is a basic Arduino-framework C++ program, written in VS Code with PlatformIO. It should port to other IDEs.

pio run -t upload
pio device monitor

Libraries: U8g2, Adafruit ADS1X15, RunningAverage and EasyButton, installed by PlatformIO. The DFRobot multi-gas sensor library (MIT licence) is included in src.

3D-print files (STL and Fusion 360): 3D-Printed-Assembly.zip

Known limitations

The code could do with some optimisation.
It's lightly tested. Check it against a known gas before trusting a reading.
CO temperature compensation is turned off in the firmware.