This is the old version (analog), for new version (digital): https://github.com/tl5915/Nitrox

Nitrox-Analyser

Nitrox Analyser (Divesoft DNA clone)

Material

• 50mL Syringe
  • cut the tip off and drill to 16mm, trim to approriate length (around 63mm)
  • keep the silicone plug on the plunger, cut holes for USB-C as end plug, drill hole for lanyard
• M16x1 Nut (can be 3D printed: https://www.thingiverse.com/thing:3428400)
• Flow Limiter (can be 3D printed: https://www.printables.com/model/52405-nitrox-analyzer/files)
  • flow_restrictor_1.stl & flow_restrictor_10mm.stl
  • measured the inner diameter of oxygen cell to select appropriate flow_restrictor file
• Oxygen Cell (any 3-Pin Molex type, e.g. Vandagraph R-22VAN)
• Seeed Xiao ESP32-C3
• LM358P Operational Amplifier
• 3mm 20mA Green LED
• 100/3.3k/47k ohm Resistors
• 0.1uF Ceramic Capacitors
• Prototype Board (cut to 7x8)
• 8-Pin Female Headers
• JST-XH (2.54mm) 3-Pin Female Connector
  • cut the locking arrows off to fit Molex header on oxygen cell

Circuit connection

• LM358P pin 8 (Vcc) connects to Xiao 3V3 pin
• LM358P pin 4 (Gnd) connects to Xiao Gnd pin and Oxygen Cell pin 1 (-)
• LM358P pin 3 (IN1+) connects to Oxygen Cell pin 3 (+)
• LM358P pin 1 (OUT1) connects to Xiao pin A2
• 3.3k ohm resistor connects between LM358P pin 2 (IN1-) and pin 4 (Gnd)
• 47k ohm resistor connects between LM358P pin 1 (OUT1) and pin 2 (IN2-)
• 0.1uF capacitor connects between LM358P pin 8 and 3
• 0.1uF capacitor connects between Xiao pin A2 and Gnd pin
• 100 ohm resistor connects between Xiao pin D7 and LED long leg (+)
• LED short leg (-) connects to Xiao pin D6
• Connect the antenna for Xiao, antenna can be sticked on the back of circuit board

UI instruction

• Power with USB-C cable

• Connects to 'Nitrox Analyser' WiFi, password: 12345678

• Browser open URL: 192.168.4.1/upload_page

  • upload 'icon.png' file

• iPhone connects to the WiFi, in Safari open URL: 192.168.4.1

  • bottom middle up arrow button: Add to Home Screen

• Click 'Cal. Low' to calibrate in air

• Click 'Cal. High' to calibrate in oxygen/rich nitrox

• Click '1-Point Calibration' or '2-Point Calibration' to change calibration mode

  • 2-point calibration available if Cal. High is higher than Cal. Low

• Click 'Setting' to enter setting page

  • Click 'Calibrate High %' to change gas used for 'Cal. High': default 99%, can be set lower
  • Click 'PGA Gain' to change gain used for voltage calculation:
    • measure oxygen cell voltage in air with a multimeter and adjust gain to match the readings
    • default 13.8 (highly dependent on resistor variability)
    • only matter if accurate voltage in required, doesn't affect oxygen percentage calculation
  • Click 'Reset Calibration' to reset all calibration values and PGA gain to default
  • Click 'Firmware Update' to update firmware by uploading .bin file

• LED blink number represent the oxygen percentage to the nearest 10%

  • i.e. 21% 2 blinks, 36% 4 blinks, 80% 8 blinks, etc.
  • LED stays off if O2 < 5%, LED stays on if O2 > 95%