Bugfix

• Use the internal Pull-up resistor to get a more consistent recording for the button press

Bugfix v1.2

• The battery monitor is fixed
• Lowered the error on the screen to have a better reading of the display

Version 1.1

This version adds the feature to calibrate the sensor also on 100% oxygen.
Oxygen sensors don't have a fully linear scale especially if they are near the end of there lifespan.

The meter calibrates at boot time to air resulting in a mV measurement for 20,9% oxygen or 0,209 ppO2
Once that is done and you measure 100% oxygen and you start a forced calibration ( by long pressing the button )
The meter is also calibrated on the 100% oxygen, thus saving 2 setpoints:

• 1 setpoint has a mV reading for a ppO2 of 0,209 Bar resulting in 20,9% air
• 2e setpoint has a mV reading for a ppO2 of 1,00 Bar resulting in 100% oxygen

Blue line is with the calibration done on 1 setpoint (air) and thus if the sensor is fully linear we would expect this line,
However when we calibrate on 100% oxygen the red line is used for the calculations.

As you can see the drift is the biggest towards the 100% point, thus this is mainly important whey you want to sample high oxygens percentages.

The 100% oxygen setpoint is stored in EEPROM to survive a reboot.
When calibrating to 100% oxygen it is advised to do this often over time to avoid introducing a bigger error then the linear drift does.

Important: by pressing the button for a long time ( > 5second) then 100% oxygen calibration is erased and a recalibration starts at air