I've now built the pressure difference measurement sensors into the machine.
One sensor is enclosed in a sealed box (ex screws!) with a tube going from the box to the back of the fan. The box is sealed airtight with tape and hot glue, there is a BMP280 inside, the other sensor which is located on the outside of the machine case is a BME280 so I can detect temperature and humidity as well.
I'm getting readings of:
Machine not running difference 34 -> 43
Running no restriction difference 5 -> 17
Running restriction 20% -2 -> 6
Running restriction 50% 0 -> 6
Running restriction 100% -20 -> -9
The difference when the machine is not running is due to the sensors reading different values so will be different for each installation and needs to be taken into account.
From the above (and yes I need to do some more sampling) it's certainly possible to detect (ice) restriction to airflow and so it looks easily possible to incorporate this into the defrost detect code.
Acuario