The multimeter is soon replaced by an Arduino with the help of an AD623ARZ ... For more HVAC projects check out this pinwheel flow meter and [Adam]’s piece on HVAC techs, hackers who make ...
It runs on an Arduino MEGA 2560 PRO (or similar microcontroller) and supports a wide array of additional hardware including pressure transducer, water level sensor, flow meter, OLED display ...