Altimeter

The Blue Fly Vario device (aka BFV) is supported by the CdaCrr app since the 4.0 version. The exact model which may be used is the Bluetooth_USB_v12.2 one (or more recently, while not tested, the Bluetooth_USB_v23). It provides altitude with a +/-10 cm precision thanks to a MS5611 pressure sensor.

The BFV sends pressure data every 0.02s (50Hz) and battery level every 10 seconds. The use of the BFV when connected to the CdaCrr app enables several features:

• your rolling average CdA is computed during rides ("Training Mode") once a given precision is reached thanks to high velocity and precise altitude (dCdA<0.005). The main interest is to assess the refine your road racing aero position. 
• your virtual mass and accurate slopes are computed during climbs. The main interest is to assess and understand the influence of the road quality on Crr. 

An interesting link to read about this pressure sensor, with tests and some info about the light sensitivity of the sensor, fixed with neoprene it the BFV:
http://blueflyvario.blogspot.com/2017/02/pressure-sensor-anatomy.html

See in this video how to connect the BFV to CdaCrr app.

Known limitations (update 2022/08/25): The pressure sensor of the BFV is highly sensitive to wind gusts which impacts the altitude measurement. The CdaCrr app try to suppress the air speed dependence, but the result is far from perfect, and precise altitude is only available during calm wind days for the moment...