The Bluefly varios measure vertical speed and altitude based on atmospheric pressure. The pressure sensor inside the Bluefly does this 50 times every second, with a resolution that enables the measurement of tiny altitude differences of just 10 cm. Turn it on and it beeps pleasantly when you go up. It is super responsive.
Flight instruments are awesome devices to help pilots know the air. The best components are cheap and the instruments should be too. The BlueFlyVario project grew from a not-for-profit hobby that got out of control. The Bluefly varios of today remain focused on meeting what pilots need to fly. This pilot focused philosophy continues to motivate the production and ongoing development of awesome little varios.