• Correct altimeter reading is a bit tricky. In a vacuum-chamber test the Jumplogger, an analog Barigo and the X2 showed different results: Barigo and X2 pretty close 4000m, the jumpLogger ca 4500m (baro).

This although the BOSCH BMP280 (probably the same as in the X2) is pretty precise plus temperature compensated. 

Temperature seems to be the problem: the "international barometric formula" for calculating altitude from pressure refers to a temperature gradient of 0,65 K per 100m. The BMP280 library calculates altitude by measuring the temperature of the sensor, in my vacuum chamber around 20ºC - this is not a temp that you see in 4000m altitude, so it reports a wrong altitude!

Problem is, that the temp of the sensor never reflects the actual situation, since jumping out of a heated aircraft will keep it warm until you reach the ground...

I didn't use the altimeter function of the library but just measure the pressure and convert to altitude with a fixed theoretical temperature of 0ºC = 273K - this gives me results pretty close to the other altimeters (+- 50m)

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[EDIT APR2022] above was WRONG. Altimeters typically conform to ICAO standard atmosphere. That is 15ºC at sea level, -0.65ºC per 100m altitude gain. So to display consistent corrected altitude, I start at 15ºC in the barometric formula and use the last corrected altitude to adjust the temp factor by the "-0.65ºC per 100m" -  this gives correct readings parallel to X2 and Barigo :-)
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• using the jumpLogger on the aisle side arm in a Cessna Caravan had problems with continuous GPS reception. Having it on the window-side arm seems to resolve this problem. Probably the same behavior as all the other GPS devices

• I added a video of a test jump with the jumpLogger on YouTube: