Edge 705 altimeter

[Thomlodz]

I have this old model bought on Internet auction. It works good but there is one strange thing - altimeter. I read how to calibrate it [Only registered and activated users can see links. ]
"To calibrate the altimeter manually:

Acquire satellites
Press Menu
Select Save Location
Set Elevation to a known value
Select OK

The elevation will now automatically correct itself any time that it is within 30 meters of the saved location."
But this is not working in my device. In the beginning the displayed altitude was about 20 meters about the real value (read from maps or smartphone). I reinstalled the firmware and now the error is less but still exists. I read in forums that if I set the elevation (e.g. of my home) manually and i press START then Edge 705 would show the exact waypoint elevation value. But my device do not do it. Why?

[Magnetron]

Have you made sure to leave the GPS on long enough to calculate the Altitude ??
Because it requires @ least 4 sats to calculate Alti.