Air pressure is dependent on the altitude, which in turn affects the vacuum performance.
The air pressure at sea level is 1013 mbar. It decreases by approx. 12.5 mbar with each
Therefore, at an altitude of 300 m/900 feet the air pressure will decrease by approx. 38 mbar down to 975 mbar.
A vacuum ejector with a stated vacuum performance of 948 mbar would thus operate at a reduced performance of 910 mbar.