the fluid level dh dt shown by the dotted line 4 is determined in order to be able to ensure a perfect evaluation of the pressure of the surroundings the fluid level should be lowered below the level s 2 to the level s 1 in order to reach the level s 1 thus the fluid level proceeding from the level s 2 must still be lowered by the height h 1 on account of the previously evaluated sinking speed dh dt one may now determine the period of time t 1 in which the pump must run further with a constant power so that with a constant sinking speed the fluid level is lowered to the level s 1 by the measure h 1 the following applies after completion of the period of time t 1 the pump is switched off and the fluid level in the interval t 2 increases again until it has again reached the level s 2 on exceeding the level s 2 the control of the pump again detects a pressure change and the detected signal for the fluid level which is represented by the unbroken line 2 in fig1 again rises after completion of the interval t 2 in the interval t 2 the measurement of the pressure of the surrounding is carried out provided that the interval t 2 is longer than a predefined interval t 2 min if the sensor signal remains constant at the value s 2 for a shorter time period than t 2min then it is the case of the fluid admission into the.