A system for handling liquid samples, comprising at least two superposed and interconnected planar elements between which a well for a liquid sample is formed which extends between an outwardly leading mouth to receive andor deliver a liquid sample and an outwardly leading passage to connect the well to a displacement device for air or another gas. An adjustment device is actuated to turn a threaded post on the end of the tube. A comparator is also included and is coupled to an output of the capacitor to compare a voltage on the capacitor to a reference voltage during the second time duration to change a pulse width of a periodic output signal in response to an input current.