The electronic balances are used for surface pressure measuring. The balance measures the force that influences on the Wilhelmy plate of certain length at the water - air interface and form electric signal that is proportionate to surface pressure P. According to the definition P=b0-b, where b0 - surface tension of clean water surface, b - surface tension of water at presence of monolayer at the two mediums interface.

The balances are fixed on the LB trough frame and are connected by cable to the U2 unit plug. The electric circuit is shown in Fig1.

Fig.1. Diagram of surface pressure sensor

Change of surface pressure in a monolayer causes change of force that draws the Wilhelmy plate into water, that results in spring deflection relative to some initial position of equilibrium. Any deflection the spring results in shift of the shutter and in change of difference between the photo diodes signals. This differential signal flows to the amplifier with emitter division circuit (transistors VT1 and VT2) and the coil and a precision variable loading multiple resistor R6. Polarity of the coil connection is chosen the way that spring and the magnet deflection induces the signal that prevents the spring deflection, i.e. a negative feedback is carried out.

Force of influence on the magnet (and consequently on the spring) is directly proportional to current in the coil, and that is why it is convenient to be calibrated by current through potentiometer R6 that is connected with the coil L1 consequently.

In order to speed up the equilibration achievement the spring is situated in the chassis in special channel with little air gap (0.1-0.2 mm) so that the spring operates as an air damper.

Besides there are differentiating and integrating elements that improve the transient characteristics of the balances in the feed back circuit.