In an appts. for controlling the phase boundary between solutions for extraction plants of the type comprising a regulator with an input sensitive to the potential of a pick-up with variation of resistance inserted in a measuring bridge and a control output with several operating zones, the further features comprising(a) a pick-up with a series of probes of different lengths dipping into the solutions, (b) means for connecting the probes to the intermediate resistance of a potential divider of the measuring bridge, (c) means for withdrawing solution operated by the control output. The appts. provides a level control system for the phase boundary between two solutions, at relatively low cost, good sensitivity and high reliability.
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