Abstract: A device is described which comprises in combination at least one resistor means placed at a determined level on the inner wall surface of the vat, said resistor means having a resistance which is variable in response to the presence or absence of the liquid at the level thereof; switching means responsive to a control signal for causing the feed pump means for the liquid to be stopped, and control means for producing the control signal for the switching means, said control means comprising impedance means which includes the resistance of the said at least one resistor means, whereby the said resistance being varied by the presence or absence of the liquid in the vat at the level thereof causes the control means to operate the switching means thereby to stop or restart the feed pump means.