the valve 37 the nozzle 28 may also communicate with a solvent receiver container or tank 40 through the valve 19 the tank 40 is vented to the atmosphere the general operation of the present apparatus 11 will now be described with reference to fig3 the signal a is applied to a non inverting input of a comparator 41 the reference voltage vr is applied to an inverting input of the comparator 41 the comparator 41 produces a high output when the signal a is higher in magnitude than the signal vr and the pressure in the nozzle 14 is above a predetermined value indicating a clogged condition the output of the comparator 41 is connected to an input of an or gate 42 the output of which is connected to an alarm 43 the signal b is applied to an inverting input of the or gate 42 thus the output of the or gate 42 will go high in response to sensing of a clog by the sensor 24 and or 26 and energize the alarm 43 the remainder of fig3 is in flowchart form and will be described appropriately during normal operation of the apparatus 11 the valve 19 connects the nozzle 14 to the pump 16 the pump 16 is on the valve 17 is open and ink is supplied to the nozzle 14.