in a cavity of screw 58 housing 40 defines a passageway 59 for the admission of air from a supply of air under pressure p 2 which supplies also passageway 34 housing 40 further defines an output chamber 60 connected by pipe 61 to upper cylinder chamber 33b referring now to fig1 and 2 and assuming that the signal pressure p 1 i e the pressure in chamber 2 is increased over the equilibrium force balance condition as expressed by the above equations this produces a decrease of the clearance between the orifice of nozzle 13 and plug member 12 and a concomitant increase of the pressure in chamber 19 the relative sizes of nozzle 13 and restriction 28 are selected typically in a 2 1 ratio approximately so that servo pressure is readily varied in response to nozzle clearance as the nozzle clearance decreases the nozzle pressure increases increasing the servo pressure in chamber 19 the pressure in chamber 19 acts on diaphragm a 3 resulting in downward movement of nozzle support 18 closing of valve 25 26 withdrawal of valve element or pintle 36 from its seat until the pressure in chamber 20 increases to such an extent as to re establish a force balance condition on nozzle support or piston assembly 18 the force balance condition is re established when the servo pressure acting on diaphragm a 3 plus the pressure of spring 24 balances the pressure acting on diaphragm a 4 typically a ratio of 3 1 of servo pressure to output pressure areas is desirable for this ratio the pressure in chamber 19 is 1 3 of output pressure in chamber 20 for equilibrium conditions assuming now that the signal or control pressure p 1 is decreased rather than.