General Terminology


  • Input Current – The electrical current to the valve which commands control flow, expressed in milliamperes (mA).
  • Rated Current – The specified input of either polarity to produce rated flow, expressed in milliamperes (mA). Rated current is specified for a particular coil configuration (differential, series, individual or parallel coils) and does not include null bias current.
  • Coil Impedance – The complex ratio of coil voltage to current. Coil impedance will vary with signal frequency, amplitude, and other operating conditions, but can be approximated by the DC coil resistance R, expressed in ohms(½) and the apparent coil inductance L, expressed in henrys(H), measured at a specific signal frequency.
  • Dither – An AC signal sometimes superimposed on the valve input to improve system resolution. Dither is expressed by the dither frequency hertz (Hz) and the peak-to-peak dither current, expressed in milliamperes (mA).


  • Control Flow QV – The flow through the valve control ports to the load expressed in in3/sec (cis), gal/min (gpm), or liters/min (lpm).
  • Rated Flow QR – Servovalves are typically rated at 1,000 psi drop, while Proportional Valves are rated at 150 psi drop.The flow under no-load condition,QNL, will vary with supply pressure.
  • Flow Gain – The normal relationship of control flow to input current, expressed as cis/mA, gpm/mA, or lpm/mA.
  • No Load Flow – The control flow with zero load pressure drop, expressed in cis, gpm, or lpm.
  • Internal Leakage – The total internal valve flow from pressure to return with zero control flow (usually measured with control ports blocked), expressed in cis, gpm, or lpm. Leakage flow will vary with input current, generally being a maximum at the zero level of null (called null leakage).
  • Load Pressure Drop – The differential pressure between the control ports (that is, across the load actuator), expressed in lbs/in2 (psi) or bars.
  • Valve Pressure Drop – The sum of the differential pressure across the control orifices of the valve spool, expressed in psi or bars.Valve pressure drop will equal the supply pressure, minus the return pressure, minus the load pressure drop.