the mass density of the material a first pressure transmitter 28 measures the pressure of the furnish pumped by fan pump 22 pressure transmitter 28 can comprise a pressure gauge and a suitable circuit for generating and transmitting a measurement signal suitable for use in a computer calculation transmitters 26 and 28 preferably are within a few feet downstream from pump 22 and pressure screen 24 but could be at other locations a second pressure transmitter 30 which can be similar to transmitter 28 measures the pressure of the furnish in headbox 10 at a location downstream from tube bank 10 a but in the wider part of the converging portion where the velocity of furnish 11 is relatively low the density and pressure measured by transmitters 26 and 28 are supplied to a calculating circuit 32 which calculates the volumetric air content fraction of the furnish at atmospheric pressure u03b1 atm using for the purpose a relationship which is discussed in greater detail below a jet velocity calculator circuit 34 uses the atmospheric pressure air content fraction u03b1 atm calculated by circuit 32 and the pressure of the furnish in the headbox as measured by transmitter 30 to calculate an ideal current jet velocity v j i and.