Abstract: A mechanical and electronic apparatus for integrating gas flow charts for determining gas volume. The mechanical apparatus includes a motor driven disc on which a recorded flow chart can be placed, with suitable pen arms, and operating arms, so that an operator can simultaneously vary the angular position of each of two pens, so that the pens may trace or scan simultaneously, the two recorded traces of pressure and differential pressure, as the disc and chart rotate under a motor drive. Optical encoders convert the rotation of the disc and the angles of swing of the pen arms to digital values. The outputs of the encoders are scaled to the particular charts and chart arms. The result of the scaled pulses go to an up/down counter for interpretation of total angle of each of the pen arms. The digital numbers representing the angles of the two pen arms then have their square roots extracted, the two outputs of square roots of pressure (P & A) and differential pressure H go to a multiplier and an accumulator.