Pump for accurate mixing of three gas components with predetermined fractions

A mixing pump that creates an accurate mixture of three gases at predetermined fractional ratios that can be set in steps of 10 ppm is described. A nearly continuous flow of each of the three component gases is produced by pistons driven by stepping motors; the gas mixture is forwarded by a fourth piston. The flow of each component gas is adjusted by the stepping frequency of the motor and a microcomputer system is used to adjust the three frequencies according to the desired fractional concentrations. The total flow of the gas mixture is adjustable between 0.1-500 ml/min and is nearly independent of the after-load. The accuracy of the pump was tested by mixing the respiratory gases, O2 and CO2, with various carrier gases (N, He or Ar) at various fractional ratios and total flow rates. The fractions of O2 and CO2 in the mixture were analysed with the Scholander technique. In the physiological range, the mixing error in the gas fractions was less than 4%. The pump is, thus, suited for producing calibration mixtures.