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.