In vitro ventilation experiment by the tandem piston type respirator through the separated pipes for inspiration and expiration has been carried out with several lengths of the connecting pipes on the condition of several frequencies and tidal volumes of oscillatory flow. A smooth pipe and a pipe with circumferential grooves were tested as the connecting pipes between the respirator and the lung model chamber. The half life of the tandem piston type respirator was half of the high frequency oscillatory ventilation respirator in the market on the similar condition. The half life of the carbon dioxide in the chamber as a lung model with the separated connecting pipes for inspiration and expiration was smaller than a single pipe. The half life with the separated connecting pipes was not varied with the increase of the length of the connecting pipes. The inner surface was less affect on the half life. The half life with the separated connecting pipes is inverse proportional against the increase of the product of frequency and tidal volume fV.
