The calibration of gas volume measuring devices at continuous and pulsatile flows

A gas circuit that was capable of passing continuous or pulsatile flows via a 350 L Collins chain-compensated gasometer was built and evaluated. Various turbine volume transducers and dry gas meters were tested with gas compositions and flows that mimicked: a) inspired pulsatile flow over the physiological range and, b) mixed expirate being withdrawn from a Douglas bag. We found the Collins gasometer to be very accurate throughout its elevation, but its mixing fan is not required and atmospheric air should be left to saturate and the added water vapour calculated. Dry gas meters can be accurate to within 1% when calibrated (60 to 150 L/min), but require at least 25 L to be passed through them. The Morgan Ventilometer is an extremely reproducible device (coefficient of variation 0-0.2%, n = 60), but an increase in calibration syringe rate will elevate the calibration factor and reduce the percentage accuracy (one unit increase in calibration factor reduces accuracy by 0.6 - 1.0%). The optimal calibration syringe rate appears to be 30 - 45 b/min. Entrainment through the attached respiratory tubing can also alter the validity of the Ventilometer's calibration procedure.