A method is presented which yields an order of magnitude increase in precision in the measurement of conductance to free molecular flow. The technique utilizes a calibrated variable conductance as a reference in a true substitution procedure. A series line was used to study balanced and unbalanced conditions and to explore the effects of various factors on precision, such as flow stability, pump behavior, ionization gauge sensitivity and response, temperature, transients, and sorption. A parallel line was used for measurements on short tubes of small diameter. For this, an iteration technique was employed for the calibration of the variable conductance on an absolute basis. Ionization gauge resolution of 0.02% and linearity of better than 0.1% were achieved. Relaxation effects were apparent. Conductance measurement precision of a few parts per 1000 was obtained. Transmission probabilities for the small diameter, short tubes showed deviations of as much as 18% from the Clausing factors at length-to-radius ratios ≥1, but approached the Clausing values as l/r→ 0. Deviations of <1% from the Clausing value required l/r ≤0.05.