Q: What is the best way to determine EO concentration in the sterilization chamber?

A: I would suggest that the most accurate way to determine EO concentration is to use a sensor inside the chamber. The most common ones use infrared spectroscopy, although it can also be measured with gas chromatography, microwave spectroscopy, and perhaps others. Using this method you get a direct measurement of the EO concentration which gives you a relatively precise measurement of the concentration (assuming that the equipment is good, calibrated, used properly, etc.) as opposed to other methods which give you an indirect calculated estimate of the concentration. Some other methods used, in order of preference are 1) pressure differential, 2) weight, and 3) volume. I don't suggest using weight as the primary determinant of EO concentration as it is subject to variability. I would not recommend using volume as it is the least accurate methodology.