The gas inventory of the Tandem Mirror Experiment Upgrade (TMX-U) must be carefully controlled, if it is to successfully create various plasma configurations for thermal barrier experiments designed to provide an improved performance for tandem mirror experiments. This paper is a progress report on the calibration methods and pressure measurements of machine conditions deriving from recently improved neutral-beam gas control, and changes to the internal baffling geometry and the gettering system.Various recorded pressure vs time histories produced by the machine's internal vacuum systems and their pumping mechanisms are presented. An upgraded computer code for calculating dynamic pressures is described. Our concluding remarks deal with the problems of establishing and maintaining a pressure measuring system for a large fusion experiment, where the quality and reliability of components must be carefully considered.