RECOMMENDED PRACTICE FOR THE CALIBRATION OF MASS SPECTROMETERS FOR PARTIAL-PRESSURE ANALYSIS

This Recommended Practice addresses issues involved in the use of partial pressure analyzers (PPAs) for quantitative analysis and describes recommended apparatus and procedures for determining resolution and sensitivity of a PPA so that the instrument can be used quantitatively for partial pressure, partial flow and gas composition analysis. This updates previous material in the AVS Standard 2.3-1972 (tentative) by including reference to current pressure transfer standards and computer controlled PPAs. This document presents an introduction to PPAs and how they work, definitions pertinent to the use of PPAs, equipment needed for calibration, instrument setup prior to calibration and the measurement of sensitivity and linearity by various methods. Four methods of calibration of a PPA are described as follows: (1) the direct comparison of the PPA output with a transfer standard pressure gauge, (2) the indirect comparison of PPA readings with readings of a transfer standard pressure gauge separated by a flow restriction (pressure divider method), (3) comparison of the PPA output with the calculated pressure generated in an orifice-flow system, and (4) calibration of the PPA response to known gas flow rates. The first three methods may be carried out on a test stand of suitable design or in situ. The fourth method requires that the pumping speed during calibration be the same as the pumping speed during use, and normally implies that the PPA is calibrated in situ. Discussion on gas interactions, sources of nonlinearity, stability of sensitivity and quality assurance methods is given.