Dynamic extension – A new principle for a vacuum primary standard for UHV-range (10−6–10−10 Pa) calibration of hot cathode ionisation gauges

The main obstacle in the construction of primary vacuum standards for very low pressures – ultra-high vacuum (UHV) – is outgassing from the walls of the apparatus. Thus, principles utilising a higher pumping rate in the calibration chamber, e.g., the pressure ratio technique, seem to be more useful for this range. However, a serious problem is inherent in this method, namely, that the gas in the calibration chamber is not in equilibrium and thus cannot be characterised by a single scalar pressure value, although such a value is then used to evaluate the measurement. This discrepancy can be overcome by using a hot cathode ionisation gauge as a transfer standard calibrated by the dynamic extension method.The dynamic extension method is explained and compared with the pressure ratio technique. The basic conditions for achieving acceptable uncertainty values are considered.

► Principle of primary standard for calibration vacuum gauges down to 1E-10 Pa described.
► Influence of outgassing suppressed by increased pumping.
► Increased pumping causes gas equilibrium disturbance.
► Correct calibration curve is obtained in spite of disturbed equilibrium.
► Solution is proper procedure and restriction to hot cathode ionization gauges only.