Calibration of Total Pressure Gauges in the UHV and XHV Regions.

Abstract

When total pressures decrease to the XHV region, measurements with ionization gauges become increasingly sensitive to disturbances such as electron-or photonstimulated desorption, outgassing of the gauges, and production of secondary electrons by X-rays. Therefore, it is very important to have a reliable pressure scale also in the XHV region. The laboratory for vacuum physics at the Physikalisch-Technische Bundesanstalt (PTB) in Germany provides a calibration system with molecular beam expansion for pressures down to 10-10 Pa and is building a new calibration system based on continuous expansion for a lower calibration limit in the 10-11 Pa decade. The advantages and disadvantages of the two systems will be discussed and calibrations with the molecular beam system will be presented. For determining the outgassing rate of the vacuum chamber of the new calibration system, a new method was applied and compared to the classical methods, the throughput method and the pressure rise method. In particular it was checked whether the outgassing rate depends on the pumping speed applied to the chamber as proposed in a theory by Maley.

Hydrogen traps in the outgassing model of a stainless steel vacuum chamber

“…Typically, dP/dt is constant at room temperature, which is in contrast to the present measurements that found P(t) to be nonlinear in time at elevated temperatures. In particular, Jousten 46 made careful measurements of P(t) in a vacuum-fired chamber made of type 316LN stainless steel, 47 and, because dP/dt was constant, he questioned whether readsorption of hydrogen was significant. This question was revisited by using the present model to calculate the outgassing at 25 C. The chamber was assumed to be made of type 316 stainless steel with no traps and having 0.1 m 3 volume, 1 m 2 wall area, and 3 mm wall thickness.…”

New, fully automated, primary standard for generating vacuum pressures between 10 10Pa and 3 10 2Pa with respect to residual pressure

“…To obtain very low outgassing rates of 10 -12 Pa l/s cm 2 , the chambers were made from austenitic stainless steel (AISI 316 LN), vacuum-fired at 950 C for 2 h. The results of outgassing rate measurements have been reported in [7,8].…”

“…The total flow is (4) and the pressure in each calibration chamber is calculated from (5) where ␥ is defined as in (1). Using these equations and dropping the assumption of isothermal conditions, we obtain (6) and (7) The first fractions on the right-hand side of ( 6) and ( 7) are the dominant factors, which describe in the numerator the approximate incoming flux and in the denominator the pumping speed, while the following two fractions describe the loss and gain of gas particles due to various backstreaming effects. The factor and the temperatures are defined as in (1) and Figure 1.…”

Measurement of Outgassing Rates of Steels

“…10 Recently, various experiments have been conducted to measure the hydrogen outgassing rate based on the RoR method using spinning rotor gauge (SRG). 1,[11][12][13] The RoR method using SRG is highly suitable for measuring very low hydrogen outgassing rates that often limit the lowest pressure achievable in a vacuum system made of steel. This is because the SRG has negligible pumping or outgassing action.…”