Vacuum technology - Vacuum gauges - Procedures to measure and report outgassing rates

2Scope / Description

This document describes a method for measuring the outgassing rate of vacuum chambers and components designed for use in vacuum chambers. The outgassing rate shall be less than 10−5 Pa m3 s−1 (10−2 Pa L s−1) at 23 °C and shall emanate from equipment suitable for high vacuum or ultra-high vacuum applications. The molecular weight of the outgassing substance or vapor is below 300u. The total outgassing rate is limited to 10−5 Pa m3 s−1, regardless of size, surface area and material state. If the outgassing rate is determined for a specific area (outgassing rate per unit area), this area is the nominal geometric area and not the specific surface area including surface roughness. When it is difficult to determine the nominal geometric surface area of the sample, such as for powders, porous materials, very rough surfaces or complex equipment, the mass specific outgassing rate (e.g. outgassing rate per gram) is used. For many practical applications, it is sufficient to determine the total outgassing rate. If the measuring instrument used is sensitive to the gas species, the total outgassing rate is given in nitrogen equivalent. However, in cases where the total outgassing rate is too high, interfering gases are identified and their outgassing rates are measured in order to improve the material. This document covers both cases. Some molecules can stay on the surface much longer than the entire measurement time and therefore cannot be detected by the detection instrument (no direct line of sight). This case is considered a surface effect and surface analysis is more useful rather than the general outgassing rate measurement considered here. In addition, molecules released from the surface by UV or X-ray irradiation are not within the scope of this document. This document aims to unify the measurement of outgassing rates in this way in order to obtain comparable values in different laboratories and methods. To this end, traceability is provided for each parameter in the described method, making it consistent with the International System of Units (SI) and adjusted to the metrological level. Outgassing rate measurements by mass loss are mainly aimed at spacecraft and satellite material testing, but they are not specific for gases. Since for acceptable measurement times, mass loss measurements require significantly higher outgassing rates (>10−5 Pa m3 s−1), which are typical in high vacuum and ultra-high-tech components. In addition, since the balance is not vacuum compatible, in situ measurements of the sample are not possible due to the large weight of the vacuum chamber. Therefore, mass loss measurements are not considered in this document. It is assumed that the users of this document are familiar with high vacuum and ultra-high technology and the corresponding measuring instruments such as ion meters and quadrupole mass spectrometers.