Description of PD ISO/TS 20177:2018 2018

This document describes procedures to measure outgassing rates from components designed for vacuum chambers and of vacuum chambers as a whole. The outgassing rates are expected to be lower than 10⁻⁵ Pa m³ s⁻¹ (10⁻² Pa L s⁻¹) at 23 °C and to emerge from devices that are suitable for high or ultra-high vacuum applications. The molecular mass of the outgassing species or vapour is below 300 u.

The upper limit 10⁻⁵ Pa m³ s⁻¹ of total outgassing rate is specified independent of the size, the total surface area and texture or state of the outgassing material. If a specific outgassing rate (outgassing rate per area) is determined, the area is not a specific surface area including the surface roughness, but the nominal geometrical one. When it is difficult to determine the nominal geometrical surface area of the sample, such as powders, porous materials, very rough surfaces, or complex devices, 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 a measuring instrument, which sensitivity is gas species dependent, is used, the total outgassing rate are given in nitrogen equivalent. In cases, however, where the total outgassing rate is too high, the disturbing gas species is identified, and its outgassing rate is measured in order to improve the sample material. This document covers both cases.

Some outgassing molecules can adsorb on a surface with a residence time that is much longer than the total time of measurement. Such molecules cannot be detected by a detecting instrument when there is no direct line of sight. This is considered as a surface effect and surface analytical investigations are more useful than general outgassing rate measurements considered here. Also, molecules that are released from the surface by irradiation of UV light or X-rays, are out of the scope of this document.

This document is written to standardize the measurement of outgassing rates in such a way that values obtained at different laboratories and by different methods are comparable. To this end, for any of the described methods, traceability is provided to the System International (SI) for the most important parameters of each method and according to the metrological level.

Outgassing rate measurements by mass loss, which were mainly developed for testing of spacecraft and satellite materials, are not gas specific. For acceptable measurement times, mass loss measurements require significantly higher outgassing rates (>10⁻⁵ Pa m³ s⁻¹) than typical for high and ultrahigh vacuum components. Also, it is not possible to measure the sample in situ due to the weight of the vacuum chamber, since the balances are not vacuum compatible. For these reasons, mass loss measurements are not considered in this document.

It is assumed that the user of this document is familiar with high and ultra-high vacuum technology and the corresponding measuring instrumentation such as ionization gauges and quadrupole mass spectrometers.

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