Vacuum technology - Vacuum gauges-Procedures to measure and report outgassing rates
1 Scope
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 1×10-5 Pa·m3·s-1 (1×10-2 Pa·L·s-1) at 23℃ 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 300u.
The upper limit 1×10-5 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 is 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 space craft and satellite materials, are not gas specific. For acceptable measurement times, mass loss measurements require significantly higher outgassing rates (>1×10-5 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.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3529 (all parts) Vacuum technology - Vocabulary
Note: GB/T 3163-2007 Vacuum technology - Terminology (ISO 3529:1981, MOD)
ISO 14291 Vacuum gauges - Definitions and specifications for quadrupole mass spectrometers
Note: GB/T 40333-2021 Vacuum gauges - Definitions and specifications for quadrupole mass spectrometers (ISO 14291 : 2012, IDT)
ISO/IEC 17025 General requirements for the competence of testing and calibration laboratories
Note: GB/T 27025-2019 General requirements for the competence of testing and calibration laboratories (ISO/IEC 17025 : 2017, IDT)
ISO/TS 20175 Vacuum technology - Vacuum gauges - Characterization of quadrupole mass spectrometers for partial pressure measurement
Note: GB/Z 42625-2023 Vacuum technology - Vacuum gauges - Characterization of quadrupole mass spectrometers for partial pressure measurement (ISO/TS 20175:2018, IDT)
ISO 27894 Vacuum technology - Vacuum gauges - Specifications for hot cathode ionization gauges
Note: GB/T 42604-2023 Vacuum technology - Vacuum gauges - Specification for hot cathode ionization gauges (ISO 27894 : 2009, IDT)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3529 (all parts), ISO 14291, ISO 27894 and the following apply.
3.1
outgassing rate
rate of molecules that leave a surface in vacuum, in units of throughput, measured at 23℃or calculated for the temperature
Note 1: For scientific investigations, it is distinguished between desorption and outgassing. The first is a surface effect, the second a bulk (diffusion) effect. For many measurement procedures, however, it is not possible to distinguish between the two effects. To measure the true outgassing rate, all molecules from a surface are removed by a bake-out or a similar procedure. A bake-out, however, in particular in industrial applications, is not possible or not desirable. For this reason, in this document, the term "outgassing rate" includes both desorption and outgassing rate.
Note 2: The temperature of the sample or the measurement chamber maybe different from 23℃, but the measured outgassing value in units of throughput shall be calculated as if released gas would have temperature 23℃. In this way, it is possible, to state a measured throughput (at 23℃) for a sample at 150℃.
Note 3: In vacuum applications, the unit of throughput is commonly used for outgassing rates, but for known outgassing gas species it is also possible to use the unit of mass flow, e.g. g/s.
Note 4: In some cases, when outgassing gas molecules of a certain species easily adsorb on the surrounding surfaces of the measurement system, it is distinguished between intrinsic and measured outgassing rate. The intrinsic outgassing rate is the outgassing rate that leaves the sample surface. It is higher than the measured one, when no equilibrium of ad-and desorption on the surfaces of the walls has been reached. In this document, it is assumed that equilibrium is established, but this may not be true for absorbing gases like water vapour.
3.2
geometrical surface area
area of a surface determined from its geometrical dimensions, not including surface roughness
3.3
mass range of outgassing
mass range of QMS from 1 to the highest m/z of all measured or expected outgassing gas species
Note: For the scope of this document, the mass range of outgassing is 300.
3.4
nitrogen equivalent outgassing rate
outgassing rate (3.1) when all gases released from the sample are assumed to be nitrogen molecules
Note: To be consistent, all quantities involved in a physical equation (e.g. conductance and pumping speed) need to be expressed for nitrogen, if pressure is measured in nitrogen equivalent. Otherwise, the same nitrogen reading of a vacuum gauge could lead to different quantities dependent on gas species (see the following example)
Example: If the throughput of hydrogen is measured by pN2C, where pressure pN2 is measured in nitrogen equivalent and C is the effective conductance (pumping speed), C shall be the value calculated or determined for nitrogen, and not hydrogen.
3.5
bake-out time
time for which a sample or chamber is maintained at a specified (bake-out) temperature
Note: Bake-out time does not include the warm-up and cool-down phase.
