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Measuring Outgassing Rates in High and Ultra-High Vacuum Applications
A few months ago we wrote about the use of plastics in vacuum applications. Plastics have a relatively high outgassing rate. One of the methods commonly used to characterize the outgassing rate of plastics is to measure the change in mass of a sample before and after exposure to vacuum. This is reported as the Total Mass Loss or TML. In high and ultra-high vacuum applications materials of construction must exhibit very low outgassing rates. Plastics are generally excluded and metal surfaces are carefully cleaned, often polished, and baked out to remove absorbed water. Outgassing rates for clean metal surfaces in UVH applications are typically 1 x 10-10 Torr-l/cm2-s or lower and outgassing rates in the range of 10-13 to 10-14 Torr-l/cm2-s have been reported. A direct measurement of mass loss in these applications would not be a practical way to assess outgassing.
The outgassing rate is measured in quantity of gas (pressure x volume) per unit surface area per unit time. Common units are Torr-Liters/cm2-s or Atm-cm3/cm2-s. Knowledge of the outgassing rate is required to relate the ultimate pressure that will be achieved in a chamber to the pumping speed. Measurements of outgassing rates may be required to test or validate bakeout procedures or surface treatments, to compare different materials, or to demonstrate that a vessel meets a customer’s requirements.
In some cases an outgassing measurement may be performed on an actual vacuum chamber. The observed outgassing rate is then an average over all of the surfaces in the chamber. In other cases it may be desirable to measure the outgassing rate of a sample. In this case, the sample is placed in a chamber having a known outgassing rate and the combined outgassing rate is measured. For this measurement to be accurate, we obviously like to have a chamber with a much smaller outgassing rate than sample. Using a sample with as large an area as possible, such as multiple stacked plates, can help to achieve this. When outgassing rates are quoted in the context of high and ultra-high vacuum systems, it is normally assumed that water has been removed from the surface, usually with the aid of a bakeout. This is not necessarily the case for the outgassing rates reported for other materials.
Two general methods have been used to measure outgassing rates for high and ultra-high vacuum materials. The simplest is to measure the rate of rise. The chamber is evacuated, then valved off and the rate of pressure rise is recorded. The outgassing rate is:
The outgassing rate is measured in quantity of gas (pressure x volume) per unit surface area per unit time. Common units are Torr-Liters/cm2-s or Atm-cm3/cm2-s. Knowledge of the outgassing rate is required to relate the ultimate pressure that will be achieved in a chamber to the pumping speed. Measurements of outgassing rates may be required to test or validate bakeout procedures or surface treatments, to compare different materials, or to demonstrate that a vessel meets a customer’s requirements.
In some cases an outgassing measurement may be performed on an actual vacuum chamber. The observed outgassing rate is then an average over all of the surfaces in the chamber. In other cases it may be desirable to measure the outgassing rate of a sample. In this case, the sample is placed in a chamber having a known outgassing rate and the combined outgassing rate is measured. For this measurement to be accurate, we obviously like to have a chamber with a much smaller outgassing rate than sample. Using a sample with as large an area as possible, such as multiple stacked plates, can help to achieve this. When outgassing rates are quoted in the context of high and ultra-high vacuum systems, it is normally assumed that water has been removed from the surface, usually with the aid of a bakeout. This is not necessarily the case for the outgassing rates reported for other materials.
Two general methods have been used to measure outgassing rates for high and ultra-high vacuum materials. The simplest is to measure the rate of rise. The chamber is evacuated, then valved off and the rate of pressure rise is recorded. The outgassing rate is:
Here, P is the pressure, t is the time, V is the volume of the chamber and A is its internal surface area.
Another widely used method is the conductance method. In this case, a pump remains connected to the vessel under test but separated from it by a known conductance. Measuring the pressure drop across the conductance yields the outgassing rate.
Another widely used method is the conductance method. In this case, a pump remains connected to the vessel under test but separated from it by a known conductance. Measuring the pressure drop across the conductance yields the outgassing rate.
Figure 1. Two methods of measuring outgassing rates in high vacuum applications.
For the conductance measurement, the outgassing rate is given by:
In some measurements a variable conductance is used. Performing the measurement over a range of conductance can improve the accuracy. To reduce errors due to gauge calibration, a system of valves may be used to exchange the gauges P1 and P2.
The conductance measurement is complicated by the need for a known conductance; however, it may be favored because the measurement can be performed in less time. Both measurements are subject to errors due to inaccurate pressure measurements. Ion gauges are generally used for measurements at low pressures. In addition to inaccurate calibration and a calibration that can change with time, ion gauges can interfere with the measurement by outgassing, or by acting as a pump. Both hot filament and cold cathode gauges can act as a pump under certain circumstances1, 2. It is also necessary to correct the ion gauge reading for the gas composition. Ion gauges are usually calibrated for nitrogen, while the residual gas in an ultra-high vacuum system is likely to be largely hydrogen. A spinning rotor gauge has been used to circumvent these errors3, however, the pressure measurement range is then limited to higher pressures.
Degassing rates are strongly temperature dependent. This can lead to errors in measurement, due to heating by the filament of an ion gauge for example, or to inaccurate comparisons of different measurements. It is also possible to take advantage of this temperature dependence to measure very low outgassing rates by performing the measurement at elevated temperatures and extrapolating back to room temperature4.
Proper fabrication techniques, cleaning and surface preparation are essential for UHV service. These are some of the ways in which we help our customers Reduce Project Risk and achieve the lowest total cost of ownership.
The conductance measurement is complicated by the need for a known conductance; however, it may be favored because the measurement can be performed in less time. Both measurements are subject to errors due to inaccurate pressure measurements. Ion gauges are generally used for measurements at low pressures. In addition to inaccurate calibration and a calibration that can change with time, ion gauges can interfere with the measurement by outgassing, or by acting as a pump. Both hot filament and cold cathode gauges can act as a pump under certain circumstances1, 2. It is also necessary to correct the ion gauge reading for the gas composition. Ion gauges are usually calibrated for nitrogen, while the residual gas in an ultra-high vacuum system is likely to be largely hydrogen. A spinning rotor gauge has been used to circumvent these errors3, however, the pressure measurement range is then limited to higher pressures.
Degassing rates are strongly temperature dependent. This can lead to errors in measurement, due to heating by the filament of an ion gauge for example, or to inaccurate comparisons of different measurements. It is also possible to take advantage of this temperature dependence to measure very low outgassing rates by performing the measurement at elevated temperatures and extrapolating back to room temperature4.
Proper fabrication techniques, cleaning and surface preparation are essential for UHV service. These are some of the ways in which we help our customers Reduce Project Risk and achieve the lowest total cost of ownership.
- Pressure Measurement with Ionization Gauges, Karl Jousten, http://cdsweb.cern.ch/record/455555/files/open-2000-271.pdf
- http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/IG1BAGapp.pdf
- A Comparison of Outgassing Measurements for Three Vacuum Chamber Materials, M.L. Stutzman, P. Adderley, B.M. Polker, M. Baylac, J. Clark, A. Day, J. Grames, J. Hassknecht, G.R. Myneni, P.M. Rutt and C.K. Sinclair, International Workshop on Hydrogen in Materials & Vacuum Systems Jefferson Lab, Newport News, Virginia November 11-13, 2002
- Measurement of Ultra Low Outgassing Rates For NLC UHV Vacuum Chambers, K. Kishiyama, s. Shen, D. Behne, J.N. Corlett, D. Atkinson, K. Kennedy, T. Miller, L. Ekiksson and M. Ross, Proceedings of the 2001 Particle Accelerator Conference
