The materials used for space systems shall be selected based upon low outgassing criteria. The outgassing criteria are based upon the micro-VCM test (ECSS-Q-70-02), and shall take into account the location of the outgassing source and its mass.

GENERAL REQUIREMENT FOR OUTGASSING:
The general requirement for materials outgassing is RML<1.0% and CVCM < 0.1 % as per ECSS-Q-70.

MORE STRINGENT REQUIREMENT FOR OUTGASSING:
The acceptance limits can be made more stringent if the materials concerned are used in critical areas. The use of materials that are deemed acceptable according to the general requirement does not ensure that the spacecraft system or component remains uncontaminated. Consequently, subsequent functional, development and qualification test shall be used where appropriate to ensure that the materials performance continues to be satisfactory. The Table provides a recommendation for outgassing requirements based on good design practice for applications around sensitive items (e.g. optics, detectors and cryogenic equipment). For a specific application, the definition of the outgassing requirements shall take into account quantity of material concerned and specific environmental conditions (e.g. available volumes and temperatures)

ACCEPTABLE HIGH TML OUTGASSING
The water vapour release (WVR) can be calculated as follows: WVR = TML - RML. The TML data for water absorbing materials such as polyamides, polyimides and polyurethanes is often above 1.0%. Water absorption is in most cases reversible and can be controlled by purging of critical hardware with dry gases. Often the water absorption of materials is not harmful with respect to contamination. Materials with high TML outgassing and acceptable RML (RML <1.0%) and acceptable CVCM (CVCM<0.1%) may be accepted for the normal cases where following is applicable:

• When no equipment at temperatures below -100C is involved;
• When the water desorption is fast (e.g. in case of polyimide films and polyurethane paints);
• When no high voltage equipment is involved;
• When dry gas purging controls the water re-absorption during ground life up to launch

MITIGATION:
Mitigation techniques, such as baking, application of protective coating or shielding, can be applied in order to protect against or remove residual contaminants, process contaminants and handling contaminants.

VARIANCE IN OUTGASSING:
Variability in materials composition or process parameters may cause a large spread in outgassing results. This can induce a need for a retest or batch to batch test.

AGE OF LAST TEST RESULT:
If the material is tested more than 10 years ago at the time of PDR, new test is required for application. If the most recent test is older than 7 years, new test recommended.