ASTM E595 is the standard screening test for vacuum outgassing, run at 125°C for 24 hours under high vacuum with the collector plate held at 25°C. Materials are typically accepted when total mass loss (TML) stays below 1.0% and collected volatile condensable material (CVCM) stays below 0.1%. These figures are screening thresholds, not a guarantee of flight performance.
TL;DR:
- A TML result above 1.0% may still qualify when RML falls below 1.0%, but CVCM must remain below 0.1% for baseline screening.
- Request raw weights recorded before and after testing, blank runs, conditioning records, calibration certificates, and full TML, CVCM, RML, and WVR results from the lab.
- For sensitive optics or assemblies where deposition timing matters, use ASTM E1559 to measure outgassing rates, and specify collector placement, temperature, and exposure duration.
- Check Goddard database entries against current supplier reports, and require lot testing when batch variation poses a risk for the material.
Table of Contents
- 1. What ASTM E595 measures and why it matters for contamination control
- 2. Standard test apparatus, specimen preparation and test parameters to specify
- 3. Acceptance criteria and how to interpret results for design decisions
- 4. Limitations of ASTM E595 and when to specify higher-fidelity methods
- 5. Using published outgassing data and the GSFC database correctly
- 6. Practical lab and procurement checklist for specifying ASTM E595 tests
- 7. How we treat outgassing requirements for optical and vacuum viewports
- FAQ
- Sources
1. What ASTM E595 measures and why it matters for contamination control
ASTM E595 weighs a specimen before and after a controlled thermal vacuum exposure and tracks three values that between them describe how much material a sample sheds and how much of that residue condenses on cooler surfaces nearby.
- TML is the percentage weight lost by the specimen during the 24 hour exposure, calculated from the difference between initial and final mass.
- CVCM is the percentage of volatile material that migrates from the specimen and condenses on a controlled-temperature collector plate.
- WVR (water vapour recovered) separates out moisture that the specimen reabsorbs after test, which lets engineers calculate RML (recovered mass loss), a figure that often reads lower than raw TML once water vapour effects are stripped out.
The method is comparative and deliberately conservative. It tells us how a material behaves against a fixed, repeatable exposure, not how it will behave over years in orbit, which is why we treat E595 as a screening gate for material selection rather than a lifetime prediction tool.
2. Standard test apparatus, specimen preparation and test parameters to specify
A lab report that only states “pass” against TML and CVCM is not enough on its own. The apparatus and conditioning details determine whether two results from different labs are actually comparable, so these are the parameters worth checking before accepting a certificate.
- Chamber temperature and vacuum level. Specimen temperature must hold at 125°C ±1°C, with chamber pressure maintained below 7×10⁻³ Pa, roughly 5×10⁻⁵ torr.
- Specimen mass and holders. Specimens are placed in aluminium boats, positioned so the collector plate geometry gives a consistent view factor to each sample.
- Collector temperature. The condensable collector is held at 25°C throughout, which is the reference point that makes CVCM values comparable across different test runs.
- Specimen count and blanks. Reports should include multiple specimens per material along with blank runs, so background contamination in the chamber itself is accounted for rather than attributed to the test article.
- Balance sensitivity. Microbalances used for pre- and post-weighing should be sensitive enough to detect small mass changes on light specimens, since these affect TML and CVCM calculations.
- Collector bake-out and pre-conditioning. Collector plates need a documented bake-out before use, and specimens should go through consistent pre-conditioning (commonly 24 hours at a specified temperature and relative humidity) so moisture content does not skew the first weighing.
- Solvent-bearing materials. Liquids and solvent-containing compounds often need pre-curing and an additional bake, commonly 65°C for 24 hours, to simulate pre-launch conditioning before the formal E595 run.
Pro Tip: Ask the lab for the raw pre- and post-test weights, not just the calculated percentages. It is the fastest way to catch a mis-recorded specimen mass before it affects a material decision.
3. Acceptance criteria and how to interpret results for design decisions
The baseline screening limits are TML below 1.0% and CVCM below 0.1%, and most materials databases and specification sheets use these figures as the default pass line. Reading the result correctly, though, means looking past the raw TML number.
- A material with TML above 1.0% can still be acceptable when RML, the mass loss after moisture reabsorption, falls below 1.0%, because the excess loss was simply water vapour rather than a contaminating volatile.
- For optics, cryogenic systems or high-voltage assemblies, limits are often tightened well below the standard thresholds, since even trace condensable films can shift reflectance or create arcing paths.
- NASA/Goddard-style specifications for sensitive optical surfaces add a witness-sample reflectance check, commonly requiring the change in reflectance to stay within 3% alongside the standard TML and CVCM limits.
- Detection limits matter: if a candidate material sits close to the threshold, request improved balance sensitivity or longer collector exposure rather than accepting a borderline pass at face value.
- Where a material genuinely exceeds the baseline but is still needed for the design, document the decision through a Materials Usage Agreement, recording the rationale, the actual test data and any mitigating design features.
4. Limitations of ASTM E595 and when to specify higher-fidelity methods
E595 answers one question well: does this material shed an unacceptable amount of mass under a fixed, accelerated exposure. It does not model the kinetics of outgassing over a mission timeline, and ECSS guidance treats it explicitly as a screening tool rather than a predictive one.
- When deposition rate and timing matter, for example near a sensitive optical surface that degrades gradually over months, ASTM E1559 uses quartz crystal microbalances to characterise outgassing kinetics and deposition rates directly, with collector temperatures and geometry set to match the actual application.
- ISO/TS 20177 standardises how outgassing rates are measured and reported for high and ultra-high vacuum components, which helps when results from different labs or suppliers need to sit on the same metrological footing.
- For critical assemblies, specify the QCM placement, collector temperature and exposure duration explicitly rather than accepting a generic kinetic test, since the value of E1559 data depends heavily on matching the test geometry to the real application.
5. Using published outgassing data and the GSFC database correctly
The Goddard outgassing database is a reference source of material TML, CVCM and WVR data, organised by material category, alphabetical listing and a low-outgassing subset for quick screening.
- Historically, materials meeting TML below 1.0% and CVCM below 0.10% were selected for spacecraft use, and the database still reflects that convention.
- Many entries date back decades, so a formulation change at the supplier’s end can make an old database value meaningless for a current batch, even when the product name has not changed.
- We recommend cross-checking any database figure against the supplier’s current technical data sheet, and requesting a recent test report rather than relying solely on a historical listing.
- Where lot-to-lot variance is a known risk for a material class, specify lot-specific E595 testing in the purchase order rather than accepting a generic certificate of compliance.
6. Practical lab and procurement checklist for specifying ASTM E595 tests
A clear test instruction in the RFQ avoids the ambiguous results that cause rework later. The following points cover what we ask for when specifying outgassing testing on a glass or coating component.
- Pre-conditioning record. State the conditioning period, temperature and relative humidity applied before the initial weighing.
- Specimen mass and mounting. Confirm the specimen mass and mounting are documented, and that mounting exposes edges consistently with the aluminium boat geometry.
- Collector plate bake-out. Require documented bake-out of the collector plate and inclusion of blank runs alongside the test specimens.
- Calibration records. Ask for current calibration certificates for the balance and the vacuum gauge used during the run.
- Full report fields. Insist on TML, CVCM, RML and WVR values, plus apparatus geometry, specimen and collector temperatures, and any blank corrections applied.
- Lab accreditation. Confirm the testing laboratory’s accreditation scope covers ASTM E595 specifically.
- Lot testing clause. Where a material has a history of batch variance, write a lot testing requirement directly into the specification rather than relying on the manufacturer’s generic data sheet.
Pro Tip: Paste the report field list straight into your RFQ as a mandatory deliverable. It costs the lab nothing extra to provide and saves a second test cycle when a report comes back incomplete.
7. How we treat outgassing requirements for optical and vacuum viewports
When we fabricate glass components destined for vacuum or optical use, specification clarity carries as much weight as the fabrication itself. We work from documented test apparatus details and batch traceability records rather than assuming a material class is automatically compliant.
If you are sourcing vacuum viewport glass or coated optical windows, request the lot certificate and the test apparatus geometry alongside the TML and CVCM figures, since those details determine whether a result actually applies to your build.
— Alexandra
FAQ
What is the NASA low outgassing standard ASTM E595?
ASTM E595 is the test method NASA and the wider aerospace industry use to screen materials for vacuum outgassing, exposing specimens to 125°C for 24 hours under high vacuum. Materials are typically screened against TML below 1.0% and CVCM below 0.1%, with results logged in the Goddard outgassing database.
What is ASTM E595?
ASTM E595 is a standard test method that measures total mass loss and collected volatile condensable material from a specimen exposed to vacuum and heat. It runs at 125°C for 24 hours at a vacuum below 7×10⁻³ Pa, with the collector plate held at 25°C, and the method is explicitly comparative rather than predictive of long-term service behaviour.
What are the ESA outgassing requirements?
ECSS and ESA guidance generally follow the same screening thresholds as NASA, accepting materials with RML below 1.0% and CVCM below 0.1%. Where mission kinetics or deposition timing matter, ECSS guidance recommends supplementing E595 with a kinetic method such as ASTM E1559.
Does PTFE outgas in a vacuum?
Fluoropolymers such as PTFE are generally regarded as low-outgassing materials and commonly appear in the low-outgassing subset of the Goddard database. Actual performance still depends on the specific formulation and batch, so a current test report or data sheet should confirm compliance rather than relying on the material family alone.
Sources
- E595 Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment
- Outgassing Database User Guide | Goddard Engineering and Technology Directorate
- ISO/TS 20177:2018 – Vacuum technology — Vacuum gauges — Procedures to measure and report outgassing rates
