Until this week, this site displayed an outgassing signature for 64 materials. Today it displays 20.
Nothing was measured differently. No material got worse. What changed is that I built a database that refuses to serve a number without its source, then pointed the site at it. Forty-four signatures did not survive the question “who says so?”
I want to explain why I consider this an upgrade, because the instinct says otherwise. A table with 64 rows looks more complete than a table with 20. Completeness is what most references optimize for. It is also how a rumor spreads.
Where the 64 came from
The old values came from where everyone’s values come from. Vendor tables that list a single number per material, no species, no temperature. Course notes. The memory of the trade. A 1e-11 for baked stainless that I have heard in three languages and never once seen attached to a measurement.
None of these numbers were wild. Most sat within an order of magnitude of something defensible. That is precisely what makes them dangerous. A wrong number gets caught. A plausible orphan gets repeated.
What sourcing them actually taught me
Building the base meant reading the primary documents, and the primary documents are stranger than the tables that cite them.
The most used materials list in gravitational wave detection publishes 95 material rows. Fourteen carry an outgassing figure. None of the fourteen states a pumping time. For water on unbaked stainless, that list gives a value nearly two orders of magnitude above what CERN publishes at ten hours of pumping. The two are not in disagreement about the steel. They are answering different questions, and only one of them says which. The CERN reference itself is blunt about this: a water outgassing rate quoted without its pumping time carries no meaning.
The same CERN lecture note publishes two values for silver bearing copper after the same 200 C bake, in two tables, credited to the same measurements. A factor of three apart. Neither retracted.
The LIGO acceptance threshold on the five hydrocarbon masses appears in two specifications of the same corpus. Same number, 2e-12. One document writes it per square centimetre. The other writes it with no area at all. Two different physical quantities wearing the same digits.
Two respected compilations disagree on unbaked molybdenum by a factor of 131. And the constant that converts a rate into monolayers per year, the one this site itself engraved in an earlier essay, exists in the literature as both 5e14 and 1e15 molecules per square centimetre, a factor of two, with neither source naming the molecule it stacks.
I did not find these problems by looking for problems. I found them by writing down, for every value, where it comes from and under which conditions it holds. The contradictions surfaced on their own. The base now publishes ten of them, side by side, each value under its key, with no arbitration. Picking a winner would just manufacture one more orphan.
The rule the site now follows
Every value in the base carries a source key, the conditions as the document states them, the species, the measurement temperature, and the pumping time or an explicit declaration that the source omitted it. A value that cannot carry all of that does not enter. A material that has no defensible value gets a declared absence with its reason, which is information, where a silently reused number is noise.
That rule is what shrank the column. The 44 signatures that vanished were never deleted. They were never really there.
The same rule runs forward into the calculators. Every result now cites the key of the value it consumed. If the base holds two conflicting values for your material, the calculator returns a range and both keys instead of a quiet choice. If your operating point falls outside the validity range a source declared, the calculator refuses and says why. Sixty-four of the 379 readings on its menu exist to be refused. A tool that always returns a number is lying about at least some of them.
The part that concerns you
Take the outgassing figure you use for stainless steel. The one in your gas budget spreadsheet, your design review slide, your mental arithmetic at the RGA screen.
Where does it come from? At what pumping time? Which species? After which bake, measured how?
If you can answer, you are holding a measurement. If you cannot, you are holding a claim, and you have been passing it along. I was. Twelve years in vacuum, and until I forced every number on this site to show its papers, I could not have told you which of my own figures were measurements and which were folklore with good posture.
The base is at /tools/outgassing, the calculators at /tools/calculators, the raw data at /data/outgassing.json under CC BY. Twenty signatures today, 258 entries, 18 sources, thirteen and a half decades from a getter pumping krypton to neoprene shedding water. It will grow. It will grow slowly, because every new number has to answer the question first.
A number is a claim. A source turns it into evidence. Everything else is rumor with units.
Sources · 8
- LIGO-E960050-v13, LIGO Vacuum Compatible Materials List, D. Coyne (ed.), 4 June 2014, Table 1 Approved Construction Materials. The table publishes 95 material rows; fourteen carry an outgassing figure, and no pumping time or measurement temperature appears anywhere in the table. Rows A14 and I9 carry a figure whose species column could not be resolved, and the base records them as declared absences rather than guessing.
- P. Chiggiato, Outgassing properties of vacuum materials for particle accelerators, CAS Glumslov 2017, arXiv:2006.07124. Source of the water rate on unbaked stainless read at ten hours of pumping, of the statement that a water outgassing rate without its pumping time is meaningless, and of the two silver-bearing copper values after the same 200 C bake, Table 3 against Table 4.
- LIGO-E960022-v24, LIGO Vacuum Compatibility, Cleaning Methods and Qualification Procedures, B. Bland, D. Coyne and J. Fauver (eds.), 27 June 2012, section 13.17.2.1: the hydrocarbon acceptance criterion on AMU 41, 43, 53, 55 and 57, written with an area in the denominator.
- LIGO-E080177-v2, RGA Test Qualification of components for the LIGO UHV, section 6d: the same five masses and the same figure, written with no area. The base publishes both under the key sc-hc-threshold-unit, with no arbitration.
- R. J. Elsey, Outgassing of vacuum materials II, Vacuum 25(8) (1975) 347-361, Pergamon Press, The Rutherford Laboratory, Chilton, England. No DOI is printed on the document. Source of the molybdenum value read at one hour of pumping that the base sets against the LIGO list.
- R. J. Elsey, Outgassing of vacuum materials I, Vacuum 25(7) (1975) 299-306. Source of the monolayer surface density of 1e15 molecules per square centimetre this site uses.
- D. K. Benson and G. A. Beitel, Development and evaluation of vacuum pressure gauge components from carbon and graphite, NASA CR-2101, Midwest Research Institute report C-2922, contract NAS 1-10738, Langley Research Center, September 1972. The footnote on page 11 defines one monolayer as 5e14 molecules per square centimetre and uses that definition throughout its table I.
- The outgassing base itself, /data/outgassing.json, published under CC BY 4.0. Every count in the closing paragraph is read from it: 258 entries, 18 source keys, 20 material signatures, and a span of 13.6 decades from a TiZrV getter pumping krypton at 1e-18 to neoprene at 3.99e-5. The ten contradictions are its site_conflicts block.
Ratings follow the Stack scale: [A] primary or standard, [B] manufacturer, [C] secondary. The counts in this essay were reconciled by hand against the base on 2026-09-01, the discipline set out in ESSAY_PUBLISH_CHECKLIST.md: an essay is a dated artifact and its numbers are not wired to the build. The gate check:outgassing verifies that the base does not contradict them.