NUMERICAL AUDIT · 8 NOVEMBER 2026
One missing digit in NeqSim's ISO 6976:2016 table
NeqSim's ISO 6976:2016 constants give n-decane a molar mass of 42.28168 instead of 142.28168 g/mol, so with 0.1 mol% n-decane the relative density comes out about 0.55% low and the Wobbe index about 0.28% high.
Xamit Kadirbekov, Daniyal Kadirbekov · AI-assisted investigation and writing
Merged into NeqSim master on 8 November 2026 (PR #4920).
Upstream pull request #4920 · GitHub catalog
Authors: Xamit Kadirbekov and Daniyal Kadirbekov (GERO Numerical Assurance). Published 8 November 2026.
Published proposal
- equinor/neqsim PR #4920 — head commit
5b0e37fa2a383b1188be01e1dcf6ab5ced1acfd4on master2f6ea70; open at submission on 8 November 2026 (Tashkent time).
Context
We cross-checked open gas-measurement code — AGA8 / GERG-2008 equations of state, ISO 5167 flow metering and ISO 6976 calorific values — against the NIST reference code and the standards themselves. Most of it agreed with the references to 13–14 significant digits. This is one of the few places where it did not.
What the table is used for
ISO 6976 gives the rules for computing a natural gas's calorific value, density, relative density and Wobbe index from its composition. NeqSim, Equinor's open-source thermodynamics library, implements the 2016 edition in Standard_ISO6976_2016, which reads the per-component constants from src/main/resources/data/ISO6976constants2016.csv.
The discrepancy
The n-decane row of that file (component nC10) has MolarMass 42.28168 g/mol. The correct value is 142.28168 g/mol (ISO 6976:2016, Table A.2; 10 × 12.0107 + 22 × 1.00794). NeqSim's own 1995-edition table already has 142.285. The first digit was lost.
- 19,"nC10",42.281680000,0.434000000,0.584000000,...
+ 19,"nC10",142.281680000,0.434000000,0.584000000,...Effect
The molar mass enters the mixture molar mass, and through it the relative density, the density, the Wobbe index and every value given per unit mass. Volumetric calorific values are not affected.
| n-decane in the gas | Relative density | Wobbe index |
|---|---|---|
| 0.1 mol% | about −0.55% | about +0.28% |
| 0.01 mol% | about −0.056% | about +0.028% |
Regression example: methane 0.999, n-decane 0.001. NeqSim returns a mixture molar mass of 16.068699 g/mol; the expected value is 0.999 × 16.04246 + 0.001 × 142.28168 = 16.168699 g/mol.
Who is affected
The impact is narrow. Only Standard_ISO6976_2016 reads this table; the main NeqSim calculation paths use the 1995 table, which is correct. Only gases that contain n-decane are affected, and the error grows with the n-decane fraction.
Fix and test
- Change the n-decane molar mass in
ISO6976constants2016.csvfrom 42.28168 to 142.28168. - New test
testMolarMassWithDecaneinStandard_ISO6976_2016_Test.java: the methane / n-decane mixture above must give 16.168699 g/mol (tolerance 1e-6). Before the fix it gives 16.068699.
Upstream status
On 8 November 2026 Even Solbraa, NeqSim's maintainer, merged PR #4920 into master (merge commit 5f19b31) after two approving reviews. The latest release at that date, NeqSim 3.28.0, does not yet contain the fix.
Limits
The full NeqSim test suite was not run locally; the effect was checked with a standalone class against the released NeqSim 3.28.0 jar and with an independent implementation of the ISO 6976:2016 formulas. The percentages above are for the example compositions; other gases give other values. We make no claim about any operator's measured volumes or invoices.
Attribution and license
AI tools (Claude) assisted with the analysis, the reference calculations, the patch and test code, and the text of this report; the results were re-run before publication. Xamit Kadirbekov and Daniyal Kadirbekov are the report authors; upstream software remains credited to its developers. Report: CC BY 4.0. Upstream patches retain their upstream licenses.