Guarantee test correction

Method, assumptions and limits

This tool corrects a measured value to reference (guarantee) conditions using the same Butler-Volmer cell-voltage model as the Curve Fitter — the correction is the model's predicted voltage difference between your test conditions and guarantee conditions, added to the measured value. The measured value itself is never edited or overridden.

Sequential decomposition (ASME PTC convention)

The total correction is broken into temperature, cathode-pressure and anode-pressure components applied one at a time, in that stated order — the same convention used in ASME Performance Test Code correction curves for turbines and compressors. The order matters slightly for the split between components (though not for the total), which is exactly why the order is stated rather than left implicit.

Uncertainty propagation

Combined uncertainty follows the root-sum-square (RSS) method from the ISO/JCGM "Guide to the Expression of Uncertainty in Measurement" (GUM) — the standard reference for combining independent measurement uncertainties:

Partial derivatives are evaluated numerically from the same model used for the correction itself, at your measured (sensor terms) or reference (model terms) operating point — not approximated from a generic sensitivity table. The model-parameter terms only appear if you've filled in j₀/α/ASR uncertainty (auto-filled when you arrive from the Curve Fitter, which already computes them from the fit's own covariance matrix). Leave them blank and the combined uncertainty reflects instrument accuracy only — which understates the true uncertainty whenever the stack model itself is loosely constrained, most obviously when you're using the "representative PEM defaults" rather than your own fitted curve.

The verdict — three contractual conventions, not one physics answer

Comparing a corrected value plus uncertainty against a guarantee is a policy choice about who bears the risk of measurement uncertainty, not something physics decides. This tool supports three:

ASME Performance Test Code practice for turbines and compressors is closer to point comparison — correct to reference conditions, compare directly, and negotiate the uncertainty band separately in the test agreement rather than folding it into an automatic verdict. The two "benefit of doubt" conventions are a common contractual shorthand when the test procedure wants a single automatic PASS/FAIL/INCONCLUSIVE output; which one your contract actually specifies is exactly the kind of detail worth confirming before a witnessed test, not after. INCONCLUSIVE (under either uncertainty-based convention) is a real, useful result — it means the band straddles the guarantee line, so neither a PASS nor a FAIL claim is honestly supportable from this test alone.

What this tool deliberately does not do

• It does not know your actual instrument calibration — the meter/sensor uncertainties are exactly what you enter, nothing more.
• It does not include model uncertainty unless you supply it — see "Uncertainty propagation" above.
• It does not correct for degradation between commissioning and test date — if meaningful time has passed, factor that in separately (see the LCOH tool's degradation model).
• It does not pick your contract's comparison convention for you — the three options above are real, different, and equally "correct" mathematically; only your test agreement says which one applies.
• It is not a substitute for a witnessed, contractually-defined test procedure — it is the correction math that procedure should be running.