Pressure-Gauge Accuracy Class → Uncertainty

Turn an ASME B40.100 grade (or IEC/EN percent-of-span class) into an absolute error band at your reading.

Input±E = (class% / 100) · FS , %reading = E / reading · 100

Gauge full-scale span (psi)or enter psi — converts to psiCurrent reading (psi)or enter psi — converts to psiAccuracy grade / classCustom accuracy (%FS) (%) — optional

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The engineering

ASME B40.100 rates mechanical dial gauges as a percent of full-scale span, not percent of reading. A Grade 1A (±1% FS) 100 psi gauge is off by ±1 psi everywhere on the dial — so at a 10 psi reading that ±1 psi is 10% of what you're measuring. This card converts the class into the fixed psi band and then shows how ugly it gets down at the bottom of the scale.

Rule of thumb: keep your operating point in the middle third of the dial. B40.100 also allows a wider tolerance band near the very ends of the range (typically the lower 25% and upper 25% for some grades) on the lowest classes — this card gives the mid-scale figure, so treat readings near the pins as worse, not better.

The percent-of-span basis is why buying a smaller-range gauge for the same process pressure buys you real accuracy: half the span means half the absolute band at the same reading.

Where this math comes from

The Bourdon tube — a coiled flattened metal tube that straightens as pressure rises — was patented by Eugène Bourdon in Paris in 1849, and it remains the mechanism behind most dial pressure gauges on a plant floor today. Early gauges were graded by shop feel and factory reputation, not a written spec, so 'accurate' meant whatever the maker said it meant.

The American Society of Mechanical Engineers put numbers to it with the B40 series, consolidating gauge grading into the letter/number accuracy classes (Grade A through 4A) that instrument techs still quote. B40.100 is the modern rolled-up standard combining the old B40.1 gauge document, so a 'Grade 2A' callout on a spec sheet points straight back to the ±0.5% FS row this card uses.

1849Eugène BourdonPatents the Bourdon-tube pressure gauge, the basis of dial gauge grading.
1852Edward AshcroftLicenses Bourdon's design in the US, spreading the mechanical gauge industry.
1920ASMEBegins standardizing gauge dimensions and accuracy under the B40 committee.
1998ASME B40.100Consolidates gauge standards, fixing the Grade A–4A percent-of-span accuracy classes.

See the full timeline of the math behind every calculator →

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