Instrument calibration check

Calibration compares the instrument with a traceable standard and records the error and uncertainty at each point on a certificate. Verification compares that error with a tolerance and decides pass or fail. Adjustment means acting on the instrument (zero and span) to reduce the error. What plants call a “calibration check” is usually a verification, which is what this page does.

Five points is standard: 0, 25, 50, 75 and 100% of span, going up to the top and then back down without overshooting (overshooting would hide hysteresis). For a 0 to 150 psi transmitter: 0, 37.5, 75, 112.5 and 150 psi. If the process always runs near one value, add that point. For a scale, use loads from minimum to capacity, including where the tolerance steps up.

Transmitter PT-101 (0 to 150 psi, 4–20 mA output, ±0.5% of span tolerance = ±0.75 psi) was found with errors up to +1.22 psi (0.81% of span) at 112.5 psi going down: 4 of 5 points were out. The error grows across the range, a sign of span error (+0.49%) plus a +0.50 psi zero offset. After adjustment the largest error fell to 0.13% of span and the record reads “Pass after adjustment”.

The standard’s or data sheet value when the instrument has a grade: an ASME B40.100 grade 1A gauge allows ±1% of span, grade A allows 2-1-2%; an IEC 60751 class A RTD allows ±(0.15 + 0.002·|t|) °C, about ±0.63 °F at 212 °F. Many plants set a process tolerance wider than the catalog accuracy depending on the use; what matters is that it is written in the calibration program.

It is the tolerance divided by the standard’s expanded uncertainty. At 4:1 the standard’s uncertainty takes only a quarter of the tolerance, so the risk of passing a bad instrument or failing a good one is small (ANSI/NCSL Z540.3). In the example, ±0.75 psi tolerance against 0.075 psi for the calibrator and 0.002 mA for the meter gives a TUR of 9.7:1. Below 1:1 the standard is worse than the instrument and the test means nothing.

With test weights, in three tests: shift or eccentricity (a load at the center and four corners), repeatability (the same load several times) and weighing (increasing loads up to capacity). NIST Handbook 44 class III maintenance tolerances are 1 d up to 500 d, 2 d up to 2,000 d, 3 d up to 4,000 d and 5 d above; acceptance tolerances are half. In the example (300 lb, d = 0.1 lb) every test passes on maintenance tolerances.

No. It organizes the in-house check and produces a record with tag, standard, readings and final status, useful for the maintenance program, ISO 9001 and audits. The standard’s own certificate must come from an ISO/IEC 17025 accredited laboratory, and scales used in trade are subject to inspection by state or local weights and measures officials.