Calibration, Testing, and Loop Checks
Calibration
Calibration compares the instrument to a traceable standard. Record as-found data before any adjustment because it documents drift; as-left data documents the condition at return to service. A five-point check (0, 25, 50, 75, 100 percent) run both upscale and downscale reveals linearity, hysteresis, and deadband. Tolerance is normally stated in percent of span. The test standard should be at least four times more accurate than the device under test, a 4:1 test accuracy ratio. For DP flow, remember the square-root extraction may live in the transmitter or in the DCS; calibrate the DP portion in inches of water, and account for static-pressure effect where specified.
Loop checks
A loop check proves the entire path: sensor, wiring, I/O card, controller, HMI, and final element. Cold checks verify continuity, polarity, and point-to-point tag matching with the process out of service. Hot checks inject a signal and confirm the displayed value, alarms, interlocks, and a full valve stroke including fail-safe action. Document every result.
Traps
Adjusting span when only zero is off, using an under-rated meter, skipping as-found data, and confusing percent of span with percent of reading.
Worked example
A 0-200 degF transmitter reads 102.4 degF at a true 100.0 degF. Error = (102.4 - 100.0) / 200 x 100 = 1.2 percent of span. If tolerance is 0.5 percent, the unit fails and needs adjustment or replacement.
Rule of thumb
Percent error equals indicated minus true, divided by span, times 100.