Calibration, Maintenance, Repair, and Troubleshooting
The Calibration, Maintenance, Repair, and Troubleshooting domain of the ISA CCST Level III exam tests whether you can run instrument work to a documented standard, not merely make a reading match. Expect questions on traceability, tolerance and uncertainty, as-found/as-left records, and structured fault isolation on loops that mix 4–20 mA, HART and discrete signals.
Calibration rules that earn marks
- Traceability: the standard you use must be traceable to a national metrology institute (NIST in the US) and carry a current calibration certificate. A common shop rule is a 4:1 test uncertainty ratio - the standard should be at least four times more accurate than the instrument's tolerance.
- Record as-found data before touching zero or span. As-found shows drift trend; as-left proves the instrument is now good. Both belong in the record with the standard's ID and ambient conditions.
- Use a five-point check (0, 25, 50, 75, 100 % of span) in both directions so hysteresis and deadband are captured.
Worked example
A 0–100 psi transmitter with a 4–20 mA output reads 12.40 mA with 50.0 psi applied. Ideal output is 4 + 0.16 × 50 = 12.00 mA. Convert back: (12.40 − 4) ÷ 16 × 100 = 52.5 psi, an error of +2.5 psi, or 2.5 % of span. If the specification is ±1 % of span, the unit fails as-found. Write the number down first, then adjust zero and span and repeat all five points until as-left is inside tolerance.
Traps
- Adjusting before recording as-found, which destroys both the record and the drift history.
- Using a multimeter whose own accuracy is close to the instrument tolerance.
- Calibrating in the wrong units or with the wrong LRV/URV, so the bench is right and the DCS is wrong.
- Skipping the decreasing pass and therefore missing hysteresis.