Chapter 11 — Calibration & Maintenance

11.1 Calibration: the reference point

A sensor's reading is meaningless without a known relationship between its output and the physical quantity. Calibration establishes and maintains that relationship.

11.2 Factory and field calibration

• Factory calibration provides the as-delivered conversion and a certificate of traceability.
• Field calibration / verification confirms the instrument still performs after transport, installation, and exposure — because field conditions shift the relationship.

Periodic re-calibration (or at least verification against a reference) catches drift before it corrupts the record.

11.3 Maintenance schedules

A maintenance plan addresses:

• Readout units and dataloggers — function, clock, power.
• Cables and connectors — continuity, water ingress, rodent damage.
• Sensor zero and span — checked against manual or reference readings.
• Environmental protection — UV, corrosion, freezing.

11.4 Cold-weather guidelines

Freezing is a major cause of data loss:

• Bury or heat cables and sensors where needed.
• Use antifreeze or heated enclosures for standpipes and readouts.
• Document a seasonal plan, including safe shutdown and fallback to manual reads.

11.5 Documentation

Every calibration and maintenance action is recorded: date, method, result, person, and any change to conversion factors. This preserves the data's defensibility over the instrument's life.

Drift is silent

An uncalibrated sensor rarely fails loudly; it drifts quietly, corrupting trends until someone notices a physically impossible reading. Schedule verification; don't wait for the impossible number.

Figure. Calibration. The input-output relationship (slope = calibration factor), with zero, span, and hysteresis (Dunnicliff, Ch. 7 and 16).

11.6 Key takeaways

• Calibration ties output to physical quantity; verify it in the field.
• Maintain on a schedule: readouts, cables, zero, environment.
• Plan explicitly for freezing and seasonal effects.
• Record every calibration and maintenance action.