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Chapter 12 — Calibration, Maintenance & Installation

12.1 Introduction

The best-planned program fails if the instruments are not installed and maintained correctly. This chapter covers the field tasks that keep instruments trustworthy: acceptance testing, calibration, maintenance, and installation, and the records that document them.

12.2 Acceptance testing

  • Pre-installation acceptance tests (Table 12-1) — performed on instruments before installation to confirm they are within specification (for example, zero stability, calibration check, and electrical continuity). An instrument that fails here must not be installed.
  • Post-installation acceptance tests — performed after installation to confirm the instrument survived installation and reads sensibly against a known condition.

12.3 Calibration

Calibration establishes the relationship between the instrument's raw signal and the engineering quantity it measures. Key points:

  • Calibrate before installation and periodically thereafter, as part of the planning process (Chapter 6).
  • Record the calibration factor, zero, and span and carry them through every reading.
  • Watch for drift — a slow change in zero or span over time.

Figure: dunnicliff-calibration

Figure. Calibration converts a raw instrument signal to an engineering value; recording the zero and span is essential to trustworthy data (after FHWA-HI-98-034, Ch. 12).

12.4 Maintenance

Instruments degrade. Planned maintenance includes:

  • protecting tubes and cables where they emerge from a borehole;
  • checking desiccant in enclosures and lightning protection;
  • verifying power and communications for automated systems; and
  • replacing consumables before they fail.

12.5 Installation

Installation determines whether the instrument measures what it is supposed to:

  • Drilling must not smear or disturb the ground around a piezometer or inclinometer casing.
  • Grouting and sealing must isolate the measurement zone.
  • Tubes and cables must be protected against construction traffic and settlement (Figure 12-1 shows typical protection arrangements).
  • Installation record sheets (Table 12-2) document as-built position, depth, seal, and the calibration values in force — the permanent record that makes the data interpretable years later.

12.6 Key takeaways

  • Acceptance-test every instrument before and after installation.
  • Calibration defines the zero and span — record and carry them through.
  • Maintain instruments on a planned schedule; do not wait for failure.
  • Installation records are as important as the readings — they make the data interpretable later.
  • Protect tubes and cables: the most common cause of automated-system failure.