Field QA/QC: Calibration Drift, Blanks & Duplicate Samples

Sampling Program

Sampling stations 10

Instrument drift rate 0.30%/day

Sensor noise (σ) 0.15

Instrument freshly calibrated.

Live QC Readouts

Days since calibration

Mean duplicate RPD

0.0%

Points out of control

0 / 0

Blank exceedance

0.000

Levey-Jennings Control Chart

How it works

Every field measurement (a pH probe, a spectrophotometer, a sonar transducer) drifts away from truth between calibrations, and its error must be tracked, not assumed. This lab runs the three checks real environmental-monitoring programs use every sampling round:

Drift model:   measured = true × (1 + r·d) + N(0, σ)
  r = drift rate / day,  d = days since last calibration

Duplicate RPD: RPD = |x₁ - x₂| / [(x₁ + x₂) / 2] × 100%
  (Relative Percent Difference between two samples at the same station;
   EPA-style guidance flags RPD > 20% as a precision failure)

Control chart:  center line = mean of in-control check-standard reads
  ±2σ = warning limit, ±3σ = out-of-control limit (Levey-Jennings / Westgard 1-3s rule)

Sampling stations — how many instrumented points sit in the monitoring grid; more stations tighten the duplicate-RPD estimate.
Instrument drift rate — how fast the sensor's calibration curve slope decays per day without recalibration.
Sensor noise — the random measurement error (standard deviation) added on top of any systematic drift.
Advance Day — runs one full sampling round: every station is read once, one random station pair is re-sampled as a duplicate, and a zero-concentration field blank is measured; the check-standard reading is plotted on the control chart.
Recalibrate Instrument — resets accumulated drift to zero, the textbook fix once a control chart shows 2 consecutive points beyond ±2σ or any point beyond ±3σ.

Station markers in the 3D view are colored by how far their reading has drifted from true: green is within tolerance, amber is a warning band, red means the reading would fail QC and the batch should be flagged or rejected.

In control Warning band Out of control Duplicate pair Field blank

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