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.
- Heatmap — tints each station marker by its drift error instead of the flat traffic-light color, so the spatial pattern of degradation is visible at a glance.
- Drag the map to pan, scroll/pinch to zoom — the grid is drawn at native scale so panning never distorts distances.
Station markers on the map 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. Marker size encodes the magnitude of the drift error, mirroring the pole height used in the 3D version of this lab.