Overview
This article explains why DPD reference values and online sensor values can differ, which deviations are typically acceptable, and when recalibration is justified.
What Is a Deviation?
A deviation is the difference between a DPD grab-sample result and the online value under comparable operating conditions.
Small differences are normal because both methods have their own measurement uncertainty.
What Is Usually Acceptable?
- Usually acceptable: small, repeatable differences with stable trend behavior.
- Not acceptable: persistent large offset, sudden jumps, or continuously increasing mismatch.
- Evaluate with repeated checks under stable process conditions, not with one single comparison.
Common Reasons for Differences
- Sampling/time mismatch between grab sample and online reading.
- DPD handling effects (reaction timing, reagent condition, cuvette quality).
- Process effects (flow, pressure, bubbles, temperature, pH changes).
- Sensor condition (fouling, incomplete stabilization after cleaning).
- Sensor aging (gradual response change over time, visible in slope trend).
Why Calibration at Small Values Can Create Larger Offsets Later
If calibration is performed at very low concentration, the combined tolerance of both methods (DPD + online sensor) has a stronger relative impact on the slope calculation.
When this slope is then applied at higher concentrations, the absolute deviation can become larger.
Best practice: calibrate in the range you operate, preferably near the upper end of the normal control range.
Why Excessive Recalibration Is Risky
Recalibration always includes reference uncertainty. If calibration is repeated too frequently for small normal differences, this uncertainty is repeatedly written into the sensor scaling.
Over time, this can cause unnecessary drift or oscillation of displayed values.
Recalibrate only when deviation is persistent, repeatable, and process-relevant.
When Recalibration Is Recommended
- After sensor replacement.
- After required manual cleaning and stabilization.
- After confirmed, repeatable deviation under stable process conditions.
- After major process matrix changes.
Quick Decision Checklist
- Check stable flow/pressure and ensure no active ASR cleaning.
- Repeat DPD correctly (fresh reagent, correct timing, clean cuvette).
- Compare trend and repeated values (not only one point).
- Calibrate only if mismatch remains consistent and relevant.
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