Why DPD3 and
Online Chlorine Monitors
Don't Always Match
A straightforward field guide for operators. Written in plain language, step by step. Version 1.0.
The Key Points
Online monitors measure only fast-reacting species
Free chlorine and monochloramine — the fast-reacting forms. DPD3 also picks up slow-reacting organic chloramines if you wait long enough.
Timing creates the mismatch
A long-wait DPD3 will measure more than any online monitor. Strict 60-second timing for calibration. Different timing = different result = calibration mismatch.
The monitor isn't wrong
Differences are caused by timing errors, slow-reacting species, cold water, and what each method detects — not a fault with the monitor.
Apparent errors are expected
The online monitor may look "too low" even though it's actually correct. This is NOT a problem — it's how the methods differ.
Perfect Agreement Is Impossible
DPD3 measures things online sensors cannot. Online sensors respond much faster to change than the chemistry of DPD3 allows. The two methods will never match exactly — and they shouldn't be expected to.
This is not a flaw in either method. It's a fundamental difference in what they measure and how fast they respond. Understanding this difference is the key to interpreting calibration results correctly.
Good Practice: Three Simple Steps
Always read DPD3 at 60 seconds when calibrating
Strict timing is non-negotiable. Set a timer. Read at 60 seconds exactly.
For investigations, compare two DPD3 measurements
60-second DPD3 (free + monochloramine) vs. 3–8 minute DPD3 (adds slow organic chloramine species).
The difference tells you what's present
If 3-minute DPD3 is 1.2 ppm but 60-second is 1.0 ppm, then 0.2 ppm is organic chloramine.
Understanding the Concept: A Real Example
The Scenario
A monitor is calibrated to 1.0 ppm using long-delay DPD3. At that moment, the chlorine species are:
Next Validation
DPD3 still shows 1.0 ppm total. But the ratio has changed:
The monitor reads 0.75 ppm — lower than expected — because what it measures (monochloramine) has decreased from 0.8 to 0.6 ppm. The DPD total stayed the same, but the ratio changed. The monitor is reading correctly. The DPD3 is also correct. They're just measuring different things.
In One Sentence
DPD3 and online monitors measure chlorine differently and at different speeds, so they will not always match — consistent timing is the key to getting reliable, reproducible calibration results.
Author: Dr Michael Strahand
November 2025