ORP or Free Chlorine? A Practical Guide to Disinfection Control
Every operator running a chlorination loop faces the same question: should the dosing pump follow a free chlorine analyser or an ORP sensor? Both are legitimate measurements, but they answer different questions, and confusing them is one of the most common causes of unstable disinfection control. This technical note explains the difference and shows where a digital ORP sensor such as the DRH790 fits best.
What ORP Actually Measures
ORP, or oxidation-reduction potential, is measured in millivolts and describes the tendency of the water to accept or donate electrons. In disinfection terms, it expresses the strength of the oxidant that is present, not its concentration. Free chlorine tells you how many milligrams per litre of hypochlorous acid and hypochlorite are in the sample; ORP tells you whether that oxidant is strong enough to inactivate microorganisms under the current pH, temperature and organic load.
That distinction matters because chlorine chemistry is pH dependent. The same 1 mg/L free chlorine residual may correspond to a very different disinfection strength at pH 7.2 and at pH 8.2. ORP captures that shift automatically, which is why many regulators and plant designers accept an ORP set point as a direct disinfection control criterion.
Side-by-Side Comparison
| Criterion |
ORP sensor |
Free chlorine analyser |
| What it reports |
Oxidising strength in mV |
Oxidant concentration in mg/L |
| Response speed |
Seconds |
Tens of seconds to minutes |
| pH sensitivity |
Reflected in the reading |
Must be corrected separately |
| Membrane / reagent |
None, platinum ring electrode |
Membrane and electrolyte, or reagents |
| Maintenance burden |
Cleaning and periodic calibration |
Membrane and electrolyte replacement |
| Best suited to |
Disinfection efficacy and reaction state |
Residual compliance reporting |
The practical conclusion for most plants is not to choose one and discard the other. Use ORP as the fast control variable for the dosing loop, and use free chlorine or DPD testing as the periodic compliance check that proves the loop is delivering the required residual.
Where the DRH790 Fits
The DRH790 is a digital ORP sensor built around a platinum ring electrode. Platinum is chemically inert and unaffected by strong alkali, so the sensing surface can be cleaned back to a reproducible state instead of being replaced. The probe covers -2000 to +2000 mV with 1 mV resolution and ±2 mV accuracy, which is sufficient for both chlorination control at positive potentials and chromium reduction at negative potentials.
Three engineering details make it suitable for control-loop duty:
- Digital Modbus output. The ORP value is digitised inside the probe, so long cable runs and electrical noise from VFDs do not corrupt the signal.
- Internal isolation. Power and communication are isolated inside the body, eliminating ground-loop offset in multi-instrument installations.
- Deviation calibration. A stable offset that develops after weeks in service can be corrected with one standard solution, avoiding unnecessary probe replacement.
Commissioning Advice
Install the probe within ±30° of vertical and never horizontally or inverted, otherwise air can be trapped in the reference chamber and the reading will drift permanently. Keep the flow past the platinum ring between 0.001 and 0.5 m/s, mount the probe downstream of the dosing point so it sees fully mixed water, and calibrate monthly in a 200 mV or 468 mV standard solution.
When ORP is used as the control variable, an offset of even 20 mV can shift the effective disinfectant residual noticeably. That is why the DRH790 supports zero-point, slope and deviation calibration rather than a single span adjustment — it lets the operator correct each error source independently and keep the control loop honest.
For a selection review of your disinfection loop, share your pH range, target residual, temperature and pipe layout with the Daruifuno technical team.
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