How ORP probes measure oxidation-reduction potential, what the reading says about an oxidizing biocide, and how to keep the probe reading true.
ORP (oxidation-reduction potential, also called redox) measures how strongly the water tends to take electrons from, or give electrons to, things in contact with it. It is reported in millivolts (mV).
- Measuring electrode. A small band or disc of platinum (sometimes gold). Unlike a pH glass, the metal does not react itself; it simply takes up the electrical potential set by the oxidizers and reducers in the water.
- Reference electrode. The same silver/silver chloride reference and junction used in a pH probe. The transmitter reads the mV difference between the two.
- Positive mV means oxidizing conditions: chlorine, bromine, chlorine dioxide, ozone, and peroxide all push the reading up. Negative mV means reducing conditions, for example sulphite or bisulphite dechlorination.
As a rough guide, water with a working level of free chlorine typically reads somewhere around +650 to +800 mV, and dechlorinated water reads low or negative. These are indications only; every system has its own relationship.
¶ What the reading does and does not represent
- ORP tracks oxidizer activity, not concentration. It is not a ppm reading. Two systems at the same chlorine level can read quite differently, and the response is logarithmic — doubling the chlorine raises the reading only a few tens of mV.
- ORP depends on pH. Free chlorine is mostly hypochlorous acid (HOCl, the strong biocide) at low pH and hypochlorite ion (OCl⁻, much weaker) at high pH. The same free chlorine reads lower ORP as pH rises. Control pH before relying on ORP to control a biocide.
- Other chemistry moves it too. Treatment products, organic load, ammonia (which forms weaker chloramines), and temperature all shift the reading.
This is why ORP is best used as a control and trend signal for oxidizing biocide, checked against periodic free or total halogen tests, rather than as a measurement of chemical residual.
There is no temperature compensation on ORP in the sense used for pH; the transmitter reports the measured mV.
- Fouling and coating of the platinum by biofilm, oil, iron, manganese, or scale. The reading becomes slow and lags behind real changes in oxidizer.
- Slow response after cleaning or when new. A clean platinum surface can take minutes to hours to condition and settle, especially going from low to high oxidizer.
- Reference poisoning or a clogged junction. As with pH, sulphides and heavy metals attack the reference, and deposits block the junction. The result is offset drift.
- pH swings. A change in pH shifts ORP even when the oxidizer level is steady.
- Air bubbles and dry-out at the electrode break the circuit.
- Electrical noise and ground loops, particularly from pumps and heaters in the same water.
- Low-conductivity water gives unstable readings for the same reasons as pH.
Install an ORP probe the same way as a pH probe:
- Tip down, at least about 15° above horizontal, so the internal electrolyte stays at the tip.
- Keep the sensing tip wet, in a flow cell or pipe section that does not drain.
- Place it where the oxidizer has mixed in, well away from the biocide injection point, but close enough that the controller sees a change without long delay.
- Steady flow across the electrode gives a more stable, representative reading than stagnant water.
- Mount pH and ORP probes together in the same sample stream where possible, since you will need the pH to interpret ORP.
¶ Maintenance
- Rinse with clean water and wipe the platinum gently with a soft tissue.
- Mineral deposits: a short soak in dilute hydrochloric acid (commonly about 0.1 M, or 5 %).
- Oil and organic film: mild detergent in warm water.
- Stubborn films on the platinum: some manufacturers allow a very light polish with a fine abrasive or toothpaste. Follow the manufacturer’s instructions; never use abrasive on the junction.
- After cleaning, rinse and soak in storage solution. Allow the reading to settle before calibrating or relying on it.
Cleaning acids and bleach can burn skin and eyes, and mixing them releases chlorine gas. Never mix cleaners, wear eye protection and gloves, and follow Chemical Handling. Isolate and depressurize before removing an insertion probe — see Isolation.
¶ Checking and calibration
- Check against an ORP standard, such as a quinhydrone solution (made up in pH 4 or pH 7 buffer) or a commercial redox standard with a stated mV value (for example, 200 mV, 220 mV (ZoBell), or 470 mV solutions are common). The standard’s mV value depends on the reference type and temperature, so use the value printed for your conditions.
- Accept a small offset. Readings within about ±10–20 mV of the standard are commonly accepted. Most transmitters allow a one-point offset adjustment; there is no slope to calibrate.
- Do not “calibrate” ORP to a chlorine test. Record both, and set the ORP control point from the relationship you observe in that system.
- Service life and storage are similar to pH electrodes: the reference is consumable, commonly lasting around a year. Store the tip wet in storage solution (typically 3–4 M KCl), never in distilled water.
Models documented in this wiki: