Positive ORP (+mV) Reading in Pool Water
ORP stands for Oxidation-Reduction Potential. In practical terms, it is a measurement to oxidize contaminants.
In some parts of the world, it is also known as Redox Potential. Sometimes, you'll see the words "oxidation" and
"reduction" spelled without the hyphen connecting them. We chose the hyphen because the two chemical reactions
are really "joined at the hip" - one cannot occur without the other also occurring.
ORP is the only practical method we have to electronically monitor sanitizer effectiveness. Every true system of
automatic chemical control depends on ORP to work.
ORP and pH sensors allow us to electronically monitor and control sanitizer residual and pH automatically. In a light
usage residential pool, this might not be a primary concern. But in a public or semi-public pool or spa - one that is
under constant observation by local health authorities - some form of dependable, accurate, automatic chemical
control may well be a necessity.
Chemicals like chlorine, bromine, and ozone are all oxidizers. It is their ability to oxidize - to "steal" electrons from
other substances - that makes them good water sanitizers, because in altering the chemical makeup of unwanted
plants and animals, they kill them. Then they "burn up" the remains, leaving a few harmless chemicals as the byproduct.
Of course, in the process of oxidizing, all of these oxidizers are reduced - so they lose their ability to further oxidize
things. They may combine with other substances in the water, or their electrical charge may simply be "used up."
To make sure that the chemical process continues to the very end, you must have a high enough concentration of
oxidizer in the water to do the whole job.
- How do we measure ORP?
An ORP probe is really a millivolt meter, measuring the voltage across a circuit formed by a reference
electrode constructed of silver wire (in effect, the negative pole of the circuit), and a measuring electrode
constructed of a platinum band (the positive pole), with the pool water in between.
The reference electrode, usually made of silver, is surrounded by salt (electrolyte) solution that produces another
tiny voltage. But the voltage produced by the reference electrode is constant and stable, so it forms a reference
against which the voltage generated by the platinum measuring electrode and the oxidizers in the water may be
The difference in voltage between the two electrodes is what is actually measured by the meter. Modern ORP
electrodes are almost always combination electrodes, that is both electrodes are housed in one body - so it
appears that it is just one "probe."
- What does an ORP meter tell us and what is a “good” ORP value ?
For practical purposes, oxidizing agents are the "good guys" in the water sanitation picture, reducing agents are
contaminants and therefore are the "bad guys."
If we had a body of water in which the concentration of oxidizers (or oxidants as chemists are apt to say) exactly
equaled the concentration of reducers (reductants), then the amount of potential generated at the measuring
electrode would be exactly zero. As you might guess, the water would be in pretty sad shape, because if any
additional contaminants were introduced into the water, there would be no oxidizer to handle it.
The industry recommends to have an ORP value of at least 600 mV. And that is also what our product
recommends. We calculate chlorine from the ORP value and a value of 600 mV corresponds to approx. 2
ppm of chlorine. In case the ORP value is lower than the 600 mV and a dosing recommendation is