You probably know that some instruments use ORP to control chlorine dosing and others use ppm chlorine sensors but did you know that…

…ORP over about 3ppm won’t work?
…swimming pools in the USA use ORP and in Europe use ppm chlorine sensors?
…the ORP of towns water can vary a great deal?

Swimming PoolIn the USA nearly all pools and spas use ORP sensors to control their chlorine dose, yet conversely in the UK and Western Europe most ORP systems have been replaced with systems that measure the concentration of free chlorine in water. Pi provides systems that utilize either or both technologies.


Oxidation reduction potential (ORP or REDOX) sensors, measure the tendency of water to gain or lose electrons from anything in the water. The more positive a reading from an ORP the greater the tendency the water has to oxidize (gain electrons from) organisms or other material in the water, thereby killing or destroying them.

Why do so many pools in the USA use ORP?

When chlorine is dosed into a pool it form OCl and HOCl. Disinfection is largely done by the HOCl and ORP responds to the concentration of HOCl in the water, which makes it a good measure of the tendency of the chlorine in the water to kill bugs. Despite this, ORP is a secondary measure of HOCl and is affected by a multitude of other factors, some of which will be touched on below. The main attractions of ORP are; low purchase cost, no calibration and little or no maintenance.

What are the problems with ORP sensors?

ORP SensorUnfortunately, what ORP sensors measure is tendency and not capacity, i.e. ORP measures the likelihood or the ability of the water to kill bugs, but not how many bugs that water can kill, a subtle but very important difference. A sample with high ORP may be able to kill a small number of bugs very quickly but then not be able to kill future pollution. What’s more, although chlorine affects ORP very strongly it is not the only variable involved. The pH of water affects ORP directly and also affects the concentration ratio of OCl/HOCl, the two main disinfectant components. A lower pH (higher acidity) will cause an increase in the relative concentrations of HOCl causing an increase in ORP.

Perhaps the biggest issue with ORP is that the ORP readings on water with no chlorine in it will be different depending on the source of that water. This means that an ORP of 750mV in one part of the country is not the same chlorine concentration as 750mV in another part of the country. Also the ORP response to HOCl is not linear and increasing residual chlorine above 3 ppm has little effect on ORP readings making control above 3 ppm extremely difficult. These issues typically lead to overdosing the water with chlorine, in order to compensate for these effects. This can be seen very clearly in US pools which often have more than 2 ppm of chlorine compared to European pools which typically operate around 0.8-1.5 ppm (The World Health Organization recommends 1 ppm residual).

ppm Chlorine

Chlorine SensorThese sensors use electrochemistry to measure the free chlorine concentration directly. They tend to be slightly more expensive than an ORP sensor, but are more reproducible and precise, and therefore tend to give better control (and therefore reduced chemical cost). They are specific to free chlorine (the disinfectant) and can be easily calibrated using a DPD test for free chlorine. Whilst the capital cost for a ppm chlorine sensor is higher, total cost of ownership tends to be lower as ORP sensors are typically replaced every year and ppm sensors last for ten years or more.

Problems with ppm Chlorine sensors

A ppm sensor measures the capacity of water to kill organisms, the only problem is that it doesn’t measure how fast the bugs are killed, a variable largely down to pH. There are two different types of ppm sensors. The first measure only HOCl, and have very similar problems to ORP sensors. The other type of sensor, in pHs below 8.0, measure both HOCl and OCl. Pi only recommends the use of sensors that (for use in pools) are independent of pH, and the use of pH control that is independent of chlorine dosage. This leads to tighter control of both pH and free chlorine meaning chlorine residuals can be more tightly controlled and reduced, which in turn leads to lower costs and a more pleasant bathing experience.




ORP Sensors

Simple (no calibration)

ORP Sensors

Doesn’t measure disinfection capacity
Affected more by pH than by free chlorine
Not reproducible (not the same from site to site)
Affected by changing water chemistry
Affected by all oxidants
Using ORP control normally leads to higher residuals and less stable control

ppm Sensors

Measure free chlorine directly
Results comparable across different sites
Linear response
Only affected by free chlorine
Using a ppm sensors leads to lower residuals, more stable control and better swimmer experiences

ppm Sensors

Requires calibration
More expensive – but not much
More maintenance – but not much

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