ORP Sensor

ORP Sensor

The ORPSense range of ORP Meters from Pi utilise the very latest and best ORP sensors available in the world today for measuring the online Redox potential of any aqueous solution. They are platinum ball based sensors with integral reference electrodes which use no reagents, are extremely stable, and have reduced maintenance and reduced whole life costs.

Online ORP meters and ORP controllers are one of the most common analysers in use today in water treatment and process plants and yet online ORP meters cause continual problems for their operators. What is needed is a stable, reliable, online ORP meter for a reasonable cost. That is exactly what you get with the ORPSense and the ORPSense Automatic Calibrator.

The ORPSense sensors and flow cells are available with different controllers giving you the same great performance with different communication, display, and control options. With the ORPSense range of residual chlorine analysers, you get everything that you need – and nothing that you don’t.

The sensor at the heart of the Process Instruments online pH/ORP meter is a patented polymer filled glass electrode. This ORP electrode is far superior to many online process ORP electrodes on the market and is designed for a long life, low drift and high reliability. Unlike many ORP analysers, you don’t need to search through hundreds of ORP electrodes to find one that works in your process as the ORPSense electrode works in all environments from pH 2 to 12 and in conductivity from 0µS upward. Although they command a higher price in the market place these sensors are more than cost effective with their longer life and considerably lower maintenance requirements, typically only needing calibration once per two or three months. Despite all of the additional functionality that this unit has to offer, the purchase costs are less than or comparable to, its competitors!

The ORPSense ORP monitor takes away the requirement for continual calibration and re-calibration of the ORP meter by operators as it utilises an extremely stable pH electrode. The ORPSense will calculate a ‘degree of confidence’ in its own calibration and let you know if its confidence is low. If the pH meter detects a fault in the ORP sensor then it gives instructions on the display as to what the fault may be, with full on board fault diagnostics.

The online ORP meter is capable of full PID control and is equipped with process control options, data-logging, relay outputs, serial communications, (TCP, Ethernet, Modbus are available now with Profibus available soon). Remote Access of the ORP meter (including remote access to all control options) is available via the internet, and via a LAN. In fact the ORPSense has all the options you could want in an ORP meter whilst maintaining a low cost and great value for money.

  • Stable and reliable ORP meter
  • On board diagnostics
  • Low purchase cost
  • Long life ORP sensors
  • Remote access and SMS text alarms
  • PID and other process controls

ORP Sensor

ORP Sensor

  • Remote Sites
  • Cooling Towers
  • Food Preparation
  • Paper Mills

Anywhere you have a requirement to measure ORP is a suitable application for the ORPSense. The ORPSense ORP meter range is particularly suited to working in sites where reliability and ease of use are most important.

As described in a separate brochure, the ORPSense can come equipped to automatically clean itself at user defined intervals with all the benefits of no operator intervention. The Autoflush is particularly useful in food preparation, pulp and paper, and many applications where there is likely to be a build up of solids in the sample.

The whole range of ORPSense ORP meters can be fitted with additional sensors such as chlorine or pH. Please ask your local distributor for more details.

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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 3 ppm won’t work?

… did you know that swimming pools in the USA use ORP and in Europe use ppm chlorine sensors?

… that the ORP of towns water can vary a great deal?

In 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 utilise 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 oxidise (gain electrons from) organisms or other material in the water, thereby killing or destroying them.

ORP ppm Swimming Pool

Swimming Pool

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 Sensor

ORP Sensor

Unfortunately, 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 Organisation recommends 1 ppm residual).

ppm Chlorine

These 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

Non- Linear

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