ORP Monitor – ORPSense

As described elsewhere, 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.

Chlorine is often removed from a process and there are a number of potential reasons why. Chlorine might damage an RO membrane or turn food brown in an unsightly manner. Chlorine removal presents several challenges to instrumentation manufacturers and engineers, but did you know that…
…ORP can effectively control sodium bisulphite dosing to remove chlorine?
…ORP can limit the dose of excess sodium bisulphite which can lead to problems later in the process?
…ORP is very sensitive to changes in chlorine at very low levels?

How do you effectively control chlorine removal?

Logically, it would be perfectly reasonable to take a standard chlorine probe and dose a reducing agent such as sodium bisulphite until the probe gets to zero. Then if the probe does increase from zero, dose more reducing agent.

Unfortunately due to the inherent electrochemical properties in chlorine sensors, this perfectly logical solution won’t work. This is because amperometric chlorine probes require the presence of some chlorine in order to function properly. This is generally called polarisation and the longer a probe has been in water without chlorine, the longer it will take to see chlorine when it is reintroduced. This isn’t due to manufacturing quality or to design flaws. This is a fundamental electrochemical property that is hard to engineer out.

Some amperometric probes are more resistant to this effect than others. They are suitable for applications where chlorine may intermittently reach zero, or where the probe can be periodically reset in chlorinated water (typically potable water). Pi’s HaloSense Zero has been built with this in mind. This amperometric zero chlorine sensor is being successfully used in RO membrane applications and many other low-level chlorine processes.

However, in applications where reducing agents are used to remove chlorine, a more elegant solution is to use ORP. An ORP sensor measures oxidation reduction potential.

The reaction between chlorine and a reducing agent is a reduction-oxidation (redox) reaction. This means that ORP can be used to control this process and can ensure the removal of chlorine, whilst simultaneously preventing the overdosing of a reducing agent.

Below is a simplified example of the reaction between HOCl (free chlorine) and NaHSO₃ (sodium bisulphite).

• 2NAHSO₃ + 2HOCl → H₂SO₄ + 2HCl + Na₂SO₄

When free chlorine is in excess, the ORP is raised and when sodium bisulphite is in excess, the ORP is lowered.

At low levels, a very small change in the concentration of chlorine will result in a large change in ORP. This is because the relationship between ORP and the concentrations of oxidants and reducing agents is logarithmic. The same also goes for the concentration of reducing agent (such as bisulphite), once an excess has been reached.

The ORPSense from Pi can be used to control chlorine removal. An ORP titration curve is necessary to choose a setpoint for ORP. The graph shows a typical ORP titration curve, though the exact values will be different for each process. It is important to test each new installation and to adjust the ORP setpoint accordingly. Once this titration curve has been determined, there is no continual calibration involved with ORP sensors.

It is worth acknowledging that ORP can be affected by many other variables in the process water, such as conductivity, pH or temperature, and that this method of control won’t be suitable for all processes. Combined with Pi’s CRONOS^® or CRIUS^®4.0 controller, Pi’s ORPSense can be installed alongside other sensors like the zero chlorine sensor, pH or conductivity sensors to help give you a more complete picture of your process.

If you need to control a process where chlorine is removed with chemicals, a carbon filter or even with UV, Pi can help you by providing world class instrumentation and application expertise.

NB. This Focus On is about the removal of chlorine, however, many principles in this Focus On can be applied to other oxidants such as chlorine dioxide.

Sensor Fouling

What is the solution?

Process Instruments’ AutoClean and AutoFlush Systems

A solution for each application

AutoClean

AutoVerify

AutoFlush

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?

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.

ORP

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.

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?

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.

Conclusion