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What is ORP and how can it be used on Swimming Pools

Operating a swimming pool or spa, without an automated controller is like operating your home heating and cooling system without a thermostat. Who would constantly turn their furnace on and off manually? The majority of swimming pool and spa operators control chemistry in exactly this manner. Water balance continually changes between too much and too little as operators attempt to maintain chemical balance Consequently, unsafe water conditions, chloramine formation, and organic loading are inevitable which leads to cloudy water eye irritation and foul “chlorine smell”.

ORP stands for Oxidation-Reduction Potential and in some places it is called Redox. In practical terms, it is a measurement on how effectively chlorine or bromine can oxidize contaminants, kill algae and kill bacteria & viruses.

Why is ORP so important in a swimming pool or spa? The answer to that is that right now, ORP is the only practical method we have to electronically monitor sanitizer effectiveness. The better you can understand ORP, the better you will be able to manage a pool or spa.

In the real world, manual chemical maintenance of pool and spa water is labor intensive and almost impossible to do effectively. You must maintain sanitizer residual at a level sufficient to protect swimmers and bathers from the invasion of unwanted - and possibly harmful - plant and animal life. You must maintain the pH of the water at a level that assures the sanitizer works effectively and at the same time protects the pool shell and equipment from corrosion or scaling and the bathers from discomfort or irritation. Not only that, but in most places, there is a health department that will occasionally check water quality and if not within published parameters, will close a swimming pool or spa.

You must also make sure that all the other levels - total alkalinity, water hardness, temperature, and total dissolved solids (TDS), to name four big ones - are also in balance or not out of the recommended range.

The two most important components of water balance are sanitizer residual and pH. By far, these are the chemical tests performed most often as these are things that we are most concerned with.

ORP and 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. However, in a public swimming pool or spa some form of dependable, accurate, automatic chemical control may well be a necessity.

"But," you might say, "I am controlling the chemistry. I've got an erosion feeder hooked up or a pump with a timer adding chlorine from a bleach tank.

The key words are "dependable" and "accurate." The methods described above may get some sanitizer in the water, but will it be enough or will it be too much?

An erosion feeder, hooked in-line with the circulation system, will dispense some chemicals whenever the system is running - whether they are needed or not. A pump hooked up to a timer will dispense some chemicals constantly - whether they are needed or not. Depending on this method to accurately treat your water is a little like playing Russian Roulette - chancy at best and downright fatal at worst.

Besides, erosion feeders and feed pumps only deal with sanitizer residual. There's still nothing there to control pH. pH, as we all know, is the thing that makes chlorine work.

If you want true chemical control, you've got to have some method of monitoring both the sanitizer residual and the pH and using that information to chemically treat the water. That's where ORP comes into play.

Chemicals like chlorine, bromine, and ozone are all oxidizers. It is their ability to oxidize 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 to be filtered out.

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.

But how much is "enough?" That's where the term potential comes into play.

"Potential" is a word that refers to ability rather than action. Potential energy is energy that is stored and ready to be put to work. It's not actually working, but we know that the energy is there if and when we need it.

In electrical terms, potential energy is measured in volts. Actual energy (current flow) is measured in amps. When you put a voltmeter across the leads of a battery, the reading you get is the difference in electrical pressure - the potential - between the two poles.

When we use the term potential in describing ORP, we are actually talking about electrical potential or voltage. We are reading the very tiny voltage generated when a metal is placed in water in the presence of oxidizing and reducing agents. These voltages give us an indication of the ability of the oxidizers in the water to effectively “kill” contaminants.

How is ORP Measured?

An ORP controller and probe is 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 or gold 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 compared. These electrodes are housed in a single plastic body. The meter circuitry itself must have very high impedance (resistance) in order to measure the very tiny voltages generated by the circuit.

What Does an ORP Controller Tell US?

For practical purposes, oxidizing agents are the "good guys" in the water sanitation picture, reducing agents are contaminants or the "bad guys."

If we had a body of water in which the concentration of oxidizers exactly equaled the concentration of reducers, 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 take care of it.

As we add oxidizer to the water, it "takes" electrons from the surface of the platinum measuring electrode. To make things a little more confusing, we need to point out that electrons are negatively charged particles. When we remove these negatively charged things from this electrode, the electrode becomes more and more positively charged. As we continue to add oxidizer to the water, the electrode generates a higher and higher positive voltage. An ORP controller will tell us how effectively the water is being sanitized. ORP is a qualitative, not a quantitative measurement. Quality not quantity.

How pH Affects ORP

Professionals are already well aware that sanitizer effectiveness can vary rather significantly with changes in pH - particularly in regards to chlorine, which is by far the most commonly used sanitizer for swimming pools and spas.

The killing form of chlorine is hypochlorous acid or HOCI, which, is a powerful oxidizer. The percentage of chlorine available as hypochlorous acid present in pool and spa water depends directly on the pH.

For example, at a pH of 6.0, 96.5 percent of the Free Available Chlorine in the water is in the form of HOCI, while at a pH of 8.5, only 10 percent is iavailable as hypochlorous acid.

A DPD test can tell you how much of the chlorine is combined and how much is free and available, but it cannot tell you what percentage is in the form of hypochlorous acid. To determine this, you must take a pH test and calculate the results. The pH or the water sample will not effect the results of a DPD test.

Although ORP does not specifically tell you the chlorine concentration in parts per million, it does indicate the effectiveness of the chlorine as an oxidizer. An ORP reading will vary as pH fluctuates. As the pH goes up, the millivolt reading on an ORP meter will go down, indicating that amount of HOCL available in the water is lessened. Bringing the pH down or adding more sanitizer will increase the millivolt reading.

To give people the full picture, most ORP controllers also incorporate an electronic pH meter - which measures the difference in electrical potential between the pool water and a sample of known pH that is contained in the probe in a small glass bulb.

How much is enough?

The instruments for measuring ORP were developed in the 1960's, which enabled researchers to work toward setting standards under which ORP measurements could be used as an accurate gauge of water quality.

In 1972, the World Health Organization adopted an ORP standard for drinking water disinfection of 650 millivolts which means the sanitizer in the water is active enough to destroy harmful organisms almost instantaneously. In Germany, which has about the strictest water quality standards in the world, an ORP level of 750 millivolts has been established as the minimum standard for public swimming pools and spas.

In its 1988 standards for commercial pools and spas, the National Spa & Pool Institute stated that ORP can be used as a "supplemental measurement of proper sanitizer activity" when chlorine or bromine are used as primary disinfectants. The recommended minimum reading under the NSPI standards is 650 millivolts, with no ideal and no maximum.

Most health codes still specify that a measurable free available chlorine (FAC) residual - usually 1.0 ppm present in the water of public pools and spas, as measured with a DPD test kit.

Swimming Pool and Spa Chemical Automation

ORP technology has received widespread application in this country as the basis of automated chemical control equipment. The reasoning is clear: Only an ORP sensor can deliver the kind of feedback needed to control feeders for sanitizer and pH adjusting chemicals.

Unlike erosion feeders or timer controlled devices, ORP based chemical controllers can dispense pool chemicals as they are needed, which not only provides for optimal sanitation but also conserves chemicals and prolongs equipment and surface life. Combined with a pH sensor, these controllers can be used to activate liquid feed pumps, or erosion type feeders for dry chemicals. They also can monitor pool water chemistry and store the data.

This type of chemical automation can result in significant savings for operators of commercial pool and spa installations because chemicals are only dispensed when they are needed. Just like a thermostat saves enegery with heating and cooling.

Further, electronic control assures that sanitizer and pH adjusting chemicals will be dispensed precisely as they are needed, eliminating the “roller coaster” effect in sanitizer residual and pH that often occur in pools and spas as bather load fluctuates.

When used with liquid chemical feed pumps, the readings from the pH and ORP probes determine when the controller activates chemical pumps. The pumps are turned on and off to achieve the desired levels.

Erosion feeders (dispensing trichlor, calcium hypochlorite tablets or bromine tablets can also be controlled by an ORP controller. The feeder is placed in a bypass line, which is opened or closed through the use of a solenoid valve.

ORP devices can also be used to measure sanitizer effectiveness and control chlorine generators.

In my 26 years of experience, chemical automation has been the only way to achieve optimal water quality and economy. Swimming pools and spas utilizing this technology are much more enjoyable to swim in and easier to maintain.