The Potential of ORP

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Davies changes controller settings on one of Mastercraft’s 200+ commercial swimming pools.
Davies changes controller settings on one of Mastercraft’s 200+ commercial swimming pools.

ORP/pH controllers have been used for decades, mostly on municipal pools, for water disinfection. In recent years, however, remote communications have created a more powerful application of this technology, enabling pool care technicians and operators to be more proactive in managing water.

When sensors on an ORP unit read a deteriorating condition in pool or spa water, the unit can alert a pool care professional, allowing for immediate preventive action that can head off a potentially dangerous health issue.

While its use as an automated system for pool disinfection is an exciting prospect, both for the commercial and residential markets, for some ORP remains a poorly understood means of water care.

Concept Confusion

Most people involved with pool and spa water quality are trained to think in terms of the amounts of sanitizer or other chemicals in the water. This approach measures quantities of elements such as chlorine or levels of calcium, CYA, dissolved solids, etc. In that regime, for instance, a free chlorine level between, say, 2 to 4 ppm is a crucial value for a safe, clean pool.

On the other hand, ORP, which stands for oxidation-reduction potential (some may remember the term "oxidation reduction reaction" from high school chemistry class), is a qualitative measurement. It measures the waters ability to oxidize and sanitize material in the pool, rather than the actual amount of sanitizer. ORP is affected by many factors including the familiar water chemistry parameters such as pH, cyanuric acid and chlorine.

The concept can be confusing to people new to the subject, says Lance Fitzsimmons of ControlOMatic. "Oftentimes, pool water may be in compliance with recommended chlorine ppm levels, yet have unacceptably low ORP to effectively oxidize and sanitize the water," he says. "Conversely, there are often pools and spas that are well below the required ppm levels but have exceptional water quality."

ORP Savvy

Getting this point across to a distracted or undertrained staff can be a challenge. Skylar Davies of Mastercraft Pool & Spa, Pocatello, Idaho, a company that services 200 to 250 commercial swimming pools throughout the year, faces that challenge on a regular basis.

"ORP is one of the toughest concepts to teach typical maintenance staff," he says. "And understandably so. It's not easy to explain that ORP tells you about the 'quality' of the pool water or how well the chlorine being added to the pool is able to disinfect the pool water. Most people think it just tells them how much chlorine is in the pool. They don't understand how the controller can tell them that the ORP is 650 today with a 2 ppm reading, but tomorrow it might be 650 with a 1.5 ppm reading.

"I try to simplify the process for these hotel managers or maintenance staff by telling them to be sure the range is between 650 and 750 — and to call me only if the readings start to go outside of this range. I also set alarms at these set points to help trigger a manager to go take a look at the controller and test the water."

A major hurdle is the high-turnover rate often seen in the pool maintenance position at commercial pools, Davies adds. With so many different people operating in the pump room, it's hard to keep training and re-training the staff on how to read and interpret the information being provided by a controller.

With frequent comings and goings, neglect and user error are inevitable, Davies says. "And then there is just the general monkeying around that maintenance staff does to a controller. They will do funny things like change the delivery time from two minutes to 30 minutes — which is way too much for a smaller commercial pool. An untrained staff member just thinks: The ORP reading is low. There isn't enough chlorine. So let's just give it more for a longer period of time. This is a typical mistake made with a controller."

Another problem is simple lack of awareness of the necessary requirements of the controller. Often the staff in charge of a pool simply doesn't realize that the chemicals still need to be checked at least once a day, even with a controller on the pool.

Despite the occasional issues with inadequate training, Davies says most of his ORP-based customers are happy with the technology.

"I have one customer who runs a nice hotel with a pool and because of staffing problems could never get the water chemistry right — which made the pool look and smell awful. They wanted us to come out daily to keep the pool in good shape because they knew it affected their business. But it was cost-prohibitive to have us come out to maintain their pool every day.

"We recommended they get a controller over a year ago and they are very pleased. They realize that the pool looks, feels and smells better and guests are coming back more because of this. Unfortunately they got so comfortable that they then forgot to check their chemical drums to make sure they didn't run out of chemicals — that is another problem they are working on now. But overall it was a big success and they still have us come in periodically to make sure everything is running smoothly. It's a win-win for both the hotel and for us."

How Different Automatic Controllers Work

By Lance Fitzsimmons, ControlOmatic

While many water care professionals are unaware of the differences between ppm and ORP water chemistry, still more are untrained on the way different kinds of controllers sanitize pool water. Below is a brief summary of the most popular types.

ORP Controllers

ORP is a qualitative measurement, not a quantitative measurement and is affected by many factors including the biggest variables in swimming pool water chemistry: pH, cyanuric acid and salt. But ORP is a measure of the oxidizing properties of chlorine or bromine sanitizer present in the body of water.

Most ORP controllers will use a bypass system, taking a small sample of water out of the plumbing, and directing it to a small container that houses the ORP and pH sensors. These output an ORP reading as a voltage, typically expressed as milli-volts.

ORP is probably the most common method used by chemical controllers to measure sanitizer level, and the most inexpensive type of system to buy. However, as stated in the main section of this article, understanding that the measurement is the water's ability to oxidize and sanitize rather than the actual amount of sanitizer can be misleading for the novice. Some controllers calculate the actual chlorine concentration based on the ORP, pH, and temperature measurements. The important thing to remember is that the ORP sensor doesn't measure anything specific; it will not differentiate between different sanitizers in the water.

Amperometric PPM

Amperometric ppm systems measure the total amount of sanitizer in the system. Like ORP, an amperometric ppm measurement is taken by a sensor consisting of a copper and a platinum element. These sensors are housed in a small chamber, usually located on a bypass from the main plumbing of the equipment set. A liquid sample is delivered to the annular space between the two fixed electrodes in the sensing cell. A small amount of potential is applied to the working electrode, made of gold wire and the counter electrode, made of silver. An electrical current is generated by the reduction of HOCL and/ or OCL- at the gold electrode. The process consumes the electrons from the oxidation of the silver electrode, creating a small amount of electrical current. This charge is in direct linear proportion to the amount of residual chlorine present in the sample.

This charge is then sent from the sensors to the controller, the electrical voltage is measured, and converted to parts per million. The residual value is displayed on the digital indicator in ppm.

As long as the pH and the flow are relatively stable, the amperometric ppm measurement is accurate. The amperometric sensor is also affected by pH, cyanuric acid, and salt systems, but the effect is less than on the ORP measurement because of the linear relationship of the sensor. Amperometric measurements also require flow to operate — when there is no flow the oxidizer is soon consumed by the measurement leaving none left to measure.

Selective Membrane PPM

This type of system provides direct readings of free chlorine in water. These are true readings of free chlorine and not indirect values derived from ORP.

Colorimetric PPM

A colorimetric ppm system measures sanitizer level in parts per million. However, it takes measurements in a very different way. The term "colorimetric" is used to describe a system of measurement where reaction based chemical indicators are used to detect the presence of a specified chemical. The indicator reacts with the specified chemical and produces a visible color change in the solution. The stronger the color of the solution, the more compound is in the solution. DPD test kits, an example of colorimetric measuring devices, are very familiar to those in the pool and spa industry.

Studies have shown that there can be significant differences in readings from these kits due to differences in color perception between individual users. However, when the eye that evaluates the test is automated, a DPD test becomes a very accurate measure. Essentially, a colorimetric system is an automated hand check. The controller is programmed to test the chemical levels in the water at certain times or time intervals each day. When it is time to test, the controller sends a signal to the Colorimetric unit, which is separate from the controller.

This signal starts the testing process. A test chamber is filled with water from the pool or spa. A beam of light goes through the test chamber and is detected on the other side to establish a base for the reading. Reagents are then added and mixed with the sample. The beam of light is again measured and the difference between the amount of light received in the base reading and the reading with reagents is the measure of the amount of sanitizer.

Colormetric systems are some of the most accurate automated sanitizer measuring systems available.

Primary Disinfection & pH Control

Primary disinfection using sodium hypochlorite, calcium hypochlorite, bromine, dichlor, trichlor or chlorine generation (salt systems) has always been a part of the automation of commercial pools and spas. However, controllers now go beyond primary disinfection and pH control, offering control and management for secondary disinfection, such as ozone and UV, as recommended by the CDC's Model Aquatic Health Code.

Some controllers can also be set up to add enzymes and shock treatments automatically. This treatment can be scheduled at a time and date when no bathers are in the pool and starts automatically. Be aware, though, that non-chlorine shock will cause a temporary increase in ORP and a false DPD reading.


Comments or thoughts on this article? Please e-mail [email protected].

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