Web Exclusive - February 2016
Find a printable version here

Understanding ORP, Sanitization & How Controllers Work

By Lance Fitzsimmons

Colorimetric PPM: 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. However, 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 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. Colorimetric systems are some of the most accurate automated sanitizer measuring systems available. The measurement is not affected by the presence of salt or cyanuric acid in the water. As there are no electrodes to corrode or gather buildup, they also work wonderfully on salt systems. Colorimetric sensors also require flushing the sample to waste; this can amount to 2 to 3 gallons of water per day.

Primary Disinfection & pH Control

Primary disinfection using sodium hypochlorite, calcium hypochlorite, bromine, di-chlor, tri-chlor 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 solutions for secondary disinfection, as recommended by the CDC's Model Aquatic Health Code (MAHC).

Secondary Disinfection Support With Controllers

More and more often, the industry is now seeing the need for a secondary disinfection system as a backup to the primary system. Controllers now control, manage and support the use of backup sanitizing systems, which is mandatory for salt water pools.

Enzymes & Shock Treatments With Controllers

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.

Using a DPD colorimetric chlorine measurement system, a water test is automatically taken in a pre-programmed frequency as often as every 15 minutes. Reagents are automatically mixed with a sample of pool water and optically read to determine total chlorine, free chlorine and combined chlorine levels. A true PPM reading is then transmitted through the controller to the Internet-based data management system. The colorimetric system is connected directly to the controller, providing a true PPM reading not subject to problems associated with PPM sensors.

Choosing the Best Controller for Your Aquatic Facility

The type of measurement system selected for a chemical controller should take all of these factors into account. The cost of any controller is often a hurdle in the decision making process; ORP measurement will require the lowest investment but will not provide chlorine concentrations as accurately as a colorimetric system. The type of measurement system selected for a chemical controller should take all of these factors in to account. The cost of any controller is often a hurdle in the decision-making process; ORP measurement will require the lowest investment, but will not give as accurate chlorine concentrations compared with a colorimetric system. Colorimetric systems will require a higher initial investment, as well as the ongoing operational costs of reagents. Cost often becomes the focus of a decision-making process; however, it is more important to focus on the needs of the facility, and to then balance those needs with any monetary limitations. Most controllers will justify the initial expense with the savings in labor and longevity of equipment and surfaces. The type of measurement chosen should reflect the other equipment on the pool. ORP measurements are the most susceptible to outside effects, and are therefore less accurate; cyanuric acid, salt, temperature and pH levels all affect ORP measurements. Amperometric measurements have small pH, temperature and cyanuric effects, but are flow-dependent. Colorimetric measurements are the least susceptible. ORP will tell you how much of your sanitizer is available for use. Amperometric and selective membrane sensor measurements tell you how much sanitizer is in the water. A controller that measures both can help a pool operator to identify and solve issues when combined chlorine is present in the water. ORP, selective membrane and amperometric PPM use electrochemical detection, which offers several advantages: simple instrumentation, ease of use, low cost and low power requirements. Colorimetric units will use more water in their testing, and require minimal power.


Every commercial swimming pool and spa should have chemical automation for maximum comfort, safety and longevity of the surfaces and equipment. Utilizing ORP controllers is the most cost effective solution, as long as the operator understands how ORP works, the swimming pool is indoors or the outdoor pool is using no more than 30 PPM cyanuric acid and it is understood that actual chlorine PPM levels will fluctuate when there is ORP control due to the variables affecting ORP.

Ideally, a facility will have ORP and PPM control, which will help ensure that the PPM levels required by local health authorities will remain constant, while also measuring the true water quality.

Lance Fitzsimmons is the inside sales manager for Pool Warden controllers manufactured by ControlOMatic. He has been in the pool industry since 1984 and has worked directly in the servicing of chemical automation products since 1991. Fitzsimmons worked with Acu-Trol, Ecolab and Leslie's in everything from technical support to sales roles. He is a CPO instructor and has a degree in sales management. For more information, visit www.poolwarden.com.