Guest Column - July/August 2006
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Three Measures of Sanitizer

Aquatic Maintenance

By Alison Smith

Colormetric PPM

Colormetric PPM also measures sanitizer level in parts per million, however it takes measurements in a very different way. The term colormetric 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 colormetric 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 colormetric 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 colormetric 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. Colormetric systems are some of the most accurate automated sanitizer measuring systems available. They are also among the most expensive.

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 well on salt systems. Colormetric sensors also require dumping the sample to waste, which can amount to five gallons of water per day.

Choosing systems

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 give as accurate chlorine concentrations compared with a colormetric system. Colormetric systems will require a higher initial investment, as well as the ongoing cost of reagents.

Cost often becomes the focus of a decision-making process, however it is more important to focus on the needs of the system and to then balance those needs with any monetary limitations. 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. Colormetric measurements are the least susceptible, affected only by temperature.

ORP will tell you how much of your sanitizer is available for use. Amperometric 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 and amperometric PPM both use electrochemical detection, which offers several advantages, including simple instrumentation, ease of use, low cost and low power requirements. Colormetric units will use more water in their testing and require additional power.


Alison Smith holds an MBA and is marketing manager for Acu-Trol Programmable Controllers, a manufacturer of pool and spa automation controllers. She can be reached at More technical information is available at