Measure for measure: electronic controllers measure sanitizer levels in several ways. This guide can help you pick the right one for the right pool

Electronic controllers are becoming more and more prevalent in the pool and spa marketplace. Their ability to continuously monitor and maintain water conditions makes them popular with health departments, facility managers and municipal organizations.

Lots of effective controllers are available now, and many use different systems to measure sanitization in pools. The three most popular methods are colormetric ppm, ORP and amperometric ppm. Understanding the basis of each measurement can assist service technicians and pool operators in managing their pools.

Colormetric

Colormetric ppm measures sanitizer levels in parts per million, but it takes its measurements in a different way.

The term "colormetric" is used to describe a system of measurement where reaction-based indicators (reagents) are used to detect the presence of a specified chemical, such as chlorine. 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 that is present.

DPD test kits, an example of a colormetric measuring device, are 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 variations in color perception between individual users. When the "eye" that evaluates the test is automated, though, a DPD kit becomes an accurate measure.

So, essentially, a colormetric system is an automated DPD test kit. The controller is programmed to test the chemical levels in the water at certain times or intervals each day. When it's time to test, the controller will send a signal to a colormetric unit.

A test chamber is filled with water from the pool or spa. A beam of light goes through the test chamber water and is detected on the other side to establish a base for the reading. Reagents are added and mixed with the sample, and the beam of light is once again measured. The difference between the amount of light received in the base reading and the one 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. In addition, the measurement is not affected by the presence of salt or cyanuric acid in the water. Because there are no electrodes to corrode or gather buildup, they work well with salt systems.

Colormetric sensors require dumping the sample to waste. This can amount to five gallons of water per day.

ORP

Oxidation-reduction potential is a qualitative measurement, not a quantitative one. It won't tell you the amount of sanitizer in the water. Instead, it gives a measurement, in millivolts, of the activity or strength of the oxidizers (substances that bum off organic matter) in relation to their concentration.

ORP is ideal for high-volume pools such as public installations, where constant, detailed sanitization monitoring is crucial. Most controllers that take ORP measurements use a bypass system. They take a sample of water from the plumbing and direct it to a small container that houses the sensors.

An ORP sensor contains two electrodes, generally a platinum one that is in contact with the water, and a "reference" electrode inside an electrolyte chamber. The electrodes are separated from the electrolyte solution by a membrane. This membrane prevents the loss of the solution and keeps contaminants from entering the sensor. To provide a reading, the sensor acts like a small battery, with a voltage output typically expressed as millivolts.

Keep in mind that ORP measures the water's ability to oxidize. Therefore, if the primary sanitizer is chlorine, ORP will be dependent on pH levels. Chlorine works best when the pH is in the appropriate range, which is around 7.4 to 7.6.

Cyanuric acid (stabilizer) levels also can impact readings taken by an ORP sensor, as can salt-pool chlorinators. When you install a controller with a salt-pool chlorination system, order a gold sensor instead of platinum. This will help prevent buildup on the sensor.

It's important to understand that ORP measures the water's ability to oxidize rather than the actual amount of sanitizer in the pool. This can be misleading for the novice because some controllers calculate the actual chlorine concentration based on the ORE pH and temperature measurements. Also, the ORP sensor doesn't measure specific sanitizers, so it will not differentiate between ozone and bromine.

Amperometric

Amperometric ppm measures the total amount of sanitizer in the system. Like ORP, an amperometric ppm measurement is taken by a sensor consisting of copper, and a platinum or gold element. They are housed in a small chamber, usually located on a bypass from the main plumbing of the equipment set.

The system works when a water sample is delivered to the annular space between the two elements in the sensing cell. The sensor generates a measurable electrical current when chlorine or another oxidizer is present. The current is then converted to an electrical voltage, measured and converted to parts per million. The value in ppm, which is probably the easiest measurement for end users to understand, is displayed on the digital indicator.

The system is easy to set up and maintain. As long as the pH and the flow are relatively stable, the amperometric ppm measurement will be accurate. Like ORE the amperometric sensors are affected by several factors, including pH. However, the effect of pH is less than it is with ORP measurement.

Amperometric measurements require flow to operate. A continual supply of sanitizer, usually chlorine, is necessary to sustain the oxidation reaction that allows the sensor to measure sanitizer levels. If water is left in the measurement chamber, but there is no flow to the system, the chlorine in the water will continue to react with the sensors.

This reaction will consume some of the sanitizer. The level of sanitizer will decrease as the limited amount available in the sample is used up in the reaction. Eventually, the reaction will consume all the sanitizer available in the sample, so it's important to maintain water flow to the system.

Making a choice

The cost of any controller often is a hurdle in the decision-making process. However, it's more important to pay attention to the needs of the system and balance them against any monetary limitations.

ORP measurement requires the lowest investment, but won't give chlorine concentration readings as accurate as that of a colormetric system. On the other hand, colormetric requires a higher initial investment and the ongoing cost of reagents.

ORP measurements are most susceptible to outside effects and therefore less accurate. Remember, cyanuric acid, salt, temperature and pH levels all impact ORP measurements. Amperometric measurements have small pH, temperature and cyanuric effects, but are flow-dependent. Colormetric measurements are least susceptible to these variables, affected only by temperature.

Smith is a sales and marketing specialist at Acu-Trol, a manufacturer of programmable controllers based in Auburn, Calif.

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