Electronic controllers have found a home in the aquatics industry. They are prized by municipal health departments, facility managers and service technicians for their ability to continuously monitor and maintain water conditions.

Most controllers measure pH and sanitizer levels by testing a small sample of water taken out of circulation. But with varying models and prices, which one should you choose?

Understanding the science behind the three most common systems ORP, colorimetric ppm and amperometric ppm can help you determine the best fit for your facility.

1 ORP. Quite simply, oxidation-reduction potential (ORP) measures the oxidizing properties of a sanitizer.

It?s probably the most common, and least expensive, measuring system used by chemical controllers.

The ORP sensor functions like a small battery. The sensor emits a voltage, usually expressed in millivolts, that gauges the water?s ability to oxidize, or destroy, bacteria.

An ORP sensor contains two electrodes: A platinum electrode is placed in the water, while a reference electrode remains in an electrolyte chamber. The platinum electrode generates a negative charge. As those electrons are taken from the electrode by oxidizing agents in the water, the electrode becomes more positively charged. The reference electrode, meanwhile, maintains a constant charge from the surrounding electrolyte solution. The charges from the reference electrode and the platinum electrode are compared, and the resulting difference represents the ORP reading.

In outdoor pools, however, the sensor?s readings can be thrown off by the presence of cyanuric acid (CYA). Cyanuric acid is a buffer that bonds to chlorine and protects it from sunlight. These cyanuric-chlorine bonds are being constantly formed and broken. But in an instantaneous test, ORP cannot detect chlorine that is attached to CYA, even though the chlorine will soon detach and be ready to oxidize.

When you begin testing CYA levels above 40 ppm, the ORP will display a lower oxidation level, despite an acceptable level of free chlorine. Keeping a log of chlorine, alkalinity and pH measurements in relation to ORP readings is important for any operator to maintain water balance.

Some newer controllers calculate the concentration of chlorine based on a combination of the ORP, pH and temp-erature measurements. Remember, the ORP sensor doesn?t measure anything specific; it will not differentiate between ozone and bromine. But it will tell you the water?s potential to oxidize contaminants. It?s important to take each of these factors into account when choosing the type of measurement system for your electronic controller.

Consider cost: ORP measurement requires a modest investment, but it won?t give as accurate a chlorine concentration measure as, say, a colorimetric system, which is more expensive to purchase and maintain.

2 Colorimetric. As in DPD tests, which determine sanitizer levels with a colored reagent, colorimetric ppm measurements use color to detect the presence of chemicals. A reagent reacts with the chemical in the water sample, causing the water sample to literally change colors.

Essentially, a colorimetric system functions as an automated hand check. A beam of light passes through the test chamber and is detected on the other side to establish a base for the reading.

A reagent the same type used in DPD test kits is then added and mixed with the sample. Another beam of light is shone through the chamber, and the difference between the two readings indicates the amount of sanitizer present.

The darker the color, the more sanitizer is in the solution.

The DPD test kit is a widely used measure for chlorine, but the same results may be read differently depending on the user. The only real way to ensure accuracy is through an automated evaluation.

Colorimetric systems are among the most expensive sanitizer measuring systems available. But they?re also some of the most accurate: Measurements are unaffected by the presence of CYA.

3 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 ORP, 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.

Ultimately, though, it comes down to balancing cost with the needs of the system.