Three Measures of Sanitizer
By Alison Smith
Electronic controllers are becoming more and more prevalent in the pool and spa marketplace. The ability of controllers to continuously monitor and maintain water conditions makes them very popular with health departments, facility managers and municipal organizations. Chemical controllers simplify water maintenance by taking over much of the work involved.
There are many effective controllers available to the general market. Most controllers will measure the levels of sanitizer and pH in a body of water. The three most popular measures of sanitizer available through chemical automation are: ORP, amperometric PPM and colormetric PPM. Understanding the basis of each measure can assist pool operators in managing their pools.
ORP is a measure of the oxidizing properties of any sanitizer present in the body of water. Most 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 sensors. An ORP sensor contains two electrodes, generally a platinum electrode 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 electrolyte solution and prevents contaminants from entering the sensor. The sensor acts like a small battery. The output from the sensor is a voltage, typically expressed as millivolts. ORP measures the waters' ability to oxidize and is therefore dependent on pH when the primary sanitizer is chlorine. Cyanuric acid levels can affect the readings taken by an ORP sensor. Salt systems also can affect ORP sensors; often special metals must be used for the electrodes in order to prevent buildup on the sensor.
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, understanding that the measurement is the water's ability to oxidize 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 ozone and bromine.
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. 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. Using a gold and silver 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 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 mg/l (PPM).
PPM is probably the easiest measurement for end-users to understand. The system is simple to set up and maintain. As long as the pH and the flow are relatively stable, the amperometric PPM measurement is accurate. The amperometric sensor also is 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.