By M. Troy McGinty
Automatic pH and oxidation reduction potential (ORP) controllers were introduced to the pool industry more than 40 years ago and have been assisting pool operators with the ongoing challenge of maintaining safe water ever since. They are no longer mysterious pieces of equipment; since their introduction, these devices have been mandated in the United States by 11 state health departments, and supported and recommended by the remaining 39 states and all Canadian provinces and territories.
With protozoa like Giardia, viruses like Norovirus and bacteria like E. Coli being introduced into recreational water every day (in most cases unknowingly), careful attention must be given when dealing with recreational water sanitation. Chemical controllers help regulate pH and sanitizer levels to eliminate and prevent the transmission of infectious diseases, which in turn saves lives.
How do they work?
An automatic chemical controller uses sensors (probes) to continually measure two parameters—pH and ORP.
Defined as the negative logarithm of the hydronium ion (H3O+) activity in an aqueous solution, pH essentially measures how much acid (hydronium ion) is contained in the water by using a zero to 14 logarithmic scale.
By virtue of its logarithmic nature, pH is a dimensionless quantity; readings can vary depending on water temperature. Pure water at 25 C (77 F) usually has a pH around seven. If readings are lower than this median level, the water is acidic, while readings above seven show the water to be basic/alkaline. In comparison, the average human tear has a pH between seven and 7.6. However, in the pool industry, the magic number is 7.5, as this is the level where bather comfort and the activity of hypochlorous acid (HOCl) and the hypochlorite ion (OCl–) work best together.
There are two reasons why swimming pool water pH should be kept in the seven- to 7.6-range. First, the water will be less likely to irritate bather’s eyes and/or skin and secondly, this range allows pool operators to maintain effective sanitization levels. A hydrogen atom (1H) bound to OCl– is more effective at inactivating disease-causing germs than OCl– on its own. Without proper pH, all other water chemistry parameters fall out of balance and safe/comfortable water becomes unattainable.
The second parameter an automated chemical controller monitors is ORP, which measures the potential of the oxidizer present in the water to remove electrons from unwanted substances. Once the unwanted substance loses its electron, its chemical structure is changed and it is incapable of being infectious.
Unlike the reading from a N, N-diethyl-p-phenylenediamine (DPD) test kit, ORP provides a true measurement of a sanitizer’s effectiveness based solely on its ability to oxidize in a specific body of water—even in the presence of other variables, such as cyanuric acid, non-chlorine oxidizers, etc. In the pool and spa industry, oxidation reduction potential is measured in millivolts (mV). A 650-mV reading has been widely accepted as the industry standard, since this level suggests the water is in satisfactory microbial condition.