Clearing up ORP misconceptions.(Letters)(Oxidation-Reduction Potential or Redox)(Letter to the Editor)

As one of the early proponents of ORP technology for sanitizer control in pools and spas, I would like to comment on the article "All Fouled Up" by Shawn Lin and David Rouse that appeared in your April 2004 issue. As pointed out by previous writers, this article shows a misunderstanding of basic water chemistry and serious misconceptions about ORP (Oxidation-Reduction Potential or Redox).

Here are five misconceptions mentioned in the article:

1 "Many controllers use a common ORP probe to gauge water balance."

ORP sensing has nothing to do with water balance. Water balance relates to the aggressiveness of water in terms of corrosive or scaling tendencies (mostly from calcium imbalance). The Saturation Index defines water balance in terms of pH, temperature, calcium hardness and total alkalinity.

2 "ORP probes ... apply a measurement of the electrical potential of the water known as conductivity."

Here again, conductivity has nothing to do with ORP or with the sanitizer. Conductivity is normally used to monitor the concentration of total dissolved solids in water. When the TDS becomes too high, the water needs to be partially or completely replaced.

3 "If ... you can't get accurate ORP readings with trichlor ..."