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ORP Automation & SWG’s

A new pool, three bodies of water. One, a 110,000-gallon vessel. The second, a 30,000-gallon play pool. The third a 2000-gallon spa. All on SWG’s, the first with 4 60K cells the second with 2 60k cells and the third with a 20k cell. The three controlled by a single ORP/pH controller capable of managing three bodies of water. A residential property with solar on the spa & the equipment in an outdoor installation.

Vassals 1&2 behaved perfectly. A set point of 700 ORP and a pH set point of 7.5. Both manually tested at 4.5 ppm of FC and 7.45 pH, consistently with by-weekly service intervals. The third vessel (the spa) was a different story.

Radical swings in ORP and ppm of FC. On Tuesday we could have an ORP of 700 & a FC reading of 2ppm so we would increase the ORP set point to 715 to increase FC. Returning on Friday to find an ORP of 625 & a FC reading of 15ppm.

After several new boards and sensor replacements it was brought to our attention that it may be due to a residual current created by the SWG.

The SWG (also known as salt cell, salt generator, salt chlorinator) cell consists of parallel titanium plates coated with ruthenium and sometimes iridium and uses electrolysis in the presence of dissolved salt (NaCl) to produce hypochlorous acid (HCIO) and sodium hypochlorite (NaClO), which are the sanitizing agents commonly used in swimming pools.

The process of passing voltage between the plates to create electrolysis can create a build-up of stray current in the water which intern can be read by the ORP probe (which reads in Millivolts) and create a false ORP reading.

It should also be noted that ORP is pH dependent. The higher the pH the lower the ORP and vice-versa. The process of controlling pH with a parasitic pump injecting muriatic acid can rapidly decrease Total Alkalinity, causing pH swings. Therefore, Total Alkalinity, must be closely monitored to insure stable pH readings. Lower acid dosages with extended mix times can also reduce pH swings on smaller bodies of water.

The most cost effective solution was decided to be the elimination of the SWG.A parasitic pump was installed in its place to inject sodium hypochlorite, (liquid chlorine). Ounce employed the ORP controller fell into line with the other two bodies of water and continues to perform reliably.


Sara Howard is Owner and President of Ultimate-Pool.com. She can be reached by email at sara@ultimate-pool.com

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