Why are CYA levels lower when using ORP?

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Rick Sass
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Joined: Wed 27 Aug, 2008 14:34
Location: SW FL

Why are CYA levels lower when using ORP?

Postby Rick Sass » Sun 14 Jun, 2009 14:21

I have a outdoor, 21,000 gal plaster pool, screened cage, no pool cover, solar heat, SWG, cartridge filter, sense & dispense system, Stenner 5 gpd variable speed acid pump, Goldline controller in SW Florida and I like to keep the FC level at 3.5.

I don't understand why with an ORP sensor/probe system the recomended CYA level should be 30-50 ppm but without an ORP sensor/probe then the recommended CYA level is 70-80 ppm. If all things being equal, except for the use of an ORP sensor, why is there a difference in CYA levels? It was my understanding that a higher CYA level would cushion/blanket chlorine from sunlight/UV and therefore I could dial back the SWG.

Thanx for the help...

chem geek
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Why are CYA levels lower when using ORP?

Postby chem geek » Sun 14 Jun, 2009 23:58

The higher CYA level without a proportionately higher FC level is a low active chlorine (hypochlorous acid) concentration resulting in a low ORP value. The ORP sensors don't do well at such lower values in terms of regulating chlorine. 3.6 ppm FC with 80 ppm is around 600-620 mV ORP depending on the sensor (ORP isn't as "absolute" as some claim and even the mV change with doubling in chlorine concentration varies by sensor manufacturer).

You are right that with higher CYA levels you would likely be able to lower the SWG on-time, but at 80 ppm CYA you should keep a minimum of 3.6 ppm FC and 4 ppm FC would be a little safer to prevent algae growth. You could also use 50 ppm Borates as an additional pH buffer (so you can keep the TA at 70 ppm to help prevent the rate of rise in pH) and its a mild algaecide.

Since you are using an SWG, why do you have an ORP sensor? Do you expect the chlorine demand to vary a lot day-to-day?

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