Anyone using pH/ORP controllers?

[pmeloy]

Just curious if anyone is using automated chemical controllers other than simple SWG? Or even a SWG system with pH/ORP controller instead of just a "rate" adjustment knob?

[PoolDoc]

They have been used, and are available.

But, using pH/ORP control, especially on outdoor pools, is very problematic. My son, who works in industrial waste water analysis, laughs at the idea of using uncalibrated pH and ORP electrodes to operate pools. In his situation, a probe that has not been calibrated in 3 days is considered to be providing bogus information. On filtered pool water (probes DOWNstream of the pool filter), you might be able to rely on an electrode for 2 weeks after calibration.

For 10 years, I sold, installed, and operated pH/ORP controllers for both indoor and outdoor municipal and school pools. I eventually replaced most of them with time clocks and calibrated feed pumps . . . and got much better results.

There are multiple problems.

+ On outdoor pools using stabilizer, ORP sensor output signals vary a LOT, when the pool is in full sun, compared to when the pool is under cloud cover. This is why Stranco and Steininger have tried to get commercial codes to ban stabilizer: their products won't work when it's present in pools exposed to sunlight!

+ With respect to both pH and chlorine levels, pH/ORP controllers tend to feed into transient conditions (someone pee-d near an skimmer => chlorine over feed; someone shocked with bleach => acid over feed), resulting in over treatment of pool water. This would be less of a problem if the feed systems were correctly sized, but typically, they are over-sized. If the pool was formally designed by engineers, the feed system over-sizing is often massive.

+ It is often the case that neither pool engineers or pool builders understand how to set up sample loops so that the sample stream accurately and RELIABLY reflects current pool water conditions, AND will fail-safe. I've seen a brand new $300,000 therapy pool nearly destroyed the first weekend it was operated ($100,000+ of damage) due to operator error combined with an over-sized feed system and a badly designed sample loop.

+ Because none of the controller makers acknowledge these and other problems, pool operators cannot be properly trained to compensate for the inherent limitations of the controllers.

And so on.

[pmeloy]

I've been using pH/ORP controllers indoors for 18 years and never saw those kinds of problems. I can't even imagine how you could do that kind of damage.

Training is definitely and issue but I've not had that hard a time getting the operators to understand the difference between ORP and PPM residuals and how it's dependent on pH. Typically a two hour session about the care and feeding of controllers and test kits is enough to keep new operators from making major mistakes.

What I'm interested in here is using pH/ORP in conjunction with CYA. Because CYA sequesters part of the HOCL residual in a form that an ORP probe doesn't react to it would be vital to keep CYA levels constant. if anyone has attempted it I'd like to hear how they made out.

Personally I think such controllers are overkill for a home pool as there just isn't the bather load or the 18+ hours of constant use that public pools have to deal with. however, some folks a tech junkies and may have one fitted.

[PoolDoc]

I've been using pH/ORP controllers indoors for 18 years and never saw those kinds of problems. I can't even imagine how you could do that kind of damage.

Neither could the design engineers, nor the pool builder . . . which is why it happened.

The pool builder DID figure it out, after it happened, and reset the valves that led to the problem. Fortunately, I'd taken photos that showed the valve positions BEFORE he worked out what had happened, and attempted to hide his error.

Of course, it was a series of design errors that made it possible for a single mispositioned valve to result in the destruction of a pool.

[chem geek]

What I'm interested in here is using pH/ORP in conjunction with CYA. Because CYA sequesters part of the HOCL residual in a form that an ORP probe doesn't react to it would be vital to keep CYA levels constant. if anyone has attempted it I'd like to hear how they made out.

The ORP sensor is measuring a voltage that will be related to the hypochlorous acid (HOCl) level. When there is CYA in the water, this active chlorine level will be very low. Roughly speaking near a pH of 7.5 the active chlorine level will be equivalent to FC/CYA compared to a pool with no CYA. So when the FC is roughly 10% of the CYA level, the active chlorine level will be around 0.1 ppm. ORP sensors tend to not do very well measuring such low active chlorine levels, even though such levels are more than enough to prevent algae growth and to kill fecal bacteria with a 99.9% reduction in around 1 minute.

As for the actual ORP level at such low active chlorine levels, this depends on the sensor since ORP is not actually absolute in spite of what the ORP manufacturers claim. Looking at their own tables shows not only differences in absolute levels, but in the slopes of ORP mV increase per doubling of FC (with no CYA). For example, With 3 ppm FC and 30 ppm CYA and a pH of 7.5 and at a temp of 80ºF, the HOCl is 0.042 ppm FC (equivalent to an FC of 0.087 ppm with n CYA) gives an ORP for Chemtrol of 689 mV, for Oakton of 647 mV, for Aquarius of 572 mV and for Sensorex (if you believe their tables) of 406 mV.