Sense and Dispense measures pH too high in spa vs pool

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guymandude

0
Jun 17, 2014
17
Austin, TX
My Hayward Sense and Dispense system is accurate in measuring pH in my pool. Manual tests match the Sense and Dispense value. However, when the system is in spa mode, the Sense and Dispense routinely measures pH as too high compare to a manual test. I am talking like 8.5 vs 7.5. This causes the Sense and Dispense to start dumping acid into the spa. This same trend happens day after day.

Here is what a trend looks like over several days:

unnamed.jpg

Why in the world would this be happening? The PB and Hayward cannot explain it.
 
You'll want to rule out a defective pH probe. Have they sent you a replacement?

Chemistry-wise, a spa will tend to have a much faster pH rise than a pool. The reason being is aeration - when the spa jets are running, the spa water will have significant amounts of aeration and thus outgas CO2 a lot faster than pool water will. Because of the lower water volume, I would expect to see the pH go up quite a bit especially if your TA is on the high side. Many hot tub owners (separate form the pool) will spend a lot of time with their initial fill water getting the TA down to 50ppm and, in some cases, adding 50ppm borates to their hot tube water to keep the pH in check. With that said, it is a bit unusual to see the pH jump up to 8.5 but that depends a lot on you alkalinity - what is your current TA?

I'm not familiar with the Hayward SnD system but I think the Pentair IntelliChem system lets you shut-off automated control when entering SPA mode. Is there a way to deactivate the SnD system in SPA mode? There's really no need for that level of automation for what amounts to less than a 1 hour soak in the spa. I would be much more concerned about sitting in a spa being continuously dosed with acid.
 
TA is 90.

I haven't asked for a new probe yet because it reads perfectly in pool mode, as validated by multiple sources.

interesting observation about the spa jets causing increase in pH, but as you say, jumping to 8.5 every day seems a bit extreme, and in any event, a manual test of the pH in the spa shows 7.5. So the issue seems to be SnD reading incorrecly when in spa mode. We'll see what the Hayward rep says. I will check if the whole thing can simply be disabled when in spa mode, and the spa water will be balanced simply by exchanging water with the pool.
 
The inaccurate reading starts immediately when the system is switched to spa mode. So at one moment in pool mode, the SnD is reading accurately, and as soon as it switches to spa mode, the SnD stops reading accurately.

Because the SnD starts reading a pH of 8.5 in spa mode, it starts dumping acid, even though it shouldn't. I dont know if the SnD performs some sort of calculation to say, Hey this is a 24,000 gallon pool that is at pH 8.5 so I need to make sure to dump a ton of acid. Or if it is smart enough to know it is in spa mode, or if it is only going off what it is sensingin real-time at the moment.
 
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