Sense and Dispense ORP Issue

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mike5906

mike5906

0
May 29, 2011
9
Keller, TX
I recently (3 weeks ago) added the Sense and Dispense system to my Pro Logic PS-8.

I have tested the water both myself (two different test kits) and taken my water to a local pool company. All of the tests show the water is near perfectly balanced.

However, my ORP sensor is reading 110mV at its lowest, and 190mV at the highest over the last three weeks. Hayward says it should be at 650mV to be correctly balanced. I am starting to suspect the ORP sensor is bad.

They were never dry for more than a 20 seconds during the install. The PH sensor is accurate, its only the ORP that is reading low.

Any ideas?
 
That is way way off, most likely a bad sensor. It could also be a bad connection between the sensor and the control unit, so triple check the cable connections before going to the trouble of getting a new sensor.
 
I find Hayward Tech Support's reply to be interesting at best. They seem to think it might be fine:

Hayward Support Response:
"The "Recommended mV Value" is the one that will allow you to achieve
a 1.0 to 3.0 reading for free chlorine. There are many factors in
the plumbing that will affect how many mV we read for the ORP value.
If you can achieve balanced free chlorine values without tripping
alarms this low reading is acceptable; if the system continues to
report errors than the probes may be in need of replacement. "

This makes me wonder... as everything I have read on the Internet says that is not true. Thoughts?
 
A great deal of what you read on the Internet is not true, and that is especially true of ORP readings. There is no such thing as an absolute ORP reading. There isn't any standard that they can be calibrated to and lots of things that have nothing to do with sanitation can change the reading. So anyone who says ORP should be X is either making things up or talking about laboratory conditions and purified water. Because of that, Hayward's point is essentially valid. If ORP varies with the FC level in the correct way, it doesn't matter what the absolute number is, and that is certainly worth looking into.

On the other hand real world variation in readings is typically +-200 or +-300 at worst. I've never seen a reading anywhere near that low in a pool that had a measurable FC level. I'm assuming you have had plausible FC readings when using a conventional test kit when you were taking those ORP readings?

By the by, I don't recommend ORP controllers for residential outdoor use, they have all kinds of potential problems that are very tricky to figure out. They do work great sometimes, but when they don't work, which is common, it is often inexplicable.
 
I am getting used to the fact the ORP is a measure of the ability to oxygenate, which is influenced by many factors other than simply chlorine level.
I keep mine set at 300, and it varies from 150 to 500 over the course of the day. The PPM levels range from 1.5 to 3, I've seen them go higher but it was because of the effective delay between dosing the pool and reading a change in ORP.
 
I just got off the phone with a different hayward rep. We went over my water test results, and the installation specs. This rep believes my problem IS a bad sensor. I now have to call a local authorized repair center to get a tech sent out with a replacement sensor.
 
Let us know how it turns out.

With my ORP meter most of the time if the value is greater than 500 my FC / Cya is OK. If my FC should drop, then ORP readings drop to around 350 - 450. The change is not linear, more like a sinusoidal curve. The reference is different for each pool, climate, and ORP meter.
 

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