PH and ORP Puzzler

[saltcrazy]

I have this weird problem that I've been trying to figure out and can use some help. My apex PH and ORP fluctuates up/down (in a matter of seconds) then goes back to a steady number. The propblem started occuring for the past 4 days and happens for several hours a day. In an attempt to fix this issue, I bought a lab PH probe... thinking that the standard probe was on its way out. I calibrated the new probe and confirmed calibration by checking it with the calibration fluid (pH 7.0 and 10.0). This was an unnecessary expense, because it didn't fix the prob. I then thought it could be stray voltage in the sump where the probes are placed (approx. 6-8 inches apart from each other as recommended in the unofficial guide). The stray voltage measured .100V AC. I installed a grounding probe and the measurement went down to .009V AC. I did all of this while the apex was acting up, but it did not resolve the issue. In my last resort to isolate the cause, I placed the PH probe in a cup of water from the tank, but it still fluctuates, which doesn't make any sense. This apex operated flawlessly for the past 6 months, and just started acting up lately. There has been no changes or additions to my system to cause any possible interference. I included a sample below from my data log to give you an idea of what's happening. I also have the latest version 4.04. I'm at my wits end. Sorry for making this so long, but I can really use some good advice to tackle this issue. Thanks for the help.
DATE PH ORP
22:00 01/28/2011 78.8 8.12 312
22:10 01/28/2011 78.8 7.98 313
22:20 01/28/2011 78.6 7.75 337

21:50 01/29/2011 78.8 7.98 316
22:00 01/29/2011 78.6 8.08 341
22:10 01/29/2011 78.6 8.10 341
22:20 01/29/2011 78.5 7.98 314
22:30 01/29/2011 78.5 8.00 312
22:40 01/29/2011 78.8 7.96 312

 

[apjohnson]

When you put the probes in a cup, was it near the place where the probes are normally mounted? I wonder if you're seeing some interference causing fluctuations in the readings. This could be motors or ballasts in the general vicinity. I know it's a stretch, but worth checking out.

--AJ

 

[saltcrazy]

Good point. I had the cup right next to the sump (running LEDs, so no ballasts). I'll give it a shot moving it further away and see what happens. Thanks for the input.