H2o2, AA: affect ORP?

[DaveMcHbg]

ORP reading vs test is WAY off. Can residual Hydrogen Peroxide, Ascorbic Acid, or Sequesterant cause this?
Orp is reading in normal range- (fas Dpt)Test is 3x higher

I shocked/ superchlorinated a spa (1000g) to break combined cl. Following that:
Used H2o2 to drop cl levels.
Used AA to remove stains.
Added Sequestrant.
Added Sod Hypo to get a residual. (<1)

Turned the auto feeder(s) back on.
The spa is fed by automatic orp/ph readers. In 12 hrs they fed the Cal Hypo to >10ppm, and the pH is testing high (off the scale) which may be due to high cl?
The ORP on the feeders (and a secondary ORP reader) read normal and are feeding cal hypo accordingly (disabled now)

So, What on earth is causing the ORP reading to be normal, when in fact the cl is 3-4x high?
and, at what cl level does pH testing become invalid.

 

[JasonLion]

Taylor PH tests are reliable up to an FC level of 10, and usable up to an FC level of about 15. Other brands tend to have lower FC limits.

ORP readings are sensitive to a number of factors. By far the largest is PH changes. If you are doing an AA treatment I would expect the PH to be low, which would drive up ORP. H2O2 will also affect ORP readings, but only if the FC level goes to zero. H2O2 and chlorine mutually destroy each other, so if you have an FC reading I would expect any H2O2 to be gone already.

It is also quite easy for ORP sensors to become fouled and require cleaning. They also fail occasionally, though that is less common. Another thing that can happen is pure chemicals getting into the test cell accidentally, which can bias the readings for days.

 

[DaveMcHbg]

Thanks Jason.
I suspect the high pH occurred due to high cl. It was actually 7.0 (w/ cl at 1) before restarting the ORP feeder, which I intentionally left low (pH) following AA treatment and knowing it would rise with the addition of cl.
I know these ORP readers are pH and Alk sensitive- one seems more so than the other (they're redundant readers, diff brands) but both are reading in the normal range while CL is very high.
would you suspect AA may have 'infected' both?

Thanks for the reliability info on pH testing--heading for the acid now!
Dave

 

[chem geek]

The ascorbic acid as a reducing agent should have been oxidized by chlorine to form dehydroascorbic acid but I don't think that will necessarily affect the ORP reading. As Jason says, ORP is sensitive to chemicals (including hydrogen gas from saltwater chlorine generators), pH, fouling, sunlight, etc. I think it's most likely the higher pH. Even with CYA in the water, going from 7.5 to 8.0 pH would move the ORP from 689 to 645 for Chemtrol or 647 to 630 for Oakton even though the active chlorine level only drops by 15%. With no CYA in the water, it would move the ORP from 803 to 757 for Chemtrol or 791 to 749 for Oakton though in this case the active chlorine level drops by around 50%. So you can see that pH is a HUGE factor which is why when using ORP one really needs to have a system that maintains the pH and then you use setpoints for your ORP at that specific pH.

 

[DaveMcHbg]

Thanks for the input guys.
(no cya use)
The controller is an Aqua Balance with Pentair label on it. The redundant reader (old controller) is Pulse. The Aqua Balance unit controls both pH and Cl and is calibrated and cleaned monthly.
It seems to be coming under control. Surprisingly the Cl (via testing) dropped almost 8ppm over 8 hrs with little use. I manually added Muratic to get the pH down this a.m. and the controller seems to be holding it.
I believe the next time I use AA and/or H2o2 I'll bring the Cl level up to normal before reengaging the controller. It really whacked the pH and Cl.
Still wondering if the sequestering agent or residual 'dehydroascorbic acid, is messing with it.