ORP controller readings too low!

[johnk50023]

I have a new ORP/Ph controller and probes. The ORP is reading very low (430-490). My chlorine levels are over 5 ppm. Thiis is the same problem I had with the last controller. pH is 7.5, CYA = 40 hardness = 260, Alkalinity = 120. Temperature is 76F. Iowa code requires ORP readings above 660. What could be impacting my readings?

[PoolDoc]

I assume you are a representative for the homeowner's association? I've put you in the "Commercial Pool" membership category.

You should know that the relationship between free chlorine and ORP is dicey; it's much more closely associated with the ppm HOCl (excluding -OCl and all chlorine/cyanurate compounds). Here's what I'd recommend.

1. Get a K2006 and retest your pool. Strip tests -- including those with 'computers' -- are not very accurate, and are horrendously bad at measuring CYA. CYA levels are critical to your problem. A CYA=40 + FC=5.0 would not normally lead to and ORP<500, but if your *real* CYA=90, it could.

2. Read the 'Best Guess' page, linked in my signature. Very roughly, the various recommended FC (free chlorine) levels correspond to a constant FC level across differing CYA levels. Understanding this page, will give you some understanding of the issue.

3. Once you have the K2006, add bleach, 1/2 tsp at a time, to a gallon of distilled water to get FC=~3 ppm. Use baking soda (1/4 tsp doses) and dry acid (sodium bisulfate) to adjust the water to a TA of 40 - 80 and a pH of 7.2 - 7.6.

4. Once you have the mix above, retest chlorine and readjust to FC = 3 - 5 ppm. THEN, put your meter probe in this water. If you don't see an ORP > 600mV, you very likely have a defective meter or probe.

5. Assuming your meter can be validated, you'll then need to look at your pool chemistry to see what the problem is. My guess is that your CYA is higher than you think.