Confused by test results.....

[Bobby72]

Split from http://www.troublefreepool.com/threads/71573-confused-by-test-results Butterfly


Your ph is so high that your chlorine has lost all it's effectiveness. Get your ph in range and bump that cya up to. 40ppm. What's % is your salt gen running? Is your pool running long enough to allow the generator to produce enough in a day

 

[JohnT]

Re: confused by test results

Your ph is so high that your chlorine has lost all it's effectiveness. Get your ph in range and bump that cya up to. 40ppm. What's % is your salt gen running? Is your pool running long enough to allow the generator to produce enough in a day

That isn't true in a pool with CYA. Read the second post in this thread for the details

 

[Bobby72]

ORP-272

"That isn't true in a pool with CYA. Read the second post in this thread for the details"

Great reading and all that logic is sound. Now let me ask a question from experience. How can we explain a 10-15 mv drop in orp on an outdoor pool with a cya of 35ppm when the ph rises up to an 8.0? we will go from a 2.0FAC to a 5.0fac without any change in orp. What is causing the ORP to drop out if the relationship between ph and effectiveness of chlorine in a cya pool is negligible. All that ORP is telling us is how effective our sanitizer is, not the amount of sanitizer. We run into this all the time when a customers acid crock runs out and they shut down due to high chlorine. Now im really thinking hard...haha

 

[JasonLion]

ORP does not directly read sanitizer effectiveness. That is a common myth, but it is simply not true. ORP varies with a wide variety of factors, some of which correlate with sanitizer effectiveness and some of which do not. For example ORP varies with active chlorine concentration, which does correlate with sanitizer effectiveness. ORP also varies with dissolved hydrogen gas concentration, which has no relationship to sanitizer effectiveness what so ever.

Various studies have shown a much higher correlation between active chlorine concentration and sanitizer effectiveness than either one has with ORP.

ORP does correlate with sanitizer effectiveness well enough that ORP based automation generally works, but not well enough to make deductions from the way you did above.

 

[Patrick_B]

ORP is affected by many factors and cannot be depended on for accurately correlating FC, with consistent reliability or accuracy. I know for a fact and agree that higher pH does negatively impact FC effectiveness, but CYA opens that window a little further. I've worked with the world's top brands of ORP units for a long time and they are notoriously inconsistent in my experience. All of them, even in nearly identical system conditions in the same facilities. Even more so in "dirty" water.

 

[chem geek]

The ORP is directly affected by pH, not just by the hypochlorous acid (HOCl). Also, if you compare the ORP movement with no CYA present vs. with CYA you will see a difference. The following shows values for Chemtrol and Oakton sensors as examples with an FC of 3 ppm.

CYA ... pH ... HOCl ... Chemtrol-ORP . Oakton-ORP
. 0 .... 7.0 ... 2.25 ......... 836 .............. 820
. 0 .... 7.5 ... 1.45 ......... 803 .............. 791
. 0 .... 8.0 ... 0.69 ......... 757 .............. 749
40 .... 7.0 ... 0.042 ....... 724 .............. 658
40 .... 7.5 ... 0.031 ....... 679 .............. 634
40 .... 8.0 ... 0.026 ....... 634 .............. 617

You can see that the Oakton sensor shows a larger ORP movement vs. pH with no CYA because its ORP reading is separately affected by pH vs. HOCl levels. The Chemtrol sensor on the other hand has a greater pH dependency at lower chlorine levels and this may be what you are seeing with the ORP sensor you are using.

If you want to know the true active chlorine (HOCl) level, there are specialized amperometric sensors with selective membranes. ORP is only a proxy for active chlorine at a constant pH level. So it can be useful for process control of chlorine when pH is separately controlled, but even then it can get interference from hydrogen gas bubbles from saltwater chlorine generators.

This post shows how for a given sensor it roughly correlates with the calculated HOCl, but 23% of the pools that had built-in ORP controllers had readings 100 mV or more different showing how much significant variation there can be even when measuring the same pool water.