ORP stuff

[bob8619]

Do I have this right? ORP is basically the effectiveness of your sanitizer? If Ph, CYA, FC are all at desirable levels in a non SWG pool, the ORP should reflect 650+ somewhere? Or are there far more variables to be considered?

 

[JoyfulNoise]

ORP mesures.....something. What that "something" is is all of the ions and chemicals in the water that affect the measured oxidation-reduction potential of the probe. It's a voltage measurement that is a proxy for ion concentrations based on the Nernst relationship in chemistry. The problem is, that voltage signal is made up of multiple, independent (and sometimes inter-dependent) values some of which are known (FC, pH, TA, etc) and some are unknown (UV light, hydroxyl radical formation, etc, etc). So the absolute value of the ORP signal is, in a general sense, meaningless. A value of 650mV for one pool might correspond to a set of pool parameters that are in balance and considered proper for sanitation while another pool might have 650mV as it's ORP measurement and be completely out of balance and under-sanitized. This is also further complicated by the fact that ORP probes from different manufacturers seem to have different response curve slopes (mV/decade) such that if you use different probes from different manufacturers, you'll get different readings from the same water sample.

So what do you need to do? You need to get a good test kit that can measure pool chemical levels accurately (we recommend two - the Taylor K-2006 from Taylor or the TF-100 from TFTestKits.net; both use the same reagent chemistries) and then you need to correlate your water parameters to what you are seeing on your ORP probe. You should also make sure the ORP probe is regularly cleaned and calibrated as stabilizer (cyanuric acid or CYA) can foul the tips of ORP probes. I would do daily measurements in the beginning to try to figure out a correlation between FC and ORP output but with the realization that you can only correlate values when all other parameters are held constant....unless you want to do a multi-factor analysis of ORP versus multiple pool water parameters.

This is why TFP does not recommend the use of ORP control systems as they are easily "fooled" and one has to operate a pool with very low CYA levels. In areas that get a lot of heat and sun, CYA needs to be higher and that is incompatible with ORP because it means that the primary oxidizer you are trying to measure, hypochlorous acid, is at very low concentrations making the signal-to-noise ratio very high.

Let us know how the ORP probe works out for you.

 

[JoyfulNoise]

Here is a very good primer on ORP - ORP Application Bulletin

I will take out one excerpt (with emphasis)-

Although a better indicator of bactericidal activity, ORP cannot be used as a direct indicator of the residual of an oxidizer due to the effect of pH and temperature on the reading. ORP can be correlated to a system by checking the oxidizer or reducer in a steady state system with a wet test, and measuring pH. If the system stays within the confines of this steady state parameter (usually maintained by inline or continuous control), a good correlation can be made. The best recommendation for ORP is to use wet tests, and over three test periods correlate the ORP values to those test parameters.