Controller Concepts: Precision in ORP Control (Or the apparent lack of it…)

(Note: Oxidation/Reduction Potential is referred to below as “ORP”, although the terms Redox, European, and High Resolution Redox (HRR), used in some high-end controllers, apply equally well in this discussion.)

Are you wondering why your fancy new pool-water automation machine seems a bit sloppy in its control? Why standardizing (calibrating) in the middle of the day sometimes doesn’t track ‘till tomorrow? There are reasons, of course — even reasonable reasons. Look at some numbers, and reason…

If you’re holding just .5 ppm in your pool (code in most states two decades ago), you can expect your controller to manage it rather precisely, within plus-or-minus .1 parts per million. Shooting for 1.5 ppm? Expect the better machines to hold plus .3, minus .2 ppm. But if you’re trying to hold more, say 4 full ppm, don’t count on tight control ? maybe +1 ppm or -.5. With the higher residual targets, you just won’t get the hair-splitting control you may have been led to expect. There is a logical reason, of course: The variations at higher residuals simply don’t mean as much! Read on.

In the world of ORP and millivolts, the standard window is from 650 mV to 850 mV, about half the potential of a flashlight battery. 650 mV, the minimum value in the fully acceptable sanitation and oxidation range, up to 850 mV, a really kick-butt “work value” for an oxidizer! (For a review, see ORP articles, PrP Issues #5 and #6.) This range of 200 numbers comprises a relative, qualitative scale of performance, for whatever oxidizer/sanitizer you’ve chosen. You can be sure if your pool’s ORP is on the left of this scale (low-end numbers) you’re just getting by. If the values are on the right of the scale your water is experiencing generous, even excessive performance from your chlorine. (Yes, it’s true for any other sanitizer, too; but only chlorine and ozone can reach the values in the mid to high 800s with ease.)

Relative work potential is in fact rather linear across the range discussed here. A ten-percent rise from a random starting point anywhere from the bottom to the top of this voltage range is predictably ten percent more “oxidative power”. But residuals (dosed quantities, or ppm) are by no means linearly distributed over that same range of values. Taking a sample cut across the old ORP vs. pH vs. PPM graph (PrP #5, developed by Uniloc and Stranco in the mid 1970s, shows us dramatically what it takes to get that example “ten percent change” at various points across the window.

Fixing the pH at 7.4 look at these twelve, 10% steps from low work value to high, comparing the residuals required to create these values:

The last two levels, representing very high ORP values, are actually “off scale”, extrapolated to show the extreme curve rise at the high end — values never needed in a public pool.)

Notice the tiny increases in the low ranges, where a ten-percent elevation in “work value” takes just small fractions of one-tenth of a part per million. In the middle ranges, a ten-percent increase in ORP is achieved by a couple of tenths chlorine or more. In the generous working ranges for busy American pools — centered around 820 mV, about 2 ppm at this pH — it takes a full ppm shift to make the ten percent ORP change. And look at values above that: From 830 mV to 850 mV it takes 2.2 ppm just to yield ten percent more work value; from 850 to 870 it takes 15.5 ppm; and for the next ten-percent jump it takes maybe an 80 ppm residual rise!! Is it any wonder that standardization is so extremely touchy — so difficult to maintain with accuracy — when you’re trying to hold 3, 4 or 5 ppm or more! Up there, you set what you hope for and take what you get.

Remember, the controller doesn’t even know you’re using chlorine. It just knows there’s a potential to oxidize out there; if the instrument can tell some pump somewhere to feed and its detected potential rises to a set point and stops, we have control. Digital controllers and especially those with internal ppm calculation and display have brought upon themselves a ready invitation for criticism. The old loosy goosy galvanometer was given the benefit of the doubt, but not those inch-high digits. 4.2 ppm and 3.5 ppm look seriously different. But they’re not.

How ‘bout selecting the “PPM control” option? Don’t even try it. You must control ORP, the real quality measure of whatever’s out there working in your water, if you want repeatable control! Even with ORP control, you must be forgiving of the expanding tolerances and your often-not-even-close ppm readings. All this appearance of sloppiness has been created by those recent high-value residual demands, not your machine or ORP technology.

The point here is not that machines are inherently inaccurate at high values, it is that it’s not important to be accurate (ppm-wise) at high values. Accuracy, and most certainly precision (PrP #5), simply aren’t available to you; nor is it necessary if — for some reason — one is compelled to hold high single-digit values of chlorine residual. Face it, the difference in kill value or oxidation value between 4 ppm and 5 ppm is insignificant, between 7 and 10 much less so, and the difference between 10 ppm and 15 ppm is darn’ near immeasurable! You can measure such values with a simple titration test kit far more accurately than you can figure them from ORP.

You don’t need to be up at those residuals, it isn’t doing you any more good, and you can’t control it anyway!

Powerful statement, that; but for those who’ve tried it, verifiably true — especially if your definition of control embodies some sort of ppm precision. Your controller is probably just fine. In the old days, when we all held .4 or .5 ppm nationwide, you could expect a nice, tight tolerance of plus / minus .1 ppm or better. At today’s 1.5 ppm desired residuals (somewhere between 770 and 830 mV at various pHs and conditions), expect minus .3 ppm / plus .5 ppm tolerance in your control. But if you’re asking for 3 ppm or more, you’ll get control something like minus .5 ppm / plus 1.0! Look at this extreme: When ORP is used for process control in the vegetable-packing industry — requiring about 50 ppm for quick sterilization — they are quite happy with a chlorine control range of 40 to 70 ppm! Their well-managed ORP hardly wiggles.

There’s so much more to discuss about ORP controllers, especially in the empirical world of qualitative values verses quantitative residuals, the many influences on ORP relating to organic load, time-delay influences when comparing test kits to readouts, the devastating effect of cyanuric acid on chlorine’s work value (PrP #6), the best times to standardize and the far greater wisdom of letting the calibration just be… But for now, just trust your controller to measure what really counts — that’s ORP or HRR — and trust it to keep your water safe. Your job? Assure more-than-adequate feed systems, keep the chemical tanks full, do confidence checks often, avoid zero chlorine at all costs, and keep very, very good records.When understood your controller will prove to be a wonderful tool, making your life much easier and your water much better — all of the time.

~kw