Controller Concepts: More on ORP

In ORP and Oxidation, PrP Issue 5, we discussed the critical importance of oxidation in pool water and the fact that the potential to oxidize — expressed as ORP in millivolts — is the only reliable measure of the quality, the “work value”, of the chlorine or bromine in the water. ORP values can be used to predict sanitation efficacy, organic consumption (water clarity), and, as we saw in the crypto update, even such things as the likelihood of inactivation of that un-predictable, extremely tough parasite called cryptosporidium.

ORP is the medium of control for industrial, medical, water, wastewater, and swimming-pool oxidants. That little platinum electrode is found in more places than you’d dream, serving the operators as a reliable measure of what the chemical is doing, not necessarily the chemical itself. In our swimming pools, the controller doesn’t have any idea how much chlorine you have in the water, indeed it doesn’t even know it is chlorine you are using! What that smart machine does know, however, is what it is that whatever you’re using is doing in the water — the quality or the resultant of the active compound doing the work. It is simply called qualitative control.

We’re lucky that there’s only one chemical doing this oxidizing work in a pool. If there were more than one variable whose change would cause a significant ORP-reading shift on the face of your controller, there would be no way to control, by instrumentation anyway, the active chemical product you’re interested in. Indeed, there would be no automation industry at all. But since chlorine, (if that’s the sanitizer in use,) is the only changeable thing in the water that makes the needle move, or the digits change, it is fully controllable. “If you can read it and make it change, you can control it,” is an old adage of the instrumentation profession. And, if what you’re really reading is a measure of the work done or about to be done, that’s better yet.

“About to be done”? Yes. ORP has another little-known quality about which pool operators should be familiar. There exists a lead time, where ORP reacts ahead of the resultant residual. In a quiescent pool (everything running, but no swimmer load) the ORP and the residual quantity should “agree”. That is, the published interrelationship between the pH value, the ORP level and the ppm required to achieve that ORP should be right on. (We use this interrelationship to set up, or “standardize” our machines, right?) However, when the swim team jumps in or, worse, the football team arrives at the pool after the sweaty game, all bets are off. The potential to oxidize drops immediately as oxidant needs are sensed and oxidation begins. But the DPD test kit tells us that the residual, that amount of un-used, ready-to-go chlorine in the water, has yet to change. Consumption has barely begun, but the ORP shows a sharp drop! You can see the value of this phenomenon, where actual “demand conditions” could be analyzed. However, don’t “standardize” your machine during those high-demand times of the day. You’ll be mis-calibrating, not making things more accurate.

What about the effect on ORP of potassium monopersulfate, or ozone, or peroxide? The “oxy” products actually don’t elevate the ORP much, so can be ignored. Ozone is a powerful elevator of ORP, however it is short-lived and usually introduced after the location of our ORP probe — in the water on the way back to the pool — and is primarily used as a supplement, not a control parameter. Peroxide does elevate ORP too, and is sometimes used instead of chlorine. While down in the realm of 500 mV or so, ORP control can nicely control hydrogen peroxide all by itself.

While we brag about ORP and the marvelous qualitative nature of its control, there is one final kicker that can monkey up the whole picture. That’s the all-too-common practice of “lying to the machine”! Again relating to the screw-turning procedure called standardization, let’s all understand something here — the only time that ORP is an honest value is when some STANDARD is used to establish the starting point. If you tweek your standardize pot or calibration screw to your liking, you quite likely are not basing the setting on anything “standard” at all. If your electrode is immersed in normal, unstabilized pool water, and you’re using a DPD kit for “free” chlorine values, you’re probably going to be close. Better yet might be an ORP standard signal from a calibrator, or calibrated “buffer solution” into which you immerse your electrode, but neither of these is commonly available. We have to count on our own judgment regarding the accuracy of our starting point.

If cyanuric acid is in the water, you can guarantee the need to lie to the instrument, as CYA creates radical drops in ORP for a given quantity of chlorine. Automation on a stabilized pool is relegated to what we call “relative control”; that is, the only thing we can count on is if we turn the knob clockwise, we’ll get more, if we turn it counter-clockwise, we’ll get less. It the instrument calculates ppm, it’s guaranteed to be wrong in the presence of CYA. In the next PrP, you’ll see this subject wrung out, showing that any water containing 70 ppm CYA or more is un-automatable ? that is, increases in residual will not produce a rise in ORP.

The last concern in the ORP story is related to bromine and bromine-compounds used to produce HOBr in your pool. We know that it takes about 2.2 times the bromine residual to match a given amount of chlorine in work value — even in test kit readings ? so we should expect the active bromine in the water may result in somewhat less an ORP than similar quantities of chlorine. As pH is very much less influential on HOBr (and less yet on OBr- ion), the chlorine nomograph – that familiar family of curves developed by STRANCO and discussed in PrP Number 6 – doesn’t apply. This industry hasn’t developed a new nomograph for bromine, so we’re whistlin’ in the wind for accurate relationships between real ORP and the bromine residuals present. When they are developed, PPOA will be among the first to know.

For those states of the union (their respective health departments, that is) now legislating minimum ORP values – a noble endeavor indeed – they are going to have one tough time insuring compliance. The World-standard 650 mV is pretty easy to achieve, even with stabilized chlorine or bromine, but 750 mV, now showing up in some western health codes, is doggone difficult to manage with bromine and near impossible with cyanuric stabilization. Environmental health specialists will have to carry an ORP standard (calibrator) to determine if the water, manually treated or the product of a controller, is anywhere near the value assumed or set by the operator.

There are conditions which can exist that encourage the formation of ineffective, sometimes offensive halide compounds as well as other even less desirable “products of incomplete oxidation” such as trihalomethanes – carcinogens beyond the scope of this article. These are generally lumped in that residual thing called “combined chlorine”.

Meanwhile, for those of us lucky enough to be automated, we can stress to our associates that the pool’s control is qualitative, not quantitative. If that doesn’t snow them, tell ‘em you’d like 750 millivolts worth of chlorine in your water, please…

~kw