One of the biggest differences between residential and commercial pools is automation. Specifically, most commercial pools have at least some form of chemical automation or controls, which allow the pool to be operated 24/7 and dose chemicals without an operator on site. You can think of the chemical controller as the brain of a pool system. Let's explore these controllers and what they are capable of.
Pool chemical controller features
While some high-end residential pools have automation systems, they are rare, and generally less complex than commercial controllers (which can be a good thing, actually). Pool controllers serve several purposes, and the top-of-the-line controllers do all (or most) of these functions.
pH sensing and pH reduction
A system can electronically measure pH in real-time using a pH probe submerged in flowing water. This pH reading is processed by the controller, which relays a certain action to adjust the pH to keep it in a desired range. In most cases, that means lowering pH in a measured way by signaling a liquid feed pump to add a specific amount of acid, or signaling a carbon dioxide injector to add CO2 gas into solution. Most chemical controllers feature a pH control at a minimum, as it is quite important to maintain pH for chlorine efficacy in non-stabilized pools.
It should be noted that controllers only lower pH and/or alkalinity. As of now, there is not a widely-adopted mechanism for automatically raising pH or alkalinity. An exception to this is when some operators take soda ash and blend it in a container with water at a certain ratio, then figure the math on how much of that high-pH solution it takes to raise the pH a certain amount in their pool. It's tedious and usually a waste of time and money. Because fortunately for everyone, raising pH is as easy as leaving the pool alone for a while and letting it off-gas CO2. Thank you, Henry's Law.
Raising alkalinity is achieved with a manual addition of sodium bicarbonate, independent of the controller. Additionally, operators who use both acid and CO2 injectors have found that injecting CO2 actually does raise the alkalinity, thanks to the carbonic acid-to-bicarbonate alkalinity conversion as pH rises. See the chart below. As the pH naturally rises, it will convert carbonic acid (which was created when the water injected with CO2) into bicarbonate alkalinity.
ORP sensing and chlorination
Also included with most controllers is an ORP probe that measures the oxidation reduction potential. ORP and chlorine levels are two different things. We want to know the ORP because it helps us gauge the speed and efficiency of chlorine. In other words, how well is your chlorine performing? How fast can oxidation occur? ORP is measured in conductivity millivolts (mV), where 650 mV is the industry-standard minimum. Over 700 mV is considered pretty good, over 750 mV is very good, and over 800 mV is exceptional. The higher your ORP, the better you chlorine is performing.
The chemical controller usually feeds the pool more chlorine as the ORP levels drop, based on parameters that the operator programs into the system. Say, if the ORP drops below 700, it's time to feed more chlorine. If the ORP drops below 650, an alarm goes off and notifies operators that there is a very high oxidant demand in the pool, and the chlorine is overwhelmed. But chlorinators are not the only thing a controller can use to correct ORP. If you have a secondary oxidizer system like Ozone, a controller can turn such a system on or off, or ramp it up and down. Same thing with a liquid enzyme feeder to add more enzymes to help handle the oxidant demand (bather waste).
Free and Total Chlorine sensing
More advanced controllers have probes that can measure both free and total chlorine in real time, which allows the controller to calculate combined chlorine levels too. Especially from an indoor air quality perspective, knowing these chlorine levels is extremely valuable. Combined with ORP, these controllers are able to give a pool operator a real-time picture of water quality in their pool. And just like ORP, chlorine sensing allows the controller to decide when and how much to chlorinate. Or in some cases with secondary systems, ramp the system up or down based on current needs of the pool. Since lowering combined chlorine takes a few things working together, chemical controllers can be programmed to do a certain sequence of things to combat the problem.
Related: Pool Water Quality Resources
Flow Rate, Water Temperature and TDS
Some controllers can measure how fast water is circulating, which is valuable for knowing if/when pool filters are under too much pressure. This means its time for a backwash (sand filters) or a bump cycle (regenerative DE filters).
Controllers can also process real-time water temperature and Total Dissolved Solids (TDS) measurements. TDS is measured in electrical conductivity, similar to how ORP is measured. As far as indoor pools are concerned, these factors play a minimal role in water quality–because temperature is maintained so consistent–but play a major role in water balance. Water balance is measured using the Langelier Saturation Index (LSI) or Ryznar Index as a way to determine the corrosive or scale-forming tendencies of water. Aggressive water will etch plaster and dissolve calcium from it, and scale forming water deposits carbonate scale in the hottest places first (like the heat exchanger).
Display independently-measured chemistry
Controllers, as of today, cannot measure alkalinity or calcium hardness without some form of titration apparatus. Such devices do exist, but are costly and rare. So the vast majority of pool operators must test for alkalinity using a test kit. Some controllers allow an operator to input the alkalinity reading, which helps the controller calculate the LSI. At the time of writing this, we do not know of any controller manufacturer that has adjusted their acid protocol per the alkalinity, but we believe it is going to be possible in the near future. This is important, because the amount of acid needed for a pH reduction depends on the alkalinity in the water...and if the controller relies on independently-measured alkalinity, the controller only knows how to follow its own programming.
As such, controllers tend to over-dose acid in pools. Which brings us to our next segment of this article.
Weaknesses of Chemical Controllers
Controllers are at the mercy of the information provided to them. They are computers that process incoming information, then they output signals for certain actions. The common example is the pH probe says the pH is too high, so the controller triggers the acid feeder to turn on.
As we just mentioned, controllers are limited by what they do not know. Alkalinity is probably the most important of these factors, but calcium hardness also matters. These factors must either be inputted to the controller, or the controller must be programmed in a way that they do not over-correct your pool chemistry.
If you have an outdoor pool, another factor we have not seen controllers measure is Cyanuric Acid (CYA). This is the 6th factor of the LSI, and can radically distort ORP readings. So beware of CYA levels if you operate an outdoor pool, and strive to keep the CYA to a minimum, ideally below 50 ppm.
Chemical controllers are valuable tools for operating pools around the clock. They are somewhat rare in residential pools, but growing in popularity. Commercial pools, however, usually have at least some form of minimal controller with pH correction and hopefully ORP. The more advanced controllers can measure more factors, like chlorine (free and total, which allow it to calculate combined chlorine), TDS and water temperature. But do not assume a chemical controller can do everything, because factors that require titration–namely alkalinity and calcium hardness–are not measured automatically. Therefore, any process that is impacted by those factors depends on regularly testing and updating the controller with it. By far and away, the most important of these from a controller perspective is alkalinity, because it determines the amount of acid it takes to make accurate pH corrections.