Why ORP/pH controllers aren't suited to spas

Shenton Aquatic
By David Watson
Monday, 30 April, 2018


Why ORP/pH controllers aren't suited to spas

Commercial spas in Western Australia require the use of a chemical controller for automated dosing of chlorine and pH correction. For many years, I have been recommending against the ORP/pH control philosophy in spa installations — a viewpoint which has not been without its detractors.

About a year ago, I received a phone call from a pool technician who challenged my view in not supporting his preferred ORP/pH controller. He asserted that these devices worked in commercial spa applications without any issue. He was wrong.

While there is a lot more at play than I can cover in this short article, there are some key factors that highlight why controllers of this type are unsuitable for spa applications. The best way to illustrate the problem is by conducting the following testing procedure.

  1. In a spa installation, turn off any present chemical controller, salt chlorinator or other chemical control system.
  2. With the spa in normal filtration mode — this means no blowers operating and no jet booster pump working (for at least five minutes) — test the pH level. Assuming the spa is balanced, the pH will likely be in the region of 7.4.
  3. Turn the blower on and start the jet pump, then wait a few minutes and test the pH again. It is likely to have increased. Depending on how long it has been running (the pH will increase with the run time), it is likely to reach up to 8.2.
  4. Turn off the blower and jet pumps, wait for the water to settle and return to normal filtration mode, then test again. You’ll find the pH level has reverted to initial test levels.

You can continue this process time and again, and the results will remain consistent. This reaction occurs in every spa — or any body of water undergoing aeration. Other examples include water slides, sprays and fountains.

The reaction being observed is the outcome of two chemical functions:

  1. The first is Le Chatelier’s principle, which states that a change (in this case, aeration) will drive elements in a chemical equation to one side of the equation in order to bring equilibrium. In the above testing scenario, by altering the amount of aeration, it drove the equation toward ‘gassing off’ and increased the hydroxide content in the water, thereby increasing the pH level. That increased pH is a real result and, while harmless to humans, ORP/pH controllers do not respond well to the change. The longer the aeration time, the more likely it is to disrupt the pH reading on an ORP controller, which leads to inaccurate chemical dosing. This is particularly problematic in commercial spas, where users are in and out often, generally requiring longer aeration times.
  2. The second chemical function at play is Henry’s law — a gas law which effectively states that the amount of gas is at equilibrium in a solution at a given pressure. Activating aeration devices alters the pressure, which results in the release of gasses. By removing the aeration, the pressure returns to previous levels and (assuming no other chemicals have been added) the spa returns to its previous state.

The entire process wreaks havoc on ORP/pH controllers, not because the equipment isn’t fit for the task, but because the underlying ORP/pH reaction requires a pH level that is more or less stable. Introducing a moving pH level adds a layer of complexity to the chemical equations, which an ORP/pH controller simply cannot handle.

To my mind, the pool technician arguing the suitability of these types of controllers for spas is the equivalent of arguing that gravity doesn’t exist. It is that extreme. So, how to explain his vastly different view in the face of inescapable chemical evidence?

The most likely reason is that it hasn’t been noticed or is assumed to be a calibration error. If a spa has a very light loading or no aeration devices fitted, then it is likely the reactions either haven’t been observed or are complete and corrected by the time the next service visit occurs.

Appreciating these chemical principles will help technicians understand why high or low readings still occur after adjusting set points in spas with an ORP/pH controller installed. Armed with that knowledge, you can develop a suitable workaround.

Image credit: © iStockphoto.com/Plainview

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