Today - we are going to talk about ORP/pH Sensor Longevity.  Working with several different Controller Manufacturers over the years, I have a pretty unique perspective of these Sensor Technologies.

In General, Sensors are like batteries; they contain a chemical (an electrolyte) that reacts with the oxidizer/or pH) in the water.  over time, the electrolyte reacting with the water will deplete, making its usefulness finite.

A common question asked of me is "how long will the sensors last?"  The answer isn't simple.  there are several factors on how long your sensors will last:

  • Level of Residual/ORP you are maintaining: the higher the level, the more rapid the reaction occurring, lessening the lifespan of sensors.
  • Cleanliness of the sensor - Sensors do need to be cleaned to allow the water sample to react with the Electrolyte.  NO TOOTHBRUSHES, Please; a simple swirl in a mild acid solution (4:1) will clear up the problem.  Remember to allow the sensors to re-acclimate with the water being tested.
  • Presence of Air/gases in the sensor: Shaking down a sensor, like a thermometer, usually corrects this issue.  every time you clean the sensor, it should be shaken down to ensure that the electrolyte is making contact with the tip of the sensor.
  • Readings drift from main calibration: this is your first indication that the Electrolyte is depleting.  ORP sensors should not deviate more than +/- 30mv, while pH should not deviate more than +/- 1.0 pH points.  Sensors can only be checked for deviation by a Service Professional (they have the equipment needed to check this.  Some controllers let you "zero" out a sensor's span to check this deviation yourself.  The farther the deviation, the slower the controller will respond, leading to overshoots of chemicals.
  • Sensor readings fluctuate: Ironically - its usually the pH Sensor's fault.  Replace the pH sensor.

I hope this helps operators understand their sensors a little better.  In general, Heavily used pools may see replacements Annually (yes, Annually).  Lower use pools with low Chlorine residuals can squeeze out at least 2 years.  Sensors claiming to be inorganic, or resistant to oxidizers, they go bad just the same - again, Regular testing of your sensors by a Professional Controller Technician will end the guesswork.

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I find the biggest problem is not sensors going "dead" but rather getting too slow. Signal magnitude can be almost perfect but if the response to chemical changes is too slow you end up over and undershooting (pump too fast or two slow results in over or under shoot, not both). I've had sensors needing replacement in as short as 1 year and as long as five years.

I find the useful life of sensors is tied more to the volume of water than the chemical levels. Large bodies of water take more time for pH and ORP to change so they can tolerate a slower sensor far better than a tiny pool that may only need 2 minutes of oxidizer feed to rise 1 PPM. I find the smaller pools(hot tubs, toddlers) need new sensors in 1-2 years while 25 meter+ pools can go 3-4 years.

This relies on regular cleaning - first some dish soap and water (just like doing dishes) soak for five minutes, wipe clean, rinse well, then soak on pH 4 buffer for 30 minutes or more (or weak muriatic acid solution - 10:1 or so) depending on how clogged the sensor glass is. Some sensors use a metal plate instead of porous glass membrane so the long acid soak isn't usually required - just a few minutes to burn off any minerals.

The easy way to check response speed is pick up some pH4 and pH10 buffer solutions at your supplier (or salt water aquarium dealer) and see how fast the readings change. Anything more than two minutes from steady low to steady high (or vice versa) is probably insufficient for a small pool while a large pool might tolerate 5-10 minutes.

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