CALIBRATION: Don't Mess with Pretty Good!

Automation, the be-all and end-all of pool-water care, is sometimes of little value and even counterproductive when well-meaning pool operators fiddle with the standardization (calibration) on a routine basis. Even more often, PPOA field analysts observe, different folks use different test kits for this sometimes daily “accuracy” check. We, along with some of the biggest equipment distributors in the country, have discovered that the use of color-comparator test kits (incorporating manual, visual comparisons to color standards) often results in differing pH readings for the same water at the same time. This is true not only because different folks have different “color vision” or use varying light sources but, shockingly, because phenol red can give us erroneous results when the chlorine residual in the sample is more than about one part per million! (and that’s even true when the reagent contains a bit of thio…) So fiddling with the standardization too often, trying to make the machine precisely match the test results, virtually guarantees inaccurate readings – therefore erroneous control – a fair amount of the time.

While this pH-testing phenomenon and its significant ramifications will be thoroughly reported on in a future PrP, leave it for now that the calibration of a reliable, functioning controller should be LEFT ALONE once your distributor’s tech or you have got it “about right”. And getting it there should be done by one person – hopefully that manufacturer’s field tech or your chief pool operator – with the following conditions: Use the most reliable kit you have, digital if possible; Ensure your water’s chlorine residual is not much more than one ppm while no less than about half a part*; Be comfortable with the pH value and setting before any chlorine-residual/ORP comparisons are made; Do this calibration test in the morning with any pool blankets off, but before the mass of humanity jumps in. Once this rare but important calibration is accomplished, QUIT. Let your controller do its job, unmolested.

As we’ve seen good controllers go a year or more with no standardization, no probe cleaning, no calibration, no fiddling for both ORP and pH… you really don’t need to tweek it every day or two!

*ORP is standardized best at moderately low chlorine values, as the “chart”, or nomograph (AFO Chapter 10 or ORP and Oxidation – Part 1, PrP Issue #5), shows. The readings between ppm and ORP at a given pH are much more accurate there than at high chlorine residuals. See Precision, Accuracy and Absurdity, PrP #5; and Precision in ORP control, PrP #16.