Water Testing Errors

There are many common mistakes that can introduce errors into water testing. Taylor Technologies discusses then in Avoiding Faulty Readings: Good Habits Of Highly Effective Water Analysts and Five Sources Of Error In Water Testing…and what to do about them.

Mistake may include:

• Taking the water sample from an area that is not representative of the whole pool or spa. The sample should not be taken at the surface or near a return line. For pools sample from elbow depth in the middle between the shallow and deep ends.
• Letting the sample sit around exposed to air before you test it. The chlorine level may change while waiting.
• Using the wrong sample volume. Even a little difference can affect the result.
• Not having the bottom of the meniscus sitting on the fill line of your sample tube.
• Holding the dropper bottle at an angle affects the drop size. The bottles should be held vertically to maintain a uniform drop size.
• Not observing the wait time with the cyanuric acid turbidity test.
• Buildup of static electricity at the bottles tip can cause a decrease in drop size. Wiping the dropper tip with a clean damp cloth or towel will eliminate the static charge.
• Not mixing reagents thoroughly between drops. Swirling or using a Speedstir Magnetic Stirrer ensures the test water and reagent is mixed during testing.
• Failing to calibrate any instruments such as a pH meter before testing.
• Using a dirty test vial and not properly cleaning test vials between uses.
• Doing color matching in artificial light or wearing sunglasses
• Testing water too soon after chemicals were added
• Water Test Interferences from high chlorine or metals
• Extrapolating test results outside the testers range - pH less then 7 or greater then 8.2; CYA greater then 100
• Interpolating values between the lines of the CYA test
• Using reagents beyond their useful life.

Note how many of these errors you observe when you watch a pool store do titration testing.