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.
Mistakes 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.
- If you don't allow the pump to run for at least 30 minutes before testing there's the chance your chemicals maybe all in one area thus giving you a false level.
- Letting the sample sit around exposed to air before you test it. The chlorine level may change while waiting.
- As the title says, the chlorine and pH levels can change when allowed to sit
- 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.
- See above
- Holding the dropper bottle at an angle affects the drop size. The bottles should be held vertically to maintain a uniform drop size.
- The dropper is designed so that each drop is very close to being the same size. If one drop is larger than the next it'll toss off the end result. It's like using 1.5 drops verses one drop.
- Not observing the wait time with the cyanuric acid turbidity test.
- You need to give the melamine time to mix with the CYA in the water. Rushing this may result in the melamine not mixing correctly and showing all your CYA
- 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.
- See above for holding bottle at an angle
- Not mixing reagents thoroughly between drops. Swirling or using a Speedstir Magnetic Stirrer ensures the test water and reagent is mixed during testing.
- Mixing help the reagents bind
- 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.
