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Water Test Fails: 12 Simple Pool Testing Mistakes

Today's pool testing guide covers the finer details of backyard water analysis, and a few common pitfalls of using an outdoor laboratory. Pool water testing can become so routine that simple mistakes could make your test results inaccurate. There are 12 pitfalls to avoid when testing your pool or spa water.

don't test in the shallow end

1. Water Sample From the Wrong Area of the Pool

Where you gather your water sample from in a swimming pool matters. Avoid areas near return lines, steps, ladders and corners of the pool. These are locations in a pool where the pool chemistry is going to be different from the pool as a whole.

In order to get an accurate sample of your pool water, don't draw the water from the surface, because that is where the highest concentration of pollutants like oils and debris gather. The surface is also where evaporation is taking place, and the interaction with the air can throw your results off. As a rule of thumb, an elbow deep depth between the shallow and deep ends of the pool is the sweet spot.

2. Not Testing the Sample Immediately

You pulled the sample from the perfect area and depth of the pool but didn’t test it right away. Life is full distractions - letting a sample sit too long gives it time to react with the air, sunlight, and even humidity. Commit to the process!

3. Tilting the Reagent Dropper

This is one of the small details that can make a big impact on the accuracy of your test results. Tilting a reagent bottle as shown, instead of holding it straight up and down will make the drop smaller and throw everything off. If your test kit includes tablets, remember to crush them fine. Just like drops, cutting corners on this step will ruin the chemical reaction. Speaking of proper measurements…

4. Too Much or Not Enough Water

Even the slightest inaccuracy of a water sample can skew your test results. It may only be a few dozen drops of water, but too little or too much can tip the scales. Precision is key when it comes to achieving the best results possible. Hold your test vial at eye level and the bottom of the meniscus or curve of the water should be lined-up with the "Fill" level line.

5. Not Swirling the Sample

You followed steps 1-4 perfectly, and added the perfect amount of reagent into the perfect amount of water and in all the excitement, didn’t swirl between drops. Take your time and mix the reagents thoroughly. Holding the comparator on the top between your thumb and forefinger, and rotate your wrist to swirl the sample within the test vial.

For the titration tests (FAS-DPD, pH demand, Alkalinity & Calcium), if you are unsure if the sample color has changed completely, add another drop. If you do not see a change, just subtract that drop from your count. Or when the color changes color briefly, and one more drop changes it completely, you can count the last drop as 5 ppm, instead of the normal 10 ppm.

6. Expired Test Reagents

Poring over every detail during the testing process could all be in vain if your pool water test chemicals were compromised before you even pulled your first water sample. It’s best to begin each season with fresh reagents. Typically, the chemicals used in test kits are considered to have a shelf life of about a year.

7. Improperly Stored Reagents

improperly stored water test reagents

The best test kit money can buy can be rendered useless due to improper storage. The shelf life of pool test chemicals can be greatly impacted by hot and cold temps. Ideal storage temperatures are between 5° to 22°C (40° to 70°F). Bear in mind that a constant fluctuation of temperature can also negatively impact the chemicals. Store pool test kits in a cool, dark place, and avoid prolonged exposure to the sun.

  • Frozen Test Reagents: Water test kit makers suspend shipments of liquid test kit chemicals during freezing weather, however water test chemicals may still be viable after freezing for a short time period. Allow frozen reagents to thaw at room temperature. If the bottle has cracked or if there are crystals around the tip, or particles floating in the bottle after shaking, you should replace the reagent.
  • Clear Reagents Turning Colors: Taylor reagents 1, 2, 3, 5, 6, 7, 9, 10 and 12 should be colorless and clear. Drop a few drops onto a flat part of the testing vial; if the drops look cloudy, have floating 'globs', or are any color other than clear transparent, you should replace the reagent.
  • Colored Reagents Changing Colors: Taylor reagents 4, 8 and 11 are colored indicator solutions. Reagent 4 or pH Indicator is red, while reagent 8 or Total Alkalinity Indicator is green and Calcium reagent 11 is a blue color. If another color, you should replace the reagent.
  • Colored Reagents Staining Bottle: Taylor reagents 4, 8 and 11 should not stain the bottle in which they are contained, which indicates a separation of the test pigment. If the reagent bottle is stained, you should replace the reagent.

8. Cracked or Faded Comparator Vials

Similar to reagents, a cracked test vial or comparator can lead to pollutants spoiling your results. Even a faded or slightly scratched test vial can open the door to particulates that have no business getting mixed up with your perfectly swirled reagents. If the test vial or optical chamber looks old, faded or perhaps slightly stained—it’s time for a new test vial.

9. Using Test Reagents from a Different Test Kit

You can't mix and match test kit reagent chemicals; this isn’t a matter of brand loyalty. Pool test kits vary by manufacturer and even the slightest variation of pool test chemicals strength and dropper orifice size can (everyone in unison) Render-Your-Test-Results-Inaccurate. Where have we heard that before? While we are on the topic of mixing and matching…

10. Mixing Test Reagent Bottle Caps

When you are done with a pool test chemical it is important to put the cap back on immediately. Not only does this help protect it from reacting with environmental variables it can prevent putting the wrong cap on the wrong bottle. Even if both caps are the same color (more reason not to delay), the chemicals under the caps are certainly not and a little bit of residue is enough to unravel the fabric of the universe - or at the very least, cross contaminate your reagents and you know that is a gateway to trouble.

11. A Dirty Test Kit

Don't touch the tip of the dropper bottles with your fingers. Exposure to these chemicals could irritate your skin and also the oils from your fingers can contaminate the drops. In addition to a clean reagent dropper bottle, thoroughly rinse out the test vials or optical chambers after testing. Lingering chemicals from a previous test is a surefire way to ruin your next test.

12. Misreading the Color Chart

As cool as you look in your sunglasses, take them off when reading discerning the hues on the optical chamber. Perhaps this is obvious, the tint of your sunglasses can make 7.5 look like 7.6 on the color scale.

Hand-in-hand with removing your sunglasses is making sure you are not holding the optical chamber up to an artificial light source, or anything other than a white background (the purpose of the white rectangle, found in some test kits). A blue sky or blue water background can lead to a green pool when you misread test results.

pH Testing Problems

1. Very high chlorine levels

20 ppm+ can turn your pH sample test a variety of colors in the red/purple spectrum. You can however, add a few (1 - 3) drops of Taylor reagent #7, sodium thiosufate to the water, to remove the chlorine from the sample before adding the pH indicator or phenol red solution. You may still however obtain an inaccurate pH test, until the chlorine level is below 10 ppm.

2. Very low alkalinity levels

-50 ppm can cause falsely higher pH test results, due to the relative pH level of the pH indicator solution, which is about 7.5. In a pool with low alkalinity levels, the pH of the test sample may change rapidly or "bounce" upwards, because of the addition of just 5 drops of the pH reagent, with a pH level of 7.5. This can raise your pH test result.

3. Out of range testing

Testing at the upper or lower limits of the test range can produce false test results. Between 7.0 and 8.0, you can discern the small changes and accurately test within that range. Below 7.0 and above 8.0 however, the test colors do not change reliably, and what may look like 9.0 is actually 8.4, or what appears as 6.6 is actually 5.8, for example. Make small additions or adjustments with pool pH chemicals, when pH is outside of the 7.0 - 8.0 range, to avoid overshooting the mark.

Total Alkalinity Testing Problems

1. Very high chlorine levels

Can produce a yellow or clear endpoint, instead of turning from green to red, like normal. As above, you can add 1-3 drops of Taylor reagent #7 (included in the kits I mentioned above), to remove the chlorine from the sample before testing for alkalinity levels.

2. Cyanuric Acid contribution

When performing a test for Total Alkalinity, the result includes carbonates, bicarbonates and also cyanuric acid. If you have any amount of cyanuric acid (chlorine stabilizer) in the water, it will add to your total alkalinity readings and produce false high test results. To compensate for the contribution, you should deduct about 1/3 of your cyanuric acid level, from your total alkalinity test. For example, if your cyanuric test shows a level of 60 ppm of cya, deduct 20 ppm from your total alkalinity test reading.

3. Very high algaecide levels

Quaternary ammonium algaecides (which is most pool algaecides), in very high concentrations in the pool can affect the alkalinity test, producing false low readings. Biguanides like Baquacil or Aqua Silk can also produce similar low alkalinity results, when levels are abnormally high.

False Chlorine Test Readings

1. High chlorine levels

Once again, high chlorine levels may be so high that the test sample "bleaches out" or turns a cloudy-clear color, after adding the reagents and swirling. In my low-range (0.25 - 2.5 ppm) K-2105 DPD test kit mentioned above, I can bleach out the test sample at less than 10 ppm, but the high-range K-2005 kit, which tests from 0.5-5.0 ppm, will go twice as high before the sample bleaches out. The problem is that high chlorine levels which don't bleach out completely, will produce false low chlorine results. To measure very high chlorine, you can dilute the sample by filling the test vial half full of pool water, and half full of bottled water, and then double the test result.

2. Combined chlorine levels

Will begin to raise the Free Chlorine test result within 30 seconds. DPD test kits use 3 bottles to test for Free, Total and the difference between the two is combine chlorine, aka chloramines. After adding DPD #1 and DPD #2, take your test reading within 30 seconds, before the inhibitors stop working on the chloramines. Add #3 reagent also within 30 seconds, and if the sample turns noticeably darker, you have a measureable amount of combined chlorine in the pool. Rinse your DPD test vial completely after testing for free and total chlorine, any traces of DPD #3 reagent left can falsely raise chlorine test results.

3. Non-Chlorine Shock

In high levels can affect a chlorine test, producing false high readings. If you have recently shocked the pool with MPS (monopersulfate), usually labeled "Oxy" - something, it can end up in the chlorine test too, and contribute to the chlorine test result, giving falsely higher total chlorine readings.

If there is one key aspect of pool maintenance where spending more will ultimately save you money in the long run, it’s using a high quality test kit from a reputable manufacturer. Cut corners in this area and you could end up spending more on chemicals attempting to restore water balance and fight water problems.

I highly recommend the Taylor K-2005 test kit (and K-2006), and the ColorQ Pro7 by LaMotte, and greatly prefer the accuracy of liquid test kits over the convenience of test strips because, in pool chemistry, accuracy is king!

In The Swim makes every effort to provide accurate recommendations based upon current ANSI/APSP/ICC-5 2011 (R2022) standards, but codes and regulations change, and In The Swim assumes no liability for any omissions or errors in this article or the outcome of any project. You must always exercise reasonable caution, carefully read the label on all products, follow all product directions, follow any current codes and regulations that may apply, and consult with a licensed professional if in doubt about any procedures. In The Swim assumes no legal responsibility for your reliance or interpretation of the data contained herein, and makes no representations or warranties of any kind concerning the quality, safety, or suitability of the information, whether express or implied, including, without limitation, any implied warranties of merchantability or fitness for a particular purpose.