How to Test Pool Water: Strips, Drop Kits and FAS-DPD Chlorine Tests

The tests that matter

CDC focuses on chlorine and pH because those two determine whether germs are being killed. It recommends testing both at least twice a day for pools and hot tubs in use, and more often with many swimmers. Many home owners with a stable outdoor pool test chlorine and pH daily and the slower numbers weekly.

Strips versus drop kits

Test strips are quick: dip, wait, compare colors. CDC says owners can use strips but notes they are less accurate than a DPD kit. Color matching is subjective, the steps between color blocks are wide, and strips go off if they get damp or old.

Drop kits use liquid reagents or tablets. For chlorine, CDC recommends DPD kits. There are two styles:

• Color-match DPD: add reagent, compare the pink color to a chart. Good for low levels; hard to read at high levels, and above about 10 ppm the reagent can bleach out and show falsely low or zero.
• FAS-DPD titration: you add a powder that turns the sample pink, then count drops of a second reagent until the pink disappears. Each drop equals a fixed amount (often 0.5 ppm with a 10 mL sample or 0.2 ppm with a 25 mL sample, per the kit's instructions). It reads high chlorine levels without bleaching and gives combined chlorine by a second step. Many pool owners prefer it for dosing decisions.

If you want a kit that covers all six tests, look for one that includes FAS-DPD chlorine plus TA, CH, pH and CYA, for example a FAS-DPD pool test kit. Replacement reagents expire, so note the date on the bottles.

Taking a good sample

1. Rinse the sample tube with pool water.
2. Turn the tube upside down, push it to about elbow depth, away from returns, skimmers and feeders, then turn it over to fill.
3. Test promptly, out of direct sunlight.
4. Fill exactly to the line. A 10% overfill means a 10% error.
5. Hold the bottle vertically so each drop is the same size, and swirl after each drop.

Reading color tests well

• Read against a white background in daylight, but not in direct sun.
• For pH, read immediately; the color can drift as the sample sits.
• High chlorine can make pH tests read falsely high on some kits. Test pH when chlorine is in its normal range when possible.

Turbidity tests for CYA

The CYA test works by making the sample cloudy. You slowly fill a viewing tube until a black dot on the bottom disappears, then read the level. It is less precise than the other tests, especially below about 30 ppm. Do it outdoors with the sun behind you, repeat it, and treat it as approximate.

Keeping reagents good

• Store kits cool, dry and out of sunlight. A hot pool shed shortens reagent life.
• Replace reagents by their expiration date or yearly.
• Keep the caps on the right bottles; cross-contamination ruins results.
• Wash hands after testing and keep reagents away from children.

Log every result

A reading on its own tells you where you are. A series tells you where you are going. Record results in the pool water log, including what you added, so trends like rising stabilizer or growing chlorine use stand out early. A simple waterproof notebook by the equipment pad works too, if you prefer paper.

When a reading looks wrong

• Chlorine reads zero right after a big dose: the reagent may have bleached. Mix the sample half and half with distilled water, test again and double the result.
• pH reads unusually high after shocking: high chlorine can interfere with some pH reagents. Retest when chlorine is back in its normal range.
• Two tests disagree: strips and drops often differ. Trust the drop test, and check its reagents' dates.
• CYA seems to jump around: the turbidity test is the least precise. Average two or three readings rather than reacting to one.
• Combined chlorine never drops: check that you subtract free from total chlorine correctly, then look at sunlight, bather load and the filter.

Common questions

Are pool test strips accurate enough?

Strips are fine for a quick check, and CDC says they can be used, but it notes they are less accurate than a DPD test kit. For dosing decisions, especially chlorine and stabilizer, a drop kit is more reliable.

Why does my chlorine test show nothing?

Either there is no chlorine, or there is so much (above about 10 ppm) that it bleached the DPD reagent. If you recently added a large dose, dilute the sample with distilled water and retest.