Every pool chemical decision you make is only as good as the test it is based on. A pool that tests at 3.0 ppm FC with a faded strip kit and an accurate 2.1 ppm FC with a Taylor K-2006 leads to two completely different service decisions — one of which leaves the pool under-chlorinated. Understanding the accuracy limitations and appropriate use cases of each testing method is foundational professional knowledge.
| Method | Accuracy | Cost | Speed | Best Use |
|---|---|---|---|---|
| Test strips | Moderate (±15–25%) | $15–30/bottle | 30 seconds | Quick screening, homeowner daily checks |
| DPD liquid drop test | Good (±5–10%) | $30–80/kit | 3–5 minutes | Standard professional testing |
| FAS-DPD drop test | Excellent (±2–5%) | $80–120/kit | 5–8 minutes | SLAM, high chlorine, professional standard |
| Digital photometric | Excellent (±2–3%) | $150–400/meter | 30–60 seconds | High-volume testing, consistent results |
| Certified lab | Reference standard | $30–100/test | 24–72 hours | Metals, disputed readings, legal/commercial |
Test strips use pad-based reagents that change color in response to chemical concentrations. The user matches the color to a reference card. Major strip brands include Aquachek, HTH, Poolmaster, and Hach.
DPD (N,N-diethyl-p-phenylenediamine) colorimetric tests use liquid reagents added to a measured water sample in a comparator block. Color is compared to a reference standard. The Taylor K-1000 series and similar kits use this method.
Standard DPD tests can measure FC and CC separately (critical for identifying chloramine problems), pH with better repeatability than strips, and total alkalinity via titration. They are significantly more accurate than strips and appropriate for most routine service testing.
DPD limitation: Like strips, DPD test reagents "wash out" at high free chlorine levels. At above 10 ppm FC, standard DPD turns colorless — giving a false zero reading. At 15–20 ppm, the test is completely invalid.
FAS-DPD (ferrous ammonium sulfate - DPD) is a titration-based method that overcomes the DPD wash-out limitation. The Taylor K-2006 Complete Kit is the benchmark professional test kit and uses FAS-DPD for chlorine measurement.
Digital photometric readers (LaMotte ColorQ Pro 9, Hach Pocket Colorimeter, Pentair Electronic Test Kit) use LED light sources and sensors to measure color change with less subjective error than human vision. They provide numerical readouts rather than requiring color comparison.
Professional water analysis labs provide comprehensive testing including metals (iron, copper, manganese), TDS, nitrates, phosphates, and confirmatory readings of standard parameters. LaMotte, Hach, and many pool supply companies offer mail-in lab services.
For a service truck serving residential pools, the practical recommendation:
SplashLens lets you record test readings for every parameter — from any test method — with timestamps and notes. Build a chemistry history for every account that travels with you to every service call. Works completely offline.
Open SplashLens Free →For field testing, the Taylor K-2006 FAS-DPD drop test kit is the most accurate widely available method for free chlorine, combined chlorine, pH, alkalinity, calcium hardness, and CYA. For absolute accuracy — particularly for metals — a certified lab water analysis is the gold standard. Digital photometric readers are accurate for most parameters with less subjective error.
Quality test strips are acceptably accurate for quick screening at normal parameter ranges. They are ±0.5 pH and ±5–10% for most parameters when fresh and used correctly. However, they wash out at high chlorine levels (above 5–8 ppm), cannot detect combined chlorine separately, and are more susceptible to color vision differences and lighting conditions than drop tests.
The Taylor K-2006 FAS-DPD Complete Test Kit is the standard professional reference kit. It measures FC (with FAS-DPD method accurate to high levels), CC, pH, TA, CH, and CYA. LaMotte ColorQ Pro 9 is the professional digital alternative for technicians who prefer photometric readings.
Send pool water to a lab when you need to test metals beyond your test kit capabilities, a customer disputes your chemistry readings, you need CYA confirmation at levels above 100 ppm, or the pool has unusual chemistry issues that field tests don't explain.