Liquid test kits use precise chemical reagents to measure pool water parameters. They are more accurate than test strips and are the professional standard for regular pool chemistry monitoring.
Key Facts
- DPD reagents measure free chlorine directly — use these for accurate chlorine testing.
- OTO (orthotolidine) measures total chlorine, not free chlorine — it cannot distinguish between active and combined chlorine.
- Taylor Technologies K-2006 is the most commonly recommended complete test kit for pool owners.
- Always compare colours in natural light using the white background on the comparator block.
DPD vs. OTO Tests
OTO (orthotolidine) produces a yellow colour in the presence of chlorine and was historically common in basic 2-way test kits. It measures total chlorine — the combined value of free and combined chlorine. OTO cannot tell you how much of that chlorine is actually active and available for sanitisation. DPD (N,N-diethyl-p-phenylenediamine) produces a red/pink colour specifically with free chlorine (DPD-1) and can be extended with a second reagent to measure total chlorine (DPD-3), allowing calculation of combined chlorine. Always use DPD reagents for meaningful free chlorine measurements.
Step-by-Step: Using a Liquid Kit
Fill the sample tubes with pool water collected 18 inches below the surface, away from jets and skimmers. For the chlorine test, add the number of drops of DPD-1 reagent specified (typically 5 drops per tube), cap the tube, invert once to mix, and immediately compare the colour to the reference comparator. Repeat with DPD-3 for total chlorine if needed. For pH, use the phenol red reagent in the same way. Always rinse tubes with pool water (not tap water) before each test and after use.
Maintaining Your Test Kit
Liquid reagents degrade with age and exposure. Replace reagents annually, or more often if you test frequently. Store in a cool, dark place away from pool chemicals, chlorine vapour, and extreme temperatures. The Taylor Technologies K-2006 includes reagents for FC, TC, CC, pH, TA, calcium hardness, and CYA — a complete set for comprehensive water testing. Reagent bottles typically have a 2-year shelf life when stored correctly. Using old or degraded reagents produces systematically incorrect readings that lead to incorrect chemical additions.
Examples
The pool smells like a public pool after a weekend of heavy use. Test: DPD-1 (free chlorine) shows 2.0 ppm (pink). DPD-3 (total chlorine) shows 3.5 ppm (deeper pink). Combined chlorine = 3.5 - 2.0 = 1.5 ppm. That is three times the 0.5 ppm action threshold. The smell is chloramines, not free chlorine. The fix is shock, not reducing chlorine. Without using both DPD-1 and DPD-3, you would have no way to make this diagnosis.
Common Mistakes to Avoid
- Using an OTO kit and treating total chlorine as if it were free chlorine — you will chronically under-chlorinate the pool.
- Comparing the colour tube to the reference under dim indoor or orange-tinted artificial light — accurate colour comparison requires natural or bright white light.
- Not rinsing sample tubes between tests, which cross-contaminates reagents and invalidates subsequent readings.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference