On a weekly service route, pools rarely need testing to determine if they need chlorine — they need testing to determine how much.

Sunlight, temperature, and bather load steadily consume sanitizer. By the time you arrive, many pools are already low. Chlorine is the one parameter where the default action — adding it — is usually correct. The real question is how to dose to last seven days.

EPA-registered sanitizer labels generally limit free chlorine to 4 ppm with bathers present, while public health guidance calls for at least 1-2 ppm minimum. In weekly residential service, that narrow range is often impractical. A pool left at 4 ppm will frequently fall below minimum before the next visit.

For once-weekly outdoor service, dosing is forward-looking. You are not targeting “in range” today — you are leaving a buffer. In practice, most pools should be left at about 5-8 ppm free chlorine. Demand is highest in the first couple of days, then slows. Without that buffer, chlorine often reaches zero mid-week.

What Chlorine Tests Measure

Chlorine tests measure oxidizing capacity, not a single compound. In pool water, chlorine exists as a small active fraction (primarily HOCl, with some OCl-) with the majority reversibly bound to cyanuric acid. Field tests report this collectively as free chlorine (FC), even though only HOCl is highly active.

Total chlorine (TC) = free chlorine (FC) + combined chlorine (CC).

Combined chlorine represents chloramines and other reacted forms. Combined chlorine indicates partially oxidized contaminants and typically requires further oxidation (breakpoint chlorination), not just routine dosing.

Test Methods

• Test strips use DPD-type chemistry on pads. They are fast but low resolution and sensitive to timing and lighting. Best for screening.

• OTO (orthotolidine) produces a yellow color and measures total chlorine only. It cannot distinguish free from combined chlorine, so it cannot indicate active sanitizer.

• ORP (oxidation-reduction potential) measures oxidizing strength (mV), not chlorine concentration. It is strongly influenced by HOCl, pH, cyanuric acid, and temperature. Because ORP primarily reflects the concentration of active HOCl, it generally tracks sanitizing strength more directly than free chlorine in stabilized water. Typical effective sanitation occurs around 650-750 mV, but ORP is best for control systems and trends — not stand-alone dosing.

• DPD and FAS-DPD use the same underlying chemistry — oxidation of DPD dye — but differ in how results are measured:

DPD (colorimetric): A visual method.

• Add DPD → free chlorine turns sample pink (FC reading by color match).

• Add iodide → combined chlorine is converted into an oxidizing form, deepening the color (TC reading by color match).

• CC = TC − FC.

Accuracy is limited by human color matching. At higher chlorine levels, color blocks compress and can mask true concentration, while FASDPD remains linear and accurate across a wider range.

FAS-DPD (titration): A measured method using the same initial reaction.

• Add DPD → sample turns pink.

• Add FAS titrant dropwise until clear → drop count = FC.

• Add iodide → sample turns pink again if combined chlorine is present.

• Titrate again → drop count = CC.

• TC = FC + CC.

Key clarification: iodide is not used in the initial free chlorine step in either method. It is only added when measuring total or combined chlorine.

For most residential work, DPD is sufficient. When precision matters —

especially at higher chlorine levels or when results seem inconsistent — FAS-DPD is the more reliable method.

Testing Interferences Chlorine results are highly condition-dependent:

• High chlorine (DPD bleaching): Excess oxidizer can destroy the DPD dye, producing a false zero.

Response: dilute the sample (e.g., 1:1 with distilled water and double the result) or use FAS-DPD.

• Combined chlorine: OTO reads it as total chlorine; DPD only reveals it if the total chlorine step is performed.

Response: run a full DPD or FASDPD test when odor, irritation, or abnormal demand is present.

• Cyanuric acid (CYA): High CYA lowers the fraction of active HOCl without changing measured FC.

Response: interpret FC relative to CYA rather than as a stand-alone number.

• pH effects (interpretation, not interference): Higher pH shifts chlorine toward the less effective OCl- form. Free chlorine reads the same, but sanitizing strength drops.

Response: correct pH before increasing chlorine.

• Reagent degradation: Heat, sunlight, and age degrade DPD reagents.

Response: replace regularly and avoid storing kits in high heat.

• Sample contamination: Oils, residues, or dirty vials can distort results.

Response: rinse thoroughly with pool water before testing.

• Technique: Timing, lighting, and drop consistency affect accuracy.

Response: use consistent sample volume; hold droppers vertical for uniform drops; read DPD colors promptly in neutral lighting; and follow consistent timing for strips.

A test result can be chemically correct yet operationally misleading if these factors are ignored.

How Much Chlorine to Add

Using liquid chlorine (10–12.5%): In 10,000 gallons:

• 1 gallon ≈ +10 ppm

• ½ gallon ≈ +5 ppm Typical weekly dosing:

• 10k pool: ½-¾ gallon

• 15k pool: ¾-1 gallon

• 20k pool: 1-1.5 gallons Adjust upward for sun exposure, low CYA, heavy use, or repeated zero readings. Adjust downward for covered pools, high CYA (80+), or light use.

When Not To Add Chlorine

Reduce or skip only when there is clear evidence that chlorine is being maintained between visits — typically when FC remains above ~5 ppm after a full week with a feeder or tablets in use. Water clarity alone is not a reliable indicator.

Most Common Mistake

Trying to stay “in range” instead of staying ahead of demand.

A pool left at 3-4 ppm often reaches zero before the next visit.

A pool left at 6-8 ppm is far more likely to maintain a residual. The difference shows up in fewer problems, fewer callbacks, and more stable water.