2.2 Testing Math: ppm, Combined Chlorine, and Records

A pink DPD vial is not yet an operating decision. You still have to turn color into a number, subtract correctly, compare the result with the local target band, and write the arithmetic where an inspector can reconstruct it. Domain 1 testing calculations on the AFO exam are short on purpose: the trap is skipping a unit, reversing free and total chlorine, or treating a strip guess as the official residual.

ppm and milligrams per liter

Pool operators live in parts per million (ppm). For dilute recreational water, 1 ppm is treated as 1 milligram per liter (mg/L). The two units are interchangeable on kit scales because pool water's density is essentially that of water. A photometer that prints "2.4" after a DPD test is saying 2.4 mg of measured chlorine per liter of sample, which you log as 2.4 ppm.

Do not confuse ppm with:

• Percent strength of a drum of sodium hypochlorite (a 12.5% solution is not "12.5 ppm in the pool").
• Pounds of product you will later add (that conversion uses basin volume, product strength, and dosage charts in the treatment chapter).
• ORP millivolts, which are an electrical reading, not a ppm residual.

If you diluted a sample because DPD bleached, multiply the instrument reading by the dilution factor before you log ppm. A 1:1 mix of sample and chlorine-free water that reads 4.0 ppm is 8.0 ppm in the pool. Logging the diluted 4.0 as the basin residual understates sanitizer by half—and can send you into an unnecessary "no chlorine" closure when the water is actually over-chlorinated.

Combined chlorine arithmetic

Combined chlorine (CC) is chlorine that has already reacted with nitrogenous bather waste and similar contaminants. It is a poorer germ-killer than free chlorine and is associated with the sharp "chlorine smell," eye sting, and indoor air complaints that patrons wrongly blame on "too much chlorine." The definition used on the exam and in DPD kits is:

CC = TC − FC

Worked example, using the same pair of readings that appears among the public 2025 AFO Candidate Handbook study questions, written here as original teaching:

• Total chlorine = 3.2 ppm
• Free chlorine = 2.1 ppm
• Combined chlorine = 3.2 − 2.1 = 1.1 ppm

That 1.1 ppm is not a rounding error. The 2024 MAHC model tells the owner to take action when combined chlorine exceeds 0.4 ppm (superchlorination, water exchange, and bather-hygiene measures are listed as examples). Some operator traditions still cite 0.2 ppm as a comfort trigger, and some AHJs publish their own number. Either way, 1.1 ppm means a chloramine problem, not a success story about a "strong" residual.

Checks that catch exam traps:

• If FC and TC are equal (for example both 2.4 ppm), CC is 0.
• If FC is recorded higher than TC, the tests were misread, reagents were added in the wrong order, or the sample was not mixed—retest. Combined chlorine cannot be negative.
• Logging only TC (the OTO habit) hides CC entirely. A pool can show 3.2 ppm "chlorine" on a yellow OTO block and still have only 2.1 ppm free—the 1.1 ppm gap is the irritation chemistry.

A second analog-comparator example: FC color matches 1.5 ppm; after DPD #3 the color matches 2.0 ppm. CC = 0.5 ppm. That value is already at or above many action levels even though free chlorine looks "in range."

Breakpoint figure: combined chlorine × 10

When CC is high, AFO coursework and the Candidate Handbook study items use a times-ten convention for breakpoint chlorination (shock):

Breakpoint-related figure ≈ CC × 10

Using the 1.1 ppm example: 1.1 × 10 = 11 ppm as the combined-chlorine-driven shock figure. Using 0.5 ppm CC: 0.5 × 10 = 5 ppm. This chapter introduces the formula so you can recognize exam items. Pounds of which chlorine product, Pool Size Factor (PSF), overnight procedure, and indoor air follow-up belong in the sanitation and oxidation chapter. Do not invent a dump amount from the multiplier alone, and do not shock during a packed public session without following your contamination and closing procedures.

Breakpoint is not "add 10 ppm to every pool every Friday." The multiplier scales with this test's CC. Indoor facilities often combine water shock with air handling and water replacement because chloramines also occupy the air. If CC is 0.1 ppm, you do not owe an 11 ppm shock just because the handbook example used 1.1 ppm.

Records an inspector can reconstruct

A commercial chemistry log is a legal and operational document. If a parameter was not written down, many investigators treat the check as if it never happened. When the test is run, record:

Keep the log in the form the AHJ requires (bound book, card, or approved electronic system). Do not back-fill yesterday's blanks with guessed numbers. When a controller prints ORP or a calculated ppm, still write the wet DPD and phenol red results beside it.

MAHC model frequencies remind you that TA, CH, CYA, total dissolved solids (TDS), and saturation index are not all hourly—but when you test them, they go on the same paper trail. Heated venues should record temperature at the same time as the DPD-FC and pH tests.

Worked example: number to "do I adjust?"

Scene. Outdoor 25-meter pool, 2:10 p.m., swim-team hour, bright sun, trichlor in the pump room.

Wet results (FAS-DPD and analog titrations):

• FC = 0.8 ppm
• TC = 1.6 ppm → CC = 1.6 − 0.8 = 0.8 ppm
• pH = 7.9 (phenol red)
• TA = 110 ppm
• CH = 280 ppm
• CYA = 40 ppm (melamine turbidity, last weekly check still valid)
• Temperature = 82°F

Sanitizer decision. CDC Healthy Swimming recommends at least 1 ppm FC in pools without CYA and at least 2 ppm when CYA or stabilized chlorine products are used (and at least 3 ppm in hot tubs). This outdoor pool has 40 ppm CYA and only 0.8 ppm FC, so FC is below a common public-health floor. Raise chlorine now (increase feed or add unstabilized chlorine per the dosage chapter) and retest after mixing time. Do not wait for closing if the venue stays open.

Combined-chlorine decision. 0.8 ppm CC is above the MAHC 0.4 ppm action model. Plan breakpoint work using 0.8 × 10 = 8 ppm as the times-ten figure, typically after hours when you can control occupancy and re-measure. Adding an unmeasured "cup of shock" during peak practice is not the AFO method.

pH decision. 7.9 sits on the high side of the common 7.2–7.8 commercial band. High pH shifts chlorine toward slower OCl-. Schedule acid toward mid-7s. TA at 110 ppm is inside a typical 80–120 ppm operator comfort band (MAHC model 60–180 ppm), so you are not fighting a zero-alkalinity problem. If you will breakpoint tonight, coordinate acid and chlorine so you are not chasing both at once without a plan.

Leave-alone items. CH 280 ppm is inside a common 200–400 ppm operator band. CYA 40 ppm is a typical outdoor working zone (local maximums vary). No hardness or stabilizer dose today.

Log line. Time 14:10; FC 0.8; TC 1.6; CC 0.8; pH 7.9; TA 110; CH 280; CYA 40 (weekly); 82°F; note "swim team / sun; increased Cl feed; acid after practice; breakpoint tonight CC×10 = 8 ppm figure"; initials.

Contrast scene. Same pool at 7:00 a.m.: FC 2.6, TC 2.7, pH 7.5, TA 110. CC = 0.1 ppm. No chemical change. Log it anyway—the blank line is the violation, not the "boring" water.

The exam will try to skip a step: reporting CC without subtracting, multiplying the wrong number by 10, or treating a strip as the official residual. Slow down: sample → valid method → arithmetic → compare to the AHJ band → act or don't → write it down.