3.3 Treatment Chemicals, PSF, and Dosage Charts

Pool water treatment on the AFO exam is not free-form chemistry. The 2025 NRPA AFO Exam Candidate Handbook includes a Pool Dosage Chart in the public study-question section and lists PSF among the terms to know. Domain 1B is Pool Water Treatment. Combined with the acid/base items in 1D, this is how you turn a needed ppm change into a quantity of chemical. The chlorine factors are 1 ppm bases. The mineral-treatment rows require more care: 18 lb sodium bicarbonate and 13 lb calcium chloride in 120,000 gallons correspond to approximately 10 ppm changes, not 1 ppm. The 2025 public chart places a shared “1 ppm” heading over all five rows, which would create an unsafe tenfold mineral dose if read literally. As a public cross-check, the 2024 MAHC Annex uses about 1.4 lb sodium bicarbonate per 10,000 gallons for a 10 ppm TA rise; scaled to 120,000 gallons, that is 16.8 lb, close to the rounded 18 lb treatment increment. On the deck, always follow the product label and local code, because product strength and hydrate form also vary.

Independent teaching here uses the public handbook chart. It does not quote the copyrighted AFO Manual, and it does not invent extra rows for muriatic acid or soda ash that the public chart does not give.

Pool Size Factor

PSF = gallons / 120,000

120,000 gallons is the chart's reference volume. A pool that holds 120,000 gallons has PSF = 1. Bigger pools scale up; smaller pools and spa-pools scale down. Write PSF on scratch paper before you touch the chemical column. Most dosage mistakes are volume mistakes, not chemistry mistakes.

Work the handbook volume once so it never slows you down: 360,000 / 120,000 = 3. If an item gives surface area and depth instead of gallons, that is a volume problem from the hydraulics chapter (length × width × average depth × 7.5 in operator math). This section assumes gallons are already in hand, the same way the handbook study prompt does.

Dosage bases and the chart-heading correction

Use the following treatment increments for 120,000 gallons:

For chlorine: total chemical = base × PSF × desired ppm change.

For bicarbonate or calcium chloride: total chemical = base × PSF × (desired ppm change / 10).

The calculation is a multiplier with three numbers, but the increment matters: 1 ppm for chlorine and 10 ppm for the two mineral corrections. Miss the increment and the answer can be wrong by a factor of ten. Units matter: liquid chlorine answers are gallons; the other four chart chemicals are pounds. An answer of "3 lb of liquid chlorine" or "3 gal of bicarbonate" is a unit fail even if the digits look familiar.

The chart is not a substitute for reading whether your liquid chlorine is 10% or 12.5% available chlorine, whether granular chlorine is calcium hypochlorite or something else, or whether calcium chloride is anhydrous or dihydrate. Exam convention first; label on the job. If the label and the chart disagree, the label plus the AHJ govern the facility. If an exam item supplies its own chart or directs you to a particular manual table, follow the item’s stated units; on the job, the label plus the AHJ govern.

CO2, muriatic acid, dry acid, and soda ash do not have base-amount rows on this public 2025 chart. Do not invent gallons-per-ppm for them from these five lines. Use those product labels and the acid/base technique in the previous sections.

Worked example: handbook 360,000-gallon pool, raise TA 40 ppm

The 2025 handbook study prompt gives a swimming pool of 360,000 gallons with alkalinity 40 ppm (and other balance numbers used later for CSI). The study task is how much sodium bicarbonate is needed to increase alkalinity to 80 ppm.

1. PSF = 360,000 / 120,000 = 3
2. Desired TA change = 80 − 40 = 40 ppm
3. Bicarbonate base = 18 lb per 10 ppm TA per 120,000 gal
4. Number of 10 ppm increments = 40 / 10 = 4
5. 18 lb × 3 × 4 = 216 lb of sodium bicarbonate

Arithmetic check in two steps so the digits cannot hide:

• 18 × 3 = 54 lb per 10 ppm in this 360,000-gallon pool
• 54 × 4 = 216 lb

A 216 lb correction is still large enough to stage over circulation periods. Retest TA and pH, watch CSI qualitatively so you do not scale the basin, and verify the product label. Treating 18 lb as a 1 ppm base would produce 2,160 lb—about ten times the chemically appropriate amount.

The same pool's chlorine and calcium lines use the same PSF. These are chart illustrations, not a full sanitizer-product lesson:

• 1 ppm liquid chlorine: 1 gal × 3 × 1 = 3 gal
• 1 ppm gas chlorine: 1 lb × 3 × 1 = 3 lb
• 1 ppm granular chlorine: 1.5 lb × 3 × 1 = 4.5 lb
• Calcium hardness from 200 ppm to 350 ppm is a +150 ppm change, or 15 ten-ppm increments: 13 lb × 3 × 15 = 585 lb calcium chloride (then stage it and read the label)

Superchlorination and breakpoint quantities belong with combined chlorine in the sanitation chapter. The dosage factors for chlorine still come from this chart when the item is a pounds-or-gallons treatment question.

Smaller realistic adjustment: 10 ppm on 60,000 gallons

Learners should see scale, not only the municipal monster. Use a 60,000-gallon spa-pool (or small indoor tank) and a modest 10 ppm bicarbonate increase.

• Volume = 60,000 gal
• PSF = 60,000 / 120,000 = 0.5
• Raise TA 10 ppm with sodium bicarbonate
• Number of 10 ppm increments = 10 / 10 = 1
• 18 lb × 0.5 × 1 = 9 lb

Nine pounds is a manageable correction. Same formula, different PSF and treatment increment. If you can produce both 216 lb and 9 lb without mixing gallons, pounds, and ppm increments, you can run the calculation.

Another check on the same 60,000-gal vessel:

• 1 ppm granular chlorine = 1.5 lb × 0.5 × 1 = 0.75 lb
• 1 ppm liquid chlorine = 1 gal × 0.5 × 1 = 0.5 gal
• 10 ppm calcium-hardness increase = 13 lb × 0.5 × 1 = 6.5 lb calcium chloride

If an item gives 60,000 gallons and you use PSF = 6 or PSF = 2, you inverted or rounded the reference volume. Slow down: gallons divided by 120,000, not 120,000 divided by gallons, and not gallons divided by 7.5 (that 7.5 converts cubic feet to gallons when you are still finding volume).

Calcium chloride raises calcium hardness

Calcium chloride is the chart chemical used to raise calcium hardness (CH). The handbook study questions ask what chemical raises calcium hardness — the answer is calcium chloride, not bicarbonate and not soda ash. CH is a CSI factor, not a pH buffer. Adding CaCl2 does not replace TA work. If TA is 40 ppm and CH is 200 ppm, you still need bicarbonate for TA; calcium chloride is the separate CH lever.

High CH plus high pH plus high TA is a scale recipe. A large CaCl2 correction still belongs in stages with rechecks of hardness, pH, and TA. Details of the saturation index wait for the CSI chapter. In the 360,000-gallon example, the +150 ppm correction is about 585 lb, not 5,850 lb; divide it into label-directed additions with circulation and retesting.

Soft fill water, plaster that is still curing, and heaters that scale at the heat exchanger are the field reasons CH is on the log sheet. The exam reason is simpler: know the chemical, know that the 13 lb base represents about a 10 ppm CH increment in 120,000 gallons, know PSF, and multiply.

Safety: never mix chemicals in a bucket

Add each chemical to the water separately. Never premix acid and chlorine, bicarbonate and acid, calcium chloride and chlorine, or "a little of everything so I only make one trip." In a bucket or a closed drum, acid plus hypochlorite can release chlorine gas. Calcium hypochlorite plus organic residue, oil, or the wrong leftover chemical is a documented fire and off-gassing pattern. OSHA hazard communication and PPE are a later chapter; the treatment rule that belongs here is simple:

• One chemical, into the pool or a flooded feeder, as the label directs
• Circulate
• Then the next chemical, if still needed after a retest
• Acid to water, never water to acid
• Store acids away from oxidizers
• Dose from the chart for a handbook-style exam item; dose from the label + AHJ for the facility

Predissolving a single product in a dedicated container of pool water is sometimes what a label describes. That is not permission to combine two treatment chemicals in the same container. The AFO-relevant sentence is: never mix chemicals in a bucket; add them to water separately.

Putting the chart on scratch paper

On an open-book exam you may have the current AFO Manual, a handheld calculator without formula memory, and the CSI slide rule. For a treatment item:

1. Write the volume in gallons.
2. Write PSF = gal / 120,000 and compute it.
3. Write the chemical and its base in 120,000 gallons: chlorine uses a 1 ppm increment; bicarbonate and calcium chloride use an approximately 10 ppm increment.
4. Write the ppm change (desired minus current), then convert it to the applicable number of increments.
5. Multiply base × PSF × number of increments. Include units (lb versus gal).
6. Ask whether the number is a full correction or a staged correction — the math does not change; the operations plan might.

That is pool water treatment: a factor, a volume, and a correctly identified treatment increment. Do not read the shared 2025 chart heading in a way that creates a tenfold bicarbonate or calcium dose. If the number is large, check the increment and arithmetic, then stage the label-directed dose on the deck.