What Each Chlorine Product Does to the Other Readings

Why this matters

Every chlorine product brings a passenger. A pound of trichlor sanitizes and also stabilizes; a pound of cal-hypo sanitizes and also hardens; a gallon of liquid chlorine sanitizes and also salts the water and nudges the pH up. One dose of any of them is invisible on the other readings. Six months of the same product on the same pool is not: it is the stabilizer at 110 ppm in September, the scale line in August, the acid bill that doubled. This card is the side-effect table and the arithmetic that turns it into a season, so the product a pool is fed is chosen on purpose rather than by habit.

The side-effect table

WHAT RIDES IN WITH 1 ppm OF FREE CHLORINE
(ratio holds for any pool volume)

product         CYA    CH     salt*  pH and TA
--------------  -----  -----  -----  ----------------
trichlor        +0.6   0      +0.8   both pulled down
dichlor         +0.9   0      +0.8   near neutral
cal-hypo        0      +0.7   +0.8   pH up a little
liquid 12.5%    0      0      +1.7   pH up a little

* as NaCl, once the chlorine is spent

How to read it: if a pool uses 10 ppm of free chlorine in a week and all of it comes from trichlor, the stabilizer reading rises about 6 ppm that week. The ratio does not depend on gallons, because the chlorine and its passenger arrive in the same proportion whatever the size of the pool. How much chlorine a pound or a gallon of each delivers per 10,000 gallons is a different question, owned by the product table in 'Shock Dosing Standard'.

Where each number comes from

Trichlor, about 0.6 ppm CYA per ppm. Trichlor is cyanuric acid with three chlorines attached. When it dissolves, each molecule releases its chlorine and leaves its stabilizer behind. By weight that works out to roughly 0.6 ppm of cyanuric acid for every ppm of free chlorine delivered. It is also the most acidic product here: as its chlorine is spent, acid is left in the water, so a pool fed on trichlor drifts down in pH and steadily loses alkalinity.

Dichlor, about 0.9 ppm CYA per ppm. Dichlor carries two chlorines per stabilizer molecule instead of three, so the stabilizer load per unit of chlorine is about half again as high as trichlor's. It dissolves fast and close to neutral, which is why it is the usual spa and quick-dissolving shock product, and why using it as the routine weekly chlorine on a stabilized pool runs the CYA up faster than anything else.

Cal-hypo, about 0.7 ppm CH per ppm. Calcium hypochlorite leaves its calcium behind. Each ppm of chlorine adds about 0.7 ppm of calcium hardness, reported as calcium carbonate. The product is strongly alkaline as it dissolves; most of that is given back as the chlorine is spent, so the net drift is a small rise in pH.

Liquid chlorine, about 1.7 ppm salt per ppm. Sodium hypochlorite is made with as much salt as hypochlorite in the jug, and the hypochlorite itself turns to salt once it has done its work. Together that is about 1.7 ppm of sodium chloride per ppm of free chlorine. It adds no stabilizer and no calcium. The jug also carries a small excess of caustic to keep the product stable, which is the reason a liquid-fed pool creeps up in pH and its acid use rises over a season.

Every product leaves chloride. Spent chlorine of any kind becomes chloride, roughly 0.8 ppm counted as salt for each ppm used. On a non-salt pool nobody tests it; on a salt pool it is part of why the salt reading drifts up even when no bag has gone in.

The salt cell, the fifth source. A salt chlorine generator makes chlorine from the chloride already in the water, and spent chlorine turns back into chloride, so the cycle adds no salt, no stabilizer and no calcium. What it does bring is a steady pH rise: the stream of hydrogen bubbles off the cell drives carbon dioxide out of the water, more so at higher alkalinity, which is why a salt pool's acid demand is a weekly line on the log and why its cell scales faster when that line is skipped ('Salt Cell Cleaning & Maintenance'). On a salt pool the table above applies only to whatever product is poured on top of the cell, and that is usually liquid chlorine for a boost, which brings its 1.7 ppm of salt with it.

Product strength does not move the ratios much. A 65 percent cal-hypo carries more of its weight as other calcium salts than a 73 percent product, so its CH per ppm of chlorine runs a little above 0.7, not below. Trichlor sticks, pucks and tablets are the same chemical in different shapes and share a ratio.

Worked season: one pool, four feeding plans

A 20,000 gallon plaster pool on a weekly route, outdoor, unscreened, opened with CYA at 40 ppm, CH at 250 ppm and salt around 500 ppm from the fill water. Through the summer it uses about 1.5 ppm of free chlorine a day, which the tech has measured as the difference between what was left after each dose and what was there seven days later. Over a 20 week season that is:

• 1.5 ppm a day times 7 days is 10.5 ppm a week.
• 10.5 ppm a week times 20 weeks is 210 ppm of free chlorine delivered in the season.

Now feed all 210 ppm from each product in turn, ignoring dilution for the moment:

Read across the rows. The trichlor plan has the pool's chlorine locked up long before September: at 166 ppm CYA the routine minimum free chlorine, which scales with stabilizer, is far above what a weekly visit can hold, and 'Cyanuric Acid Management Standard' sends it to a partial drain. The dichlor plan reaches the same 166 ppm about two-thirds of the way through the season. The cal-hypo plan lands near 400 ppm CH, which on plaster with warm water and a high pH is scaling territory ('Calcium Hardness and Scale Management'). The liquid plan adds salt nobody will ever notice on a non-salt pool and a pH that wants more acid each month.

Dilution, and why the table is an upper bound. Rain overflow, backwashing and splash-out replace water all season. If this pool replaces, say, a quarter of its volume across the summer, every passenger lands lower than the table, and a backwash-heavy sand filter pool lower still. The table is the ceiling; the readings are the truth. That is also why the CYA reading is the one that decides, not the arithmetic: trust a turbidity test taken in good light over a projection.

The realistic mixed plan, worked

Most route pools are not fed one product. The same pool fed a trichlor floater for the base and liquid for the top-up, with the log showing the floater supplying about 4 ppm a week and liquid the other 6.5:

• Trichlor share: 4 ppm a week times 20 weeks is 80 ppm; times 0.6 is 48 ppm of CYA. Opening 40 plus 48 is about 88 ppm by season end, before dilution.
• Liquid share: 6.5 times 20 is 130 ppm; times 1.7 is about 221 ppm of salt. Harmless.
• Check the split: 80 plus 130 is 210 ppm, the season total. It closes.

At 88 ppm the pool is near the outdoor stabilizer recommendation that 'Shock Dosing Standard' quotes from the CDC's Model Aquatic Health Code (at or below 90 ppm, binding only where a state or county has adopted it for regulated pools). That is the reason the floater gets turned down or pulled once CYA passes about 70: every further ppm of rise comes from the floater's share, and the liquid can carry the load with no stabilizer at all.

Choosing the product from the readings

The table never overrides a reading. A cal-hypo "help" on a soft-water pool is one-way: once CH is at target, the same product keeps adding to it.

What a product switch does to the readings you are used to

When a pool moves from trichlor to liquid chlorine, two things change that look like faults and are not. The pH starts rising week to week where it used to fall, so the acid demand that was near zero becomes a weekly line. And the alkalinity stops being eaten, so the bicarbonate the pool needed every month stops being needed. A tech who keeps adding bicarbonate on the old schedule after the switch pushes TA and pH up together. Write the switch date on the pool's route notes so the next tech reads the drift as expected.

The reverse switch, liquid to trichlor, does the opposite and adds stabilizer from the first week. Note the opening CYA on the day of the switch so the climb has a baseline.

How to verify the drift you predicted

At each monthly full panel, compare the CYA, CH and salt readings with the table's projection from the logged chlorine. A reading within one test step of the projection says the log is honest and the dilution assumption is close. A reading well below says the pool is losing more water than anyone has noticed (a leak, an autofill running hard, heavy backwashing). A reading well above for CYA says someone, possibly the customer, is adding tablets or dichlor between visits. Either surprise is a conversation before it is a chemical.

References

• See related: Shock Dosing Standard (per-unit chlorine strength, stabilizer ratio and the MAHC cyanuric acid figure)
• See related: Cyanuric Acid Management Standard; Calcium Hardness and Scale Management
• See related: The Dose Per 10,000 Gallons Factor Card; How to Size the Weekly Liquid Chlorine Dose to Last the Week
• Product labels and safety data sheets for available chlorine content and composition