Water chemistry

Pool Chlorine Types Compared for Service Techs

Liquid chlorine, cal hypo, trichlor and dichlor compared by dose, CYA added and side effects, with worked dose and CYA examples.

Four products cover most route stops: liquid chlorine, cal hypo, trichlor and dichlor. Salt chlorine generators make chlorine on site. They differ in dose, in what they leave behind in the water and in how they change pH. This table sums it up.

Comparison table

Also useful: 1 lb of 65% cal hypo raises FC about 7.8 ppm per 10,000 gallons.

What each one does to the pool

• Liquid chlorine leaves nothing behind except a little salt, and it is easy to dose in small steps. Buy it fresh and check the date, since it weakens over time and in heat.
• Cal hypo is strong and dry, so it stores in a small space. It adds calcium, so it needs a watch on calcium hardness in pools that already run high. Dissolve it in a bucket of water before use, adding the product to the water. Never toss it where it can sit on vinyl.
• Trichlor is convenient in a feeder or floater, and it is the usual reason pools drift to low pH, low TA and high CYA.
• Dichlor is fast and handy for a quick boost or a start-up. It adds more CYA per ppm of chlorine than trichlor.
• Salt cells make chlorine from salt. They do not add CYA, so you set CYA yourself. See salt pool service.

Worked example 1: what each product does to CYA

Say a pool at CYA 30 gets 30 ppm of FC over a month from one product. This is an illustration, so use your own pools' numbers.

Splash-out, backwash and rain dilute CYA a little, so real numbers drift lower than the table. The direction is the point. Trichlor takes a 30 ppm pool to the top of the 30 to 50 range in about one month at that rate. Once CYA gets high, only dilution brings it down. See what to do about high CYA.

Worked example 2: dose a 22,000 gallon pool

Readings: FC 2 ppm, CYA 40. The minimum FC is 7.5% of 40, which is 3 ppm. You decide to raise FC to 4 ppm, a gain of 2 ppm. The multiplier is 22,000 / 10,000 = 2.2.

• Liquid chlorine, 12.5%: 10.2 fl oz x 2 x 2.2 = about 45 fl oz.
• Cal hypo, 65%: 2 oz x 2 x 2.2 = about 8.8 oz by weight.

Cross-check the cal hypo number. 1 lb raises FC about 7.8 ppm per 10,000 gallons, which is about 3.5 ppm per lb in 22,000 gallons. For 2 ppm you need about 0.57 lb, which is about 9 oz. The two figures agree.

The chlorine dose tool does this scaling for any pool.

Picking a product by account

• Plaster pool with a trichlor feeder: test TA and CYA every visit. Add liquid chlorine to cover spikes and keep the feeder from carrying the whole load.
• Pool with high CYA: switch the sanitizer to liquid chlorine or cal hypo until CYA comes back into range.
• Vinyl or fiberglass pool: dissolve cal hypo first and keep granules off the surface. Calcium targets are lower on these pools at 150 to 250 ppm, so watch for build-up.
• Salt pool: the cell is the base. Liquid chlorine covers heavy use and cold-water periods when the cell makes less.
• Tight schedules: a feeder carries chlorine between visits. Liquid chlorine is quick to pour and needs no prep.

Mistakes

• Using dichlor or trichlor as the only source for months, then wondering why the pool needs more chlorine each week.
• Storing trichlor, cal hypo and liquid chlorine together, or sharing a scoop between them.
• Reading FC against the 1 to 4 ppm range and ignoring CYA.
• Skipping the date check on liquid chlorine, then dosing as if it were full strength. Weak product means a bigger volume for the same ppm.

Safety

Follow the product label and SDS. Never mix chemicals, including different chlorine products, and never mix chlorine with acid. Add chemical to water, never water to chemical. Keep every container closed, dry and in its own place on the truck.

When combined chlorine climbs, see combined chlorine and breakpoint.