Pool size matters, however. The same tablet would raise free chlorine by approximately:

10,000 gallons 4.5 ppm 15,000 gallons 3.0 ppm 20,000 gallons 2.25 ppm 30,000 gallons 1.5 ppm

Of course, trichlor tablets dissolve gradually rather than all at once. The actual increase in free chlorine depends on feeder settings, circulation, water temperature, bather load, sunlight exposure, and chlorine demand.

Still, the calculation helps illustrate how much sanitizer a tablet contains.

The same exercise can also help explain chlorine costs.

Using the representative pricing Most pool professionals know what a trichlor tablet costs.

Far fewer know exactly how much chlorine it delivers.

That may sound like a strange statement. After all, trichlor tablets are chlorine. But just as a gallon of liquid chlorine is not 100 percent chlorine, neither is a trichlor tablet. To understand what a tab is really delivering to the pool, it helps to look at the amount of available chlorine it contains For the purposes of this article, we'll use a standard 3-inch, 8-ounce trichlor tablet containing 90% available chlorine.

An 8-ounce tab weighs onehalf pound. Because trichlor is approximately 90% available chlorine, each tablet contains: 0.5 pounds × 90% = 0.45 pounds of available chlorine In other words, a standard trichlor tablet contains just under one-half pound of available chlorine.

That means two standard tablets contain nearly the same amount of available chlorine as a gallon of 12.5% liquid chlorine under the assumptions used elsewhere in this issue.

About Tablet Sizes

While the industry commonly refers to a 'standard' trichlor tab, tablet weights are not perfectly standardized across all manufacturers. Most 3-inch trichlor tablets weigh approximately 8 ounces, but some products are sold in 7-ounce or 7.5-ounce sizes. Certain commercial tablets may be slightly heavier. Smaller 1-inch tablets are also available and typically weigh between one-half ounce and one ounce each. For simplicity, the calculations in this article are based on a standard 3-inch, 8-ounce trichlor tablet.

What does that mean in practical terms?

A useful rule of thumb in the pool industry is that one pound of available chlorine raises free chlorine by approximately 10 parts per million (ppm) in 10,000 gallons of water.

Using that benchmark, a single trichlor tablet containing 0.45 pounds of available chlorine can raise free chlorine by approximately 4.5 ppm in a 10,000-gallon pool.

Pool Volume

Approximate FC Increase from

One 8-Ounce Trichlor Tablet

from SIN's chlorine analysis, a 50-pound bucket of trichlor costs approximately $155. Since a 50-pound bucket contains about 100 standard 8-ounce tablets, each tablet costs roughly $1.55.

Viewed another way, that $1.55 purchases approximately 0.45 pounds of available chlorine.

For a 15,000-gallon pool, a single

tablet contains enough chlorine to raise free chlorine by about 3 ppm. If the tablet dissolves over the course of a week, it contains enough chlorine to supply an average of about 0.4 ppm of free chlorine per day, assuming all of the available chlorine ultimately reaches the water.

The calculation helps explain why some pools can be maintained with only a few tablets per week while others require much more supplemental chlorination. The number of tablets required depends not only on pool volume, but also on chlorine demand, sunlight exposure, water temperature, and stabilizer levels.

Ultimately, a trichlor tablet can be viewed in several different ways. It costs about $1.55. It contains about 0.45 pounds of available chlorine. And in a 10,000-gallon pool, it contains enough sanitizer to raise free chlorine by approximately 4.5 ppm.

All three statements describe the same tablet. They simply provide different ways of understanding what it contributes to the pool.