Bob Lowry Explains Trichlor Tablets and Pool Acid Cautions

Pool chemistry rewards careful thinking and punishes assumptions. One of the stickiest myths is that cyanuric acid declines when water evaporates, so trichlor tablets can be used freely in hot, dry climates. Distillation physics says otherwise: evaporation removes pure water only, leaving behind calcium, salts, cyanuric acid, and everything else dissolved. When trichlor adds 10 ppm of free chlorine, it also contributes about 6 ppm of cyanuric acid. At a typical 10 ppm chlorine demand per week, CYA can rise roughly 25 ppm per month and 100 to 150 ppm over a season. That buildup weakens chlorine’s effectiveness and forces partial drains or backwashing to reset levels, because splash-out and leaks are the only real exits for CYA, not evaporation.

Every chlorine choice carries a byproduct tradeoff. Trichlor and dichlor add cyanuric acid, liquid chlorine adds salt that becomes part of total dissolved solids, and calcium hypochlorite adds calcium hardness. The question isn’t whether byproducts appear—they all do—but how quickly they accumulate and how much they matter. Liquid chlorine typically adds around 20 to 30 ppm of salt per gallon depending on pool size; even a gallon per week might lift TDS by only about 1,000 ppm a year. In terms of the saturation index, TDS must shift by about 1,500 ppm to move LSI 0.1, so that TDS rise is modest for scale concerns. Meanwhile cal hypo adds about 7 ppm calcium for every 10 ppm chlorine when using 65 percent product, which can raise hardness by 70 to 100 ppm in a few months. Choosing a sanitizer means deciding whether you’d rather manage CYA, salt/TDS, or calcium—and how you plan to restore balance when levels creep up.

Acid selection is another place where details matter. Muriatic acid (hydrochloric) delivers hydrogen and chloride ions without adding sulfate. Dry acid (sodium bisulfate) lowers pH but introduces sulfate, which can accumulate and form calcium sulfate scale once levels surpass a few hundred ppm. That scale is tougher to remove than calcium carbonate and does not show up on the Langelier Saturation Index, meaning you can see scale while your LSI looks fine. Sulfate can also enter through monopersulfate shock, so using both dry acid and MPS compounds the risk in hard-water regions. Because most service pros do not routinely test for sulfate, problems can develop quietly until rough plaster, cloudy water, or heater issues surface. This is why many technicians prefer muriatic for routine pH control and reserve dry acid sparingly or for situations that specifically call for it.

Storage and handling add another wrinkle. Dry acid is deliquescent—it absorbs moisture from the air and liquefies, which can lead to product loss, messy decks, and inconsistent dosing if containers are not sealed tightly. Acid “strength” and cost comparisons also matter. Enhanced hydrochloric blends may be safer to handle but often require larger doses per adjustment and cost more per effective unit of acidity delivered. When total alkalinity is high, achieving a pH drop with dry acid can require more product than with standard muriatic, so labels commonly recommend using muriatic to bring alkalinity into range, then maintaining pH thereafter. If you operate a saltwater chlorine generator, legacy warnings against dry acid have been traced to outdated documents; it can be used, but be mindful that sulfate accumulation still raises the risk of calcium sulfate scale on cells and heat exchangers.

Good management ties all of this together. Track your chlorine demand and relate it to how much CYA, salt, or calcium you’re introducing over weeks and months. Keep an eye on water loss methods that actually remove byproducts—backwashing, splash-out, leak repair, and controlled draining. Use the pH and alkalinity adjustment tools that reflect complex chemistry rather than rough guesses, and favor acids and sanitizers that match your region’s hardness and your maintenance style. Most importantly, recognize that myths persist because they are convenient, not because they are true. Evaporation is not your CYA exit. All sanitizers leave a footprint. Sulfates can scale even when LSI says you’re fine. When you plan for these realities, your pools stay clearer, equipment lasts longer, and clients notice the difference.