Bob Lowry Explains Chlorine Strength

Pool chemistry myths spread fast because the labels look simple and the numbers feel definitive. One of the most common claims in pool maintenance is that trichlor tablets are “the strongest chlorine” because the bucket says 99% trichlor and often lists 90% or so available chlorine. The key detail most pool service pros and homeowners miss is that the big percentage is typically about product purity, not the percent of elemental chlorine that ends up doing sanitizing work. When we treat pool water balance like a shortcut, we end up overdosing, underdosing, or ignoring side effects like stabilizer buildup, rising TDS, and long-term water quality issues.

The episode breaks down why “available chlorine” exists as a comparison tool, not a straightforward statement of how much chlorine is inside the product. Historically, chlorine gas became the reference point and got treated as 100% even though, in water, one part forms hypochlorous acid (HOCl) and the other forms hydrochloric acid (HCl). That historical convention forces other products to be expressed in a way that matches the old yardstick. So when a trichlor label says roughly 90% available chlorine, it is describing how it compares to a pound of gas chlorine, not claiming the product is 90% pure chlorine by weight. This is why reading the entire label matters: “99% trichlor” means 99% pure trichlor as a chemical, while the pool impact includes cyanuric acid (stabilizer) as part of what you add every time you feed tablets.

From there, the conversation shifts to another frequent point of confusion: cal-hypo and calcium hardness. Cal-hypo is popular in many regions, but the side effect is real and measurable over time. A useful rule of thumb shared here is that for every 10 ppm of free chlorine you add via calcium hypochlorite, you also add about 7 ppm of calcium hardness. That makes the long-term outcome predictable, especially in hard-water markets where fill water already arrives at 400+ ppm calcium. Even if a single dose seems small, weekly dosing compounds quickly, and the results show up as scaling, a calcium ring at the waterline, and tighter limits for keeping CSI and overall water balance in range.

The episode also highlights why “percent strength” across products changes how much you must use, which impacts both cost and side effects. Cal-hypo products come in multiple strengths, and newer cal-hypo tablets may be far lower in available chlorine than trichlor. Pound for pound, a lower-percentage product requires more material to hit the same free chlorine target, which means more leftover ingredients landing in the water as calcium, chloride, sodium, or general TDS. The practical takeaway for pool care is to compare sanitizers by outcome: desired FC, added stabilizer, added calcium, and true cost per equivalent chlorine delivered. A simple spreadsheet approach to cost comparison can keep buying decisions grounded in math instead of marketing, and it helps you explain these choices clearly to clients who only see the front-of-bucket percentage.