How much CYA per 10K gallons per Tri-Chlor Puck?

CliffTex

Member
Jun 9, 2007
9
Leander Texas
I have seen posts about how using tri-chlor pucks adds CYA to the water. What I am unclear of is how much CYA I can expect each puck add to the water. If the 3" pucks are 45% available chlorine, what would the approximate PPM per puck increase be per 10K gallons? My CYA reagents are so old they don't work and I really don't want to keep running to the pool store all the time to have it checked.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
Trichlor is 91.5% available chlorine -- don't forget it has THREE chlorine attached to it and each produces a hypochlorous acid molecule. The molecular weight of Trichlor is 232.4103 g/mole while that of chlorine gas (which is how all chlorine is measured relative to) is 70.906 so 3*(70.906/232.4103) = 91.5%. Cyanuric Acid (CYA) has a molecular weight of 129.075 g/mole. So Trichlor is 129.075 / 232.4103 = 55.5% CYA by weight. These add up to more than 100% because chlorine is measured relative to chlorine gas, not to a single chlorine atom or to hypochlorous acid.

The way to look at it is that Trichlor does the following:

Trichlor + 3 Water --> CYA + 3 Chlorine (hypochlorous acid)

The 3" pucks unfortunately vary in weight being 6, 7 or 8 ounces (there are also 1" 3-ounce tabs and 1/2" 1-ounce tabs). If I assume 8-ounce pucks, then one in 10,000 gallons adds 5.5 ppm FC and 3.3 ppm CYA. The general rule regardless of puck weight or size is that for every 1 ppm FC you add using Trichlor, you also increase CYA by about 0.6 ppm (note that 55.5% / 91.5% = 0.61).

Richard