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

#### CliffTex

##### Member
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
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

Thanks Richard!