Calcium Hypochlorite
Commonly called "Cal-hypo", calcium hypochlorite was patented in 1799 and called "bleaching powder." It comes in a large granular form or as 1-inch or 3-inch tablets and is produced by passing chlorine gas over slaked lime. The resulting powder or granules provide 65 to 70 percent available chlorine. Cal-hypo will support combustion and one needs to avoid mixing it with acids, ammonia, soda pop, oil, trichlor or just about anything but water. Mixing with organics will cause a fire.
Because it is slow to dissolve, it should be used either in a feeder, or pre-dissolved in water and then added as a liquid. Cal-hypo can temporarily cloud the water, because the calcium takes a long time to dissolve completely. It can also cause calcium scaling and deposits on surfaces and in circulating equipment. Cal-hypo tends to increase the water hardness level quickly and has a high pH of 11.8. Neutralizing the pH requires approximately 4 ounces of muriatic acid per pound of cal-hypo.
Sodium dichloro-s-triazinetrione (Sodium Dichlor or Dichlor from short)
Sodium Dichlor is the only type of chlorine that should be used in spas. It's fast dissolving, will not cloud the water, is relatively pH neutral and has a long shelf life. Because of it's granular makeup, there's no way to automatically dispense it and it must be added by hand.
Sodium Dichlor is chemically produced by adding soda ash and cyanuric acid to a solution of trichlor. When dried the result is a granule that provides 56% - 62% available chlorine, depending on the method of manufacture.
