Hot tub water chemistry, level by level

Five numbers run the water, and each one leans on the others. Think of sanitizer as the cleaner, pH as the condition it works in, and alkalinity as the shock absorber that keeps pH from swinging.

• Sanitizer (chlorine or bromine) kills germs. The part still able to work is the free or residual level; once it has combined with sweat and oils it is spent. See hot tub chlorine levels.
• pH is how acidic the water is. Low pH corrodes metal and eats sanitizer; high pH clouds the water, lays scale on the heater and slows chlorine down. Master Spas' manual notes that above pH 7.5 less than half of the free chlorine is in its active form. See hot tub pH.
• Total alkalinity buffers pH. Too low and pH bounces; too high and pH won't move. See hot tub alkalinity.
• Calcium hardness is dissolved calcium. Too much scales the heater and clouds the water; too little is hard on equipment and makes water foam. See calcium hardness.

Because each maker writes for its own tub and its own water care system. The widest gap is calcium: Hot Spring sets 75 to 150 ppm, Master Spas 180 to 250 ppm. Jacuzzi's 0.5 to 1.0 ppm chlorine applies only with its cartridge sanitizing system; without one, its manual points owners to the Pool & Hot Tub Alliance figures. Sundance allows a lower chlorine floor while its UV system runs.

Follow your own manual when it is tighter than CDC. Warranties lean on it: Bullfrog's says its limited warranty may be voided by damage from not maintaining the spa as the manual describes.

Alkalinity first, then pH, then calcium, and sanitizer last. Bullfrog says to bring alkalinity into range before touching pH, even if that pushes pH further off for a while. Hot Spring's start-up sets alkalinity and calcium, then pH. Master Spas adds chlorine only after pH, alkalinity and calcium are where they belong.

The wait between adjustments varies by manual. Master Spas says wait 15 minutes, test and adjust again; Bullfrog says 6 hours before retesting balancing chemicals. Use your maker's figure.

Before you get in, and a few times a week at least. Master Spas says test before and after each use, and with strips three times a week. Hot Spring says to test sanitizer daily in the first month to learn how much your household uses. CDC asks public hot tub operators to test chlorine and pH at least twice a day, which shows how fast a busy tub moves. More on test strips and kits.