What do hot tub test strip readings mean?
The short answer
Along with a sanitizer reading in its range. The other pads on a strip help explain why those two drift.
A test strip has a pad for each thing it measures. Dip it, wait the time on the bottle, and match each pad to the chart. These are the ranges most hot tub guidance uses.
How acidic the water is. Low stings eyes and wears on equipment; high makes sanitizer work poorly and water cloudy.
What holds pH steady. When it is low, pH swings; when it is high, pH is hard to move.
The chlorine available to sanitize. Public health guidance gives 3 ppm as the minimum for a hot tub.
The same job in a bromine tub.
Calcium in the water. Low can wear on surfaces; high leaves scale.
Cyanuric acid, which some chlorine products add. It builds up over time and only a drain lowers it.
If your strip shows a color, not a number
Some systems, mineral and cartridge sanitizers among them, come with strips where a pad is only compared with one printed color. Read those as the bottle says. The ranges above are for strips that give numbers.
What to do with a reading
This page does not tell you how much of anything to add, and neither does TubNudge. The product you use to adjust a reading has the amount for your tub's size on its label. Change one thing at a time, let the water circulate, and test again.
Your sanitizer system and your tub's maker may give their own target ranges. Where they differ from these, theirs come first.
This is general guidance. Your hot tub's owner's manual comes first, and so do the instructions on your products.
Sources: CDC: Controlling Legionella in hot tubs; CDC: Operating public pools, hot tubs and splash pads.