Oxidation-Reduction Potential
The water's actual sanitising power — the truest single measure of whether the pool is safe to swim in.
Good to know
There is no chlorine reading here because the ICO cannot take one. It has no chlorine sensor — it measures ORP, the water's oxidising power, which is what chlorine actually delivers. That is arguably the better number: ORP reflects sanitiser that is working, whereas a chlorine test only tells you how much is present. Chlorine locked up by cyanuric acid still shows on a test strip while doing very little. ORP cannot be converted to a reliable ppm figure, because pH, cyanuric acid and temperature all shift the relationship — so use a test kit when you need an actual chlorine number, and use this to know whether the water is sanitising. Your free-chlorine target on the setup page is what sizes the doses recommended here.