Oxidation-reduction potential is what most commercial hot tubs actually use to confirm safety. Here is what it tells you and when home owners should care.
ORP (oxidation-reduction potential) measures how strongly the water can oxidise things, in millivolts. It is a single number that summarises the overall sanitising power of the water, accounting for chlorine level, pH, and a dozen smaller factors. Above 650 mV, the water is considered safe. Below 600 mV, it is not. Home owners do not need to measure it routinely, but understanding it explains a lot.
Free chlorine alone does not tell you the whole story. 3 ppm of free chlorine at pH 8.0 has much weaker actual sanitising power than 3 ppm at pH 7.4. ORP captures that.
Commercial pools and hot tubs use ORP as the primary measure for compliance because it integrates everything that matters into a single value.
At pH 7.2, free chlorine is roughly 60% in its active form (hypochlorous acid, HOCl). At pH 7.8, it is around 25%. At pH 8.5, it is below 5%.
The test strip says 3 ppm regardless. But the water at pH 8.5 is doing about a fifth of the disinfection work of the same chlorine at pH 7.2. ORP would show this. Test strips for chlorine alone would not.
For a domestic UK hot tub, no. Keep your pH between 7.2 and 7.6 and your free chlorine between 3 and 5, and your ORP will be in the right range.
For a commercial setting (rental, gym, hotel), yes. ORP-based controllers cost about 400 pounds and pay for themselves in chemical savings inside a year.
For a heavily-used domestic tub where you want to optimise, a basic ORP pen is around 50 pounds and is fascinating to play with for a month before you settle into trusting your pH and chlorine readings again.
No. TDS measures dissolved solids in mg/L, ORP measures sanitising potential in millivolts. They are unrelated, though high TDS slightly suppresses ORP.
It cannot be both. One of the readings is wrong. Recalibrate the meter or change strip batches. ORP and free chlorine should track together broadly.