How ORP Testing Works

• The ORP meter uses an electrical charge between two poles to measure resistance in the water. This resistance is due to the exchange of electrons when chlorine oxidation takes place. A good resistance reading for a pool is in the range of 600 to 700 mV. The meter also adjusts for water temperature and pH. Balancing free chlorine residual and pH range are two of the most important factors in good chemical balance. If the pH is either too high (above 8 or too low (below 6.8), chlorine becomes less effective, even if there is plenty in the water.
  
  

What an ORP Reading Tells You

• An ORP reading doesn't tell you how much chlorine or pool acid is in the water. Rather, it tells how effective the chlorine is in oxidizing organic materials and sanitizing the pool. This is the whole purpose of having chlorine in the pool, so an ORP reading is a better measurement than one that merely measures chlorine quantity. For a home pool owner, knowing there is an effective level of chlorine in the water should be the primary goal of testing.