Free Chlorine Lower Than Total Chlorine. Total chlorine is the sum of free chlorine and combined chlorine. Total chlorine = free chlorine + combined chlorine.
If free chlorine measurements are higher than total chlorine using a dpd method, check the alkalinity and ph of the sample. Otherwise, algae can grow faster than chlorine can kill it (regardless of phosphate and nitrate levels). Like chlorine, chloramine will disinfect pool water.
The level of total chlorine should always be greater than or equal to the level of free chlorine.
Your total chlorine level is high (and your free chlorine is low) but ineffective due to a “chlorine lock”, which happens when too much cyanuric acid is added to the pool. When adding the dpd reagent, a ph of 6.3 should be observed. Subtract the free chlorine measurement from the total chlorine measurement to calculate the amount of combined. Why do you want to measure free chlorine as opposed to total chlorine?
