Answer:

OTO measures only total chlorine and therefore you will not know how much is free and how much has already been used up. Our test strips us the FACTS method (syringaldazine) that is specific to free chlorine and will not react to the combined chlorine present so that you can determine how much is available to sanitize/oxidize. DPD is the same way and could also be used to determine free chlorine. Which of these methods you prefer is up to you, but you should definitely switch to a method that will tell you how much is free available chlorine.