By Patricia Fitzgerald
This myth likely started because orthotolidine (OT) solutions have a longer shelf life than liquid N, N-diethyl-p-phenylenediamine (DPD) reagents. If the pool/spa is sanitized with bromine, an OT test is fine to use; however, OT is not the best choice if the water is sanitized with any form of chlorine. This is because chlorine users must keep track of how much chemical is in its strongest germ-killing form and how much of it is past its prime.
The most effective form of chlorine is referred to as free chlorine. When free chlorine has done its job eliminating ammonia and nitrogen compounds introduced to the water by bathers, chloramines (combined chlorine) are formed.
Besides being a weaker, slower-acting disinfectant than free chlorine, chloramines give off a strong, unpleasant odour, which can irritate bathers, even at low levels. Together, free chlorine and combined chlorine make up the total chlorine residual.
Read the full article: Busting Water Maintenance Myths