Existing models for calculating the minimum effective dosage for scale control have been applied to industrial and oil field scale control treatment optimization since the 1970s. Standard correlations are routinely used in developing the models. The models typically apply to a single inhibitor. There is a driving force limit for each inhibitor, above which scale control cannot be achieved regardless of the inhibitor dosage. Knowing the upper limit is critical for selecting the optimum treatment program and in specifying control limits for a system such as an open recirculating cooling tower or membrane system. Limits for individual inhibitors have been well documented. Studies have been conducted to determine the impact of blending inhibitors on the upper driving force limit. Upper driving force limits, as expressed by calcite saturation ratio, were measured for calcium carbonate inhibition by individual inhibitors and combinations. Results were eva