The link between the collection efficiency of a Venturi scrubber and its operating pressure drop was first established by Semrau in 1958. Friedrich Loffler used it as a starting point for the predictive model for which he won the Filtration Society Gold Medal in 1980. The approach taken in that study was not widely followed by subsequent modellers but, in conjunction with previous work, it does lay the foundation for a useful design approximation which allows grade efficiency curves to be predicted from empirical data determined at other operating conditions. In this paper, a range of detailed grade efficiency measurements conducted on a full-scale plant are reported. The curves are shown to depend only upon pressure drop across the scrubber, there being no independent effect of gas velocity, liquid to gas ratio, or dust size distribution. The implications of this for design are discussed and the grade efficiency prediction technique is demonstrated.