Volumetric Collection Efficiency and Droplet Sizing Accuracy of Rotary Impactors

Measurements of spray volume and droplet size are critical to evaluating the movement and transport of applied sprays associated with both crop production and protection practices and vector control applications for public health. Any sampling device used for this purpose will have an efficiency of collection that is a function of the sampling device itself the droplet size of the spray being sampled, and the airspeeds under which the sampling is conducted. This study focuses on two rotary impaction devices, the Hock and the FLB samplers, that were evaluated under two droplet sized sprays and four airspeeds. The collected spray concentrations were compared to standard passive samplers whose theoretical collection efficiency was calculated and used to estimate the actual spray volume sampled. Additionally, droplet sizing information derived from image analysis of droplet deposits on the rotary impactor collection surfaces was compared to actual measurements of droplet size of the sampled spray cloud. Generally, overall collection efficiencies ranged from 2.5% to 20%, with the FLB being more efficient than the Hock and with lower efficiencies at higher airspeeds for both samplers. Comparison of the droplet sizing data showed that the FLB sampler tended to underpredict the D(V10) and D(V50) data, while the Hock tended to overpredict the D(V50) and D(V90) data.