Errors in determining the flow rate of Hirst-type pollen traps

Standardisation of methods of pollen monitoring networks is vital for data quality. In pollen monitoring networks in Europe, the Hirst-type trap is standard. Hirst traps are calibrated with handheld rotameters. We detected a systematic error in the flow rate calibrated by these standard handheld rotameters. We measured the flow rate of 19 Hirst traps from three commercial brands during calibration but also during monitoring. We used three different rotameters supplied by the manufacturers of the traps, respectively. The actual air flow rate was measured using an electronic heat anemometer with negligible air flow resistance. After calibration to 10 l/min, the rotameter was removed, which led to a significant increase in the flow rate in the range of 10.5-17.2 l/min, a systematic error between 5 and 72%. No significant difference was found between the different commercial trap brands. The analysis revealed that the error depended on the type of the rotameter and the individual trap. The error may be explained by the additional air flow resistance of each rotameter. The total resistance of the system-trap plus rotameter-is higher during calibration when the rotameter is held on the inlet compared to the routine monitoring without the rotameter. Depending on the characteristic curve of the suction pump in the trap (fan), the air flow rate increases to values considerably higher than 10 l/min. Thus, monitoring is done under a higher flow rate than that was calibrated. In order to obtain comparable data within a monitoring network, a solution for correction of this systematic error seems advisable, preferably in cooperation with the manufacturers.