Digital Detector Array for the Study of Non-specifically Tagged Bees and Wasps

A 16-channel array of digital metal detectors was designed to record a magnetically permeable foil ‘tag’ at various locations of interest. The system was simple, inexpensive (less than US $150 material costs), and recorded data could be easily downloaded and analysed. The array required a low amount of power (less than 0·6 W) and the detectors induced small levels of magnetic field (magnetic induction less than 5 μT DC and less than 1·5 μT at 576 kHz). To assist search coil design, an empirical equation for the inductance of relatively flat air-wound coils, based on magnetic field theory, was presented and fit to data taken from coils ranging from 8·75 to 40 mm in radius. Using search coils with radii of 8·75 and 15·5 mm in a continuous 6-day trial of the prototype device, presence of a 25 mg tag was correctly recorded 126 times, with a single false positive. However, interference between channels prevented more than three channels from being active at once, and temperature dependences of the detectors prevented them from being used outside of a relatively narrow temperature range estimated to be approximately ±7°C about the temperature at which the detectors were tuned. Suggestions were made for improving the stability, sensitivity, and temperature compensation of the detectors for future prototypes.