Graham Hearn

Development and application of an electrostatic microprobe

Abstract Powders with a wide particle size distribution often display bipolar tribocharging characteristics. Also, triboelectrification of solid insulating surfaces generally produce a non-uniform charge density. The standard Faraday cup method of measuring charge is incapable of detecting bipolar charging and non-uniformity in surface charge density. An electrostatic microprobe has been developed which is capable of detecting charge on individual particles as small as 50μm as well as resolving charge density variations on continuous surfaces. This paper describes the microprobe and the results of its application.

Characterisation of electrostatic discharges from insulating surfaces

This paper investigates how the presence of a grounded backing plane influences the electrostatic discharge (ESD) characteristics from a charged polystyrene sheet. High-speed digital sampling techniques are used to capture discharge current waveforms via a specially designed current probe, the resultant scans giving clear indication of how the surface charge distribution contributes to the ESD. Data obtained indicate that the presence of a grounded backing plate close to a charged plastic surface has a significant effect on the resultant discharge from the surface for a given initial surface charge density. The results suggest that the presence of an earthed metal backing plate can mean the difference between an incendiary and a non-incendiary ESD. This has implications for the design of plastic components for use in sensitive flammable atmospheres.

The application of electrostatics in forensic science

Abstract During forensic science investigations, it is necessary to segregate materials of interest such as minute fragments of paint and glass from other debris recovered from clothing. Such debris typically consists of short fibres, particulate metal, sand, soil, etc. This paper describes the development of electrostatic techniques which principally include: 1. (i) A high resolution electric field probe to identify, by virtue of their electrostatic properties, the individual fibres or particles encountered in a forensic investigation. 2. (ii) The resolution of mixed debris into constituent components by manipulation in a precisely controlled variable frequency electric field. The paper goes on to describe the development of a novel prototype separator which by using a combination of electrostatic and mechanical forces enables the forensic scientist to achieve rapid and efficient recovery of small fragments of household and vehicle paint from mixed forensic debris.

Development of a novel instrument for use in electrostatic powder coating processes

Abstract This paper describes the development of a diagnostic instrument capable of evaluating the electrical parameters vital in ensuring optimised performance of electrostatic powder coating. The kit consists of an electrometer, Faraday cup and high voltage detector probe and is capable of measuring the electric charge on the powder, the gun voltage at the corona point and the current flow to the workpiece, all of which are critical parameters affecting plant performance. It is also capable of detecting the onset and severity of the very important phenomenon of back ionization.