Letter to the Editor of ?Threshold? ? July/August 2003 p10


Looking through a few recent issues of Threshold it seemed to me a pity that opportunity is not being taken to use the magazine to discuss technical views and queries. There used to be some Letters to the Editor, but this seems to have disappeared. In the hope of generating some useful discussion I offer the following thoughts:

How do we assess whether the materials we recommend and use in static sensitive areas are in fact suitable?

1)     The feature of practical interest is surface voltage

2)     The voltage on surfaces is limited by having an adequately fast route for charge to migrate of the material and away to earth. The time for charge migration needs to be short compared to the timescale of separation of rubbing action on the material surface

3)     Resistivity
does not measure how quickly charge moves over surfaces. It is particularly inappropriate for materials like clean room clothing that include surface conductive threads

4)     Measurement of charge decay tells you how quickly charge can migrate on materials. The method of measurement must give results that match behaviour observed with tribocharging

5)     Surface voltages can also be limited if surface charge experiences a high capacitance on a material. This effect is particularly helpful with materials like cleanroom garment fabrics that include core conductive threads. With such materials charge decay times can be very long so one might conclude the materials are not suitable - but if maximum surface voltages are low then can the materials be assessed as good ?

Yours sincerely, John Chubb