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,