Elcometer 280 Guia Del Usuario página 11

10 MANUALLY CALCULATING THE TEST VOLTAGE (continued)
The voltage at which a particular thickness of material breaks down is
termed the dielectric strength. This is usually expressed as the
voltage per unit distance, e.g. kV/mm (kV/inch). Its value depends on
the type of applied voltage (AC, DC or pulsed), temperature and
thickness.
The dielectric strength of coating materials usually lies in the region
of 10 kV/mm to 30 kV/mm (250kV/inch to 750kV/inch). The dielectric
strength of air ranges from 1.3 kV/mm to 4 kV/mm (33kV/inch to
100kV/inch).
Establishing the lower voltage limit
The lower limit for effective operation is that required to breakdown
the thickness of air equivalent to the coating thickness. The
breakdown voltage of a given thickness of air varies with humidity,
pressure and temperature.
If the coating thickness is known, or can be measured, the lower
limit value can be determined from the breakdown voltage for air at
that dry film thickness.
If the coating thickness is not known then the minimum value has to
be established experimentally. Reduce the voltage setting to minimum
and position the probe over an unprotected area of substrate at the
normal height of the coating surface. Increase the voltage slowly and
steadily until a spark is produced. Make a note of this voltage - it is the
lower voltage limit.
Establishing the upper voltage limit
The upper voltage limit may be determined by:
The job specification - if available and a test voltage is stated.
The dielectric strength - if specified for the applied coating.
Measure the thickness of the layer and determine the voltage by
multiplying the dry film thickness by the dielectric strength.
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