Elcometer 266 Manual Del Usuario página 41

APPENDIX B: CALCULATING THE CORRECT TEST VOLTAGE (cont)
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.
The Upper 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 refer to the graph on
page en-39. Alternatively, calculate the maximum voltage,
allowing for variations in the coating thickness. Note that 1 kV
per mm is equivalent to 25.4 V per mil (thou).
Note: This method is only suitable if the dielectric strength values were
determined for a DC voltage.
Experiment - Touch the probe on an unimportant area of the work

piece. Increase the voltage slowly and steadily until a spark
passes through the coating. Make a note of this voltage - it is
the upper voltage limit. (The dielectric strength can be calculated
by dividing this voltage by the coating thickness).
Tables and formulae - from established Codes of Practice, e.g.

NACE and ASTM. Examples of tables are given below (see
Table 1, Table 2 and Table 3). See also Section 9.1 'Automatically
Setting the Voltage' on page en-17 and Appendix A 'Standards' on
page en-35.
Once the lower and upper voltage limits have been established,
set the voltage approximately halfway between the two values.
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en-40