Boltek EFM-100, Installation And Operator'S Manual

Page: 20 / 46

C H A P T E R 2 - I N S T A L L A T I O N
14
thunderstorms routinely exceed 20 kV/m (the limit of the field mill) you should change to
a lower value Sensitivity Plug (after the storm and when it is safe to do so.)
Calculating a Precise Sensitivity Value
If you require a precisely calibrated field mill for scientific or other precision applications
you will need to calculate a precise correction factor, then order or assemble a custom
Sensitivity Plug.
The most accurate method for calculating an exact geometric correction factor is to
temporarily run two field mills simultaneously, a temporary reference field mill and the
permanent field mill. The temporary reference field mill is mounted flush with the surface
of the ground to prevent the electric field enhancement which occurs when objects are
mounted above the ground. The permanent field mill is mounted in its permanent
location. The correction factor needed is equal to the reference field mill reading divided
by the permanent field mill reading.
If two field mills are not available an approximate correction factor can be determined by
moving a single field mill from one location to another during a period of stable electric
field.
Figure 15: Correcting a Field Mill using a Reference Field Mill