Boltek EFM-100 v.2 - Manual (Page 20)

C H A P T E R 2 - I N S T A L L A T I O N
14
Sensitivity Connector
The sensitivity connector accepts a sensitivity plug to reduce the sensitivity of the field mill.
Several different values are provided so you can choose an appropriate value for your
installation. See Chapter 1: Intensification of Electric Field for a description of why
this is necessary.
Figure 15: Sensitivity Plugs
With no plug installed the field mill has a relative sensitivity of 1.0
Sensitivity plugs reduce the sensitivity to compensate for the enhanced electric field of a
field mill mounted above the surface of the ground. Lower numbers represent lower
sensitivities. For example, the 0.25X plug will reduce the sensitivity to ¼ of the field mills
normal sensitivity.
Fortunately most installations won’t require a precisely calculated correction factor. On a
clear day simply choose the sensitivity jumper which produces field values closest to the
normal fair-weather electric field of 0.1 kV/m. If electric field readings during
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.