Locating the Electric Field Mill (EFM) so that it best supports the desired application is paramount. As with any instrument or detector used to define the potential for lightning or its presence, a clear field of vision is best. Some specific elements that can affect readings of a Field Mill include; emissions from gasoline or diesel engines, overhead power lines, and the presence of excessive personnel foot traffic, all of which can carry resident charge or produce particles that influence EFM readings.
Of primary concern when determining the location of a Field Mill is not to locate it near artificial sources of charged ions, which can cause the unit to reflect higher than actual electrostatic field readings. As storms develop or approach the local area, a buildup in the electrostatic field takes place. It is this change that the Field Mill is designed to monitor. However, close proximity of the unit to potential sources of charged ions can distort the electrostatic field, requiring that such sources be given due consideration when determining the location for an EFM. Such sources of charge can be either manmade or natural objects, such as antennas and trees.
Taking care to place the sensor at a distance from these potential sources should be considered. The distance from such objects should be based on a ratio between the intended location and height of the Field Mill, and the distance to, and height of, the object.
In the case of manmade objects a ratio of 1:5 has proven to be acceptable in most cases. When dealing with natural objects such as trees, consideration must be given to the fact that their height may increase as they grow. Thus, a ratio of 1:10 would be more prudent.
If there is some uncertainty as to the best site for the unit, it is recommended that personnel refer to the manufacturers instructions for more details and/or contact the technical staff here at Mission Instruments for consultation. If desired Mission Instruments has available a contractor with over 20 years of field meteorological and applications experience with regards to the use of such devices and can assist in providing both a site survey and if needed a complete analysis of local weather patterns that affect your location.
Suitable hardware for mounting the device either on the ground or on a roof location has been designed and is available from Mission Instruments.