VLF means Pretty Very low Frequency and is similar engineering used in detectors constructed for coins, jewelry, and relics. VLF detectors broadcast at frequencies measured in kilohertz ranging from 3 kHz to 70 kHz. Listed below are five main reasons why you may decide on a VLF about a PI http://opensource.platon.org/forum/projects/viewtopic.php?p=12846136#12846136