The overall length of the probe depends on the number and length of profiling segments per probe. The MP-917 uses a transmission line embedded in the soil as a probe, and transmits a step pulse into the probe. By measuring the time it takes for the pulse to travel across a small portion, or segment of the probe, the moisture content of that segment can be determined. A diode is an electrical device that conducts current in only one direction. By recognizing this property, diodes can be used to introduce short circuits across a transmission line.
Diodes separate the probe into discrete segments allowing single probe soil profiling.
The positioning of the diodes at the start and end of each segment offers the ability to more precisely measure the time delay, and thus more accurately calculate volumetric moisture over each segment. Under most soil conditions, the probes are inserted by first making a pilot hole with a pilot rod of approximately the same cross sectional dimensions as the probe, and a pilot rod driver. The pilot rod makes it possible to penetrate difficult soils and displace rocks prior to insertion of the probe. This initial step prevents excess stress being placed on the probe which, upon extraction of the pilot rod, is inserted into the hole. The technique just described will vary with soil types - sandy soil may allow direct manual insertion of a 30 cm probe, while rocky soils require substantial effort with the pilot rod and driver.