GroPoint TDT System
The precision TDR technology used in MoisturePoint has led to the development by ESI of the "Gropoint" TDT system. Both methods measure the time an electromagnetic wave propagates along a given length of a transmission line in the soil, which is determined by the moisture content of the soil. TDR is based on a reflected wave whereas with TDT the wave or pulse is measured at the other end of the line from the transmitter. With Gropoint, a connection is required at each end of the transmission line, and this signal is simpler and less expensive to analyse than a TDR signal.
The Gro•Point. TDT wave will measure the differences in soil moisture up to 4 inches(10 cm) from the rods along all sides. A typical Gro•Point. measurement will interrogate an oval shaped area approximately 6 inches (15 cm) high X 9 inches (23 cm) wide X 15 inches (37 cm) long. The soil closely surrounding the rods will have a greater influence on the soil moisture reading than the soil three inches (8 cm) away from the rods. The soil must be in direct contact with the rods to ensure an accurate reading.
The sensors can be installed vertically or horizontally. The horizontal position, is the preferred method for a shallow sensor and vertical for sensors deeper than 12 inches (30 cm). With horizontal orientation, soil can be more easily packed around the elements, and this method also gives a more precise measurement of soil moisture in a specific range. In horizontal installation, the sensor will be measuring the soil moisture in an area about 3 inches (7 cm) above and below the sensor.
With vertical placement, the sensor will report average soil moisture over its entire active length of 16 inches (40 cm). This may be the desired orientation for covering the majority of the root zone of a crop, with one sensor.
There are three types of Gropoint probe, each one calibrated to work in a specific range of soil conductivity. ESI’s in-house technical studies have indicated a difference in coating on the probe is required for a highly conductive/saline environment in order to achieve a high degree of accuracy independent of soil type.
Accuracy & Calibration
Gro•Point. has an accuracy ± 1% (±0.01m3m-3) volumetric soil moisture in controlled laboratory conditions, and is factory calibrated for most agricultural soils. In field applications, accuracy may slightly decrease due to the inevitable heterogeneity of soil texture, soil compaction, moisture and fluctuation in soil temperature. The accuracy may also decrease in difficult soil conditions (higher clay and salinity content). In normal conditions, Gro•Point. will maintain its accuracy from permanent wilting through field capacity in Sandy Loam through Clay soils with less than 60% clay particles. Under moderately saline conditions. Gro•Point will maintain its accuracy up to 6ds/m.
Other Gropoint system options
The system can then be read by using either a battery-operated hand-held reader unit, or can be logged into a 3 or 4-channel logger. This datalogger can then be downloaded directly by using a laptop, or through the use of a field data shuttle, which can then itself be connected to a PC or laptop and downloaded. Soil and air/canopy temperature measurement can also be built into the logger.
Additional development has also recently taken place with the Gropoint system. The use of radio telemetry for transmitting the logged data is now available, incorporating the use of hopper-style radios attached to each datalogger unit and the use of a base station. The data can then be transmitted via land-line, cellular or further radio link to the required location. Software can be set up on the client’s computer for easy viewing of the data and can also be integrated with meteorological data to provide a more complete picture of all environmental variables.