Moisture [Soil]
Moisture Written Description
Moisture Sensing at MnROAD (pdf)
EW Sensor
An "EW" sensor measures volumetric water content in soil. It is one of the 3 parts of an EC sensor. See the EC sensor for more details. Data is automatically collected from each of the three sensors every 15 minutes. Soil water content is determined by an EW sensor using capacitance/frequency domain technology to measure the dielectric constant of the soil.
Manufacturer: ECHO, ECH2O-TE, and 5TE Soil Moisture sensors manufactured by Decagon.
Location and Quantity: EW sensors have been installed in several test cells, typically at multiple depths to measure moisture in pavement base and subgrade layers.
Current Status: Early EW sensors did not function well. Later models continue to function.
MnROAD Database Table: Data is being collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database tables: MNR.EW_VALUES, MNR.EC_VALUES, ET_VALUES.
HD Sensor
An "HD" sensor uses a heat dissipation method to indirectly measure soil water matric potential. Data is automatically collected from each sensor once every 12 hours. A Heat Dissipation (HD) Matric Water Potential sensor consists of a cylindrically-shaped porous ceramic body, with a heating element and thermocouple inside. Soil water matric potential is determined using the principals of thermal conductivity, heat dissipation, and water content.
Manufacturer: Campbell Scientific model CS229.
Location and Quantity: HD sensors were installed in test cells 27 and 28.
Current Status: HD sensors continue to function.
MnROAD Database Table: Data is being collected automatically every 12 hours over the life of each sensor. Data can be found in MnROAD database tables: MNR.HD_VALUES.
NP Access Tube
An "NP" was not an electronic sensor, but a vertical access tube that allowed a neutron moisture sensing probe to be lowered within various soil layers to measure soil moisture content. The Neutron Access (NP) Tube consisted of a 2 inch diameter, 8 foot long, PVC tube vertically installed into various base and subgrade layers. A neutron moisture sensing probe was then periodically lowered to measure the moisture content at various layers in a pavement test cell. When the fast neutrons produced by the probe collide with hydrogen present in the soil, they lose much of their energy. The detection of slow neutrons to the probe then allows an estimate of the amount of hydrogen present. Since water contains two atoms of hydrogen per molecule, this therefore gives a measure of soil moisture.
Manufacturer: The PVC tubes were fabricated by MnROAD staff. The neutron probe device manufacturer was Troxler.
Location and Quantity: NP access tubes were installed in many of the original test cells.
Current Status: NP readings were discontinued shortly after they were begun. The tubes filled with water and it was found PVC was a material not suitable for such measurements.
MnROAD Database Table
No data has been loaded into the MnROAD database.
RE Sensor
A "RE" sensor measures volumetric water content using time-domain measurement methods. Data is automatically collected from each sensor once every 12 hours. A Water Content Reflectometer (RE) sensor consists of two parallel stainless steel rods (attached to electronics) that can be inserted into a soil and used to measure the dielectric constant, which can then be converted to volumetric moisture content. This particular sensor does not require the typical reflectometer (cable tester) used for similar sensors.
Manufacturer: Campbell Scientific model CS616.
Location and Quantity: RE sensors were installed in test cells 19, 27 and 28.
Current Status: RE sensors continue to function.
MnROAD Database Table 14
Data is being collected automatically every 12 hours over the life of each sensor. Data can be found in MnROAD database tables: MNR.RE_VALUES.
Time Domain Reflectometer - TD
A "TD" sensor measures the moisture content of base and subgrade materials. Data was collected manually from 1993-1998, but has since been collected automatically every 15 minutes. The Time Domain Reflectometer (TD) [or TDR] sensor measures changes in the dielectric constant of the soil due to changes in the soil moisture content. The device is excited by a high frequency electromagnetic pulse, which is sent down 2 or 3 parallel wave-guides or prongs. The velocity of the reflected wave (located at its "apparent length [LA]") is measured and correlated to soil moisture content.
Manufacturer: Original TD sensors were designed by MnDOT researchers and fabricated by a consultant hired during the original construction of MnROAD. More recent installations have used Campbell Scientific models CS605, CS610, or CS645.
Location and Quantity:TD sensors were originally placed in each of the test cells in the outer wheelpath and/or at the centerline of the roadway. Some cells had replicated installations that included additional TD sensors in the shoulder area.
TD sensors are typically installed in a vertical stack of sensors to capture the moisture content at various layers below the pavement surface.
Current Status: Data from the TD sensors was manually collected from 1993-1998. The operational status of the original TD sensors is unknown at this time. Since 1999, many new TD sensors have been installed and connected to an automated data collection system.
MnROAD Database Table
Data can be found in MnROAD database tables: MNR.TD_VALUES.
Moisture Block - WM
"WM" sensors at MnROAD have been used to measure both soil moisture content and frost depth in base and subgrade layers. Data is automatically collected from each sensor every 15 minutes.
Description 15: The Watermark (WM) sensor measures changes in electrical resistance due to changes in the soil moisture content.
Manufacturer: Watermark 200-x manufactured by the Irrometer.
Location and Quantity: WM sensors were originally placed next to the TD sensors in each of the test cell. This included replicated installations in several test cells.
Similar to TD sensors, WM sensors were installed in a vertical stack of 7 sensors to capture the moisture content at various layers below the pavement surface.
Current Status: These sensors perform very well, with many of the original WM sensors still functioning. These sensors continued to be installed into new test cells to measure moisture and frost depth.
MnROAD Database Table
Data was been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database table: MNR.WM_VALUES.