Fiber Optic Relative Humidity and TDR Sensors for the Cone Penetrometer


Soil moisture content is the amount of water in soil when it is not saturated. Soil pore pressure is an indicator of soil density and its potential to hold water. Both are critical parameters for modeling the fate and transport of contaminants in the subsurface. Sandia National Laboratory has engineered two sensors for cone-penetrometer technology that permit real-time, continuous measurement of these parameters: the Time Domain Reflectometry (TDR) sensor and the Fiber Optic Relative Humidity (RH) sensor.

The TDR sensor employs an electromagnetic pulse technique to quantify moisture content. It measures the time for a pulse to travel between the beginning and end points of the probe and converts this transit time to a distance. As the moisture content of the soil increases, the distance increases.

The RH sensor quantifies capillary pore pressure in dry unsaturated soils. Two optical fibers measure optical energy across a porous polymer sensor. When water vapor is present, the optical energy is altered. The pore pressure is calculated from this relative humidity measurement.

Limitations and Concerns

No limitations or concerns have been identified.


These sensors are used to characterize moisture in soil. They are not contaminant specific.

Technology Development Status

Efforts to develop these sensors are pilot projects.

Web Links

Other Resources and Demonstrations

See SCAPS and Cone Penetrometer.