- Current observations
Optical fiber distributed temperature sensing network
- John Selker
- Barbara Bond
Fiber-optic distributed temperature sensing (DTS) has great potential for spatial monitoring in hydrology and atmospheric science. DTS systems have an advantage over conventional individual temperature sensors in that thousands of quasi-concurrent temperature measurements may be made along the entire length of a fiber at 1 meter increments by a single instrument, thus increasing measurement precision and spatial representativeness of the measurements. However, like any other temperature sensors, the fiber temperature is influenced by energy exchange with its environment, particularly by radiant energy (solar and long-wave) modulated by wind speed.
The objective of this research is to perform an energy-balance based calibration of a DTS fiber system that will reduce the uncertainty of air temperature measurements in open and forested environments.
To better understand the physics controlling the fiber temperature reported by the DTS, alternating black and white fiber optic cables were installed on vertical wooden jigs inside a recirculating wind tunnel. A constant irradiance from six 600W halogen lamps was directed on a two meter section of fiber to permit controlled observations of the resulting temperature difference between the black and white fibers as wind speed was varied.
The net short and longwave radiation balance of each fiber was measured with an Eppley pyranometer and Kipp and Zonen pyrgeometer. Additionally, accurate air temperature was recorded from a screened platinum resistance thermometer, and sonic anemometers were positioned to record wind speed and turbulence. Relationships between the temperature excess of each fiber, net radiation, and wind speed were developed and will be used to derive correction terms in future field work.
Preliminary results indicate that (1) Temperature difference between the fiber and temperature reference and fibers themselves increases with increased radiation and decreases with increased wind speed, (2) the temperature difference between dry bulb and fiber is likely to be several degrees in conditions typical for forests, and (3) twisted fiber temperatures are not significantly different from each other – independent of albedo effect due to thermal conduction between fibers.
Subsequent work will require field verification to confirm that the observed wind tunnel correction algorithms are applicable in both open and forest canopy settings. Our ultimate goal is to use atmospheric DTS measurements of 3D temperature fields in a small steep-walled forested watershed to gain a better understanding and rigorous description of the processes governing air circulation (cold air drainage etc) in the canopy. Such knowledge will assist in the interpretation of observed biological responses.