Solar radiative heating of fiber‐optic cables used to monitor temperatures in water

Abstract

[1] In recent years, applications of distributed temperature sensing (DTS) have increased in number and diversity. Because fiber-optic cables used for DTS are typically sheathed in dark UV-resistant materials, the question arises as to how shortwave solar radiation penetrating a water column influences the accuracy of absolute DTS-derived temperatures in aquatic applications. To quantify these effects, we completed a modeling effort that accounts for the effects of radiation and convection on a submersed cable to predict when solar heating may be important. Results indicate that for cables installed at shallow depths in clear, low-velocity water bodies, measurable heating of the cable is likely during peak solar radiation. However, at higher velocities, increased turbidity and/or greater depths, the effects of solar heating are immeasurable. A field study illustrated the effects of solar radiation by installing two types of fiber-optic cable at multiple water depths (from 0.05 to 0.8 m) in the center and along the sidewall of a trapezoidal canal. Thermistors were installed at similar depths and shielded from solar radiation to record absolute water temperatures. During peak radiation, thermistor data showed small temperature differences (∼0.003°C-0.04°C) between depths suggesting minor thermal stratification in the canal center. DTS data from cables at these same depths show differences of 0.01°C-0.17°C. The DTS differences cannot be explained by stratification alone and are likely evidence of additional heating from solar radiation. Sidewall thermistor strings also recorded stratification. However, corresponding DTS data suggested that bed conduction overwhelmed the effects of solar radiation.

Practical considerations for enhanced-resolution coil-wrapped distributed temperature sensing

“…Vercauteren et al, 2008;Oldroyd et al, 2013). Colour of the cable coating and direct exposure to solar radiation can have influence on the temperature measurement up to several degrees (De Jong et al, 2015), and is also relevant underwater (Neilson, 2010). …”

“…Others have signalized the effect of heat conduction between a DTS cable and bedding material in streams (O'Donnell Meininger and Selker, 2015), and inaccuracies in air and water temperatures due to solar radiation (Neilson et al, 2010;De Jong et al, 2015). The auxiliary constructions of coil-wrapped DTS are expected to deteriorate the measurement accuracy in a similar fashion.…”

Investigating riparian groundwater flow close to a losing river using diurnal temperature oscillations at high vertical resolution

“…During sunny days, solar radiation caused warming of the fiber-optic cables exposed to air and very shallow river water as reported by Vogt et al (2010b). However, the impact of solar radiation is similar for standard temperature sensors (Neilson et al, 2010).…”

Heat Transfer in the Environment: Development and Use of Fiber-Optic Distributed Temperature Sensing

“…Fiber-optic cables, as well as other thermal sensors, can absorb solar radiation and monitor higher temperatures than those of the surrounding ambient. This issue is more important when air or water velocities are small, and when the magnitude of solar radiation is large (Neilson et al, 2010). The effect of solar radiation absorption on cables can be reduced using reflective coatings or shielding the cable (Suárez et al, 2011).…”

“…Other environmental applications related with stream dynamics include the processes that controls thermal regime of saltmarsh channel beds (Moffett et al, 2008), development of distributed stream temperature models (Westhoff et al, 2007), response of stream temperatures in different riparian vegetation (Roth et al, 2010), and quantification of heat retardation along streams . The effects of radiative heating on fiberoptic cables used to monitor water temperatures have also been evaluated (Neilson et al, 2010;Suárez et al, 2011). Early DTS work in hydrogeology was focused on thermal monitoring of geothermal wells and boreholes (Hurtig et al, 1994;Förster et al, 1997).…”