Measurement of very slow flows in environmental engineering.

Abstract

Many of the flow metering techniques used in industrial applications have finite limits at slow fluid velocities in the order of 10 mm/s. By comparison, many environmental flow rates occur two or more orders of magnitude below this, examples being the rate of sap flow in plants, the percolation rate of rainfall into soil and through the landscape, flows in the benthic boundary layer of lakes, the movement of water through sandy river banks or in the swash zone of beaches, or the seepage rate of groundwater into river beds. Unlike well-defined industrial flow measurement systems, nature is extravagant with her variability. To counter this, sensor systems in environmental engineering have to be widely flung, inexpensive and highly matched. ‘Smart’ sensors must therefore be simple designs having calibration techniques that can be highly automated. Additionally, such sensors must be able to compute real data locally, apply temperature corrections, compensate for inherent non-linearity and integrate without fuss into environmental logging systems. This thesis describes the development of sensors and experimental techniques in five very slow flow rate applications in environmental engineering via three published papers and two papers in submission: -

¹Gravitational flows in a large stratified water body were identified using smart temperature strings; these sensors demonstrated new techniques for low-cost but high-precision thermistor temperature measurements, sensor temperature matching, the generation of complex algorithms within a simple sensor and a method for obtaining two-point calibrations for non-linear sensors. Field work with these sensor strings identified ‘short-circuiting’ of an urban reservoir during a storm event over the catchment which led to denser cold-water inflows moving along the bottom boundary layer of the lake.

²The movement of ‘wetting fronts’ in the soil below plants mobilizes toxic salts left behind in the soil profile by crop evapotranspiration processes that take up only fresh water. These problems are exacerbated in semi-arid areas under crops irrigated with brackish water. Automatic recording of soil salinity levels is possible using an instrument based on the combination of an EC (electrical conductivity) sensor with a platinum resistance temperature sensor within a funnel shaped ‘wetting front detector’ buried in the soil. These two combined sensors extend the usage of the low-cost 16-bit charge-balance analog-to-digital converter developed for use in stratification measurements.

³Measurement of sap flow in irrigated agriculture for determining when to irrigate crops was found to be of limited use for determining ‘when to water’ because the flow signal is masked by the plant’s genetically-coded regulatory systems. A new ‘double bridge’ analog control circuit for a self-heating thermistor was designed and described as a thermal diffusion sensor to study plant water status and the onset of irrigation stress in grapevines once sap flow had ceased. A laboratory experiment on a cut vine cane demonstrated that this thermal diffusion sensor was sensitive enough to track the response of the living cane to external forcing events that changed its plant water status.

⁴The same double-bridge thermistor control circuit was used to investigate the lower limits of very slow upward flow measurement for use in the funnels of automatic seepage meters designed to monitor groundwater flows into the bottom of rivers and lakes. Theoretical, CFD (computational fluid dynamics) and two different experimental studies showed that flows between 0.03 mm/s and 3 mm/s could be measured in the presence of buoyant thermal plumes from the self-heated spherical sensor in free water.

⁵A new type of null-buoyancy thermal flow sensor is described; it is designed specifically for the measurement of downward flows below 3 mm/s using a single thermistor. A typical application of such flow meter technology would be in the measurement of the hydraulic conductivity of soil to determine the rate at which rainfall can enter the landscape without run-off and erosion. The thermistor power dissipation is adjusted so that the upward thrust of the buoyant thermal plume from the warm thermistor sensor exactly counter-balances the downward bulk fluid velocity, resulting in flow stagnation at the sensor tip characterized by a corresponding local peak in the sensor’s temperature response. Power dissipation must increase with the square of an increasing flow velocity to maintain this null-point.

¹Skinner, A.J. and Lambert, M.F. (2006). ‘Using smart sensor strings for continuous monitoring of temperature stratification in large water bodies.’ IEEE Sensors, Vol. 6, No. 6, December 2006 ²Skinner, A.J. and Lambert, M.F. (2009). ‘An automatic soil salinity sensor based on a wetting front detector.’ IEEE Sensors, in submission, July 2009 ³Skinner, A.J. and Lambert, M.F. (2009). ‘A log-antilog analog control circuit for constant-power warm-thermistor sensors – Application to plant water status measurement.’ IEEE Sensors, Vol. 9, Issue 9, September 2009 ⁴Skinner, A.J. and Lambert, M.F. (2009). ‘Evaluation of a warm-thermistor flow sensor for use in automatic seepage meters.’ IEEE Sensors, Vol. 9, Issue 9, September 2009 ⁵Skinner, A.J. and Lambert, M.F. (2009). ‘A null-buoyancy thermal flow meter: Application to the measurement of the hydraulic conductivity of soils.’ IEEE Sensors, in submission, August 2009.