Evaluation of a warm-thermistor flow sensor for use in automatic seepage meters
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(Accepted version)
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2009
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Skinner, A.
Lambert, M.
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IEEE Sensors Journal, 2009; 9(9):1058-1067
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Andrew J. Skinner and Martin F. Lambert
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Abstract-A warm-thermistor flow sensor is evaluated for use in an automatic seepage meter measuring very slow groundwater inflows into open water bodies. A novel control circuit allows a single self-referencing thermistor to operate with a constant heat output of 36.8mW, while monitoring the thermistor's internal temperature T<inf>S</inf>and T<inf>F</inf> under still-water and flow conditions respectively. The resultant temperature difference T <inf>S</inf>-T<inf>F</inf> is the output signal from the instrument. This device is particularly sensitive to very slow fluid flows in the range 0.03 to 3 mm/s, where buoyancy problems have traditionally prevented the use of warm-thermistor flow meters. For flow speeds below 3 mm/s the sensor response was shown in the laboratory to be nearly linear with no offset term. Two flow-calibration setups were investigated; a precision plunging-probe apparatus and a single-step flow calibration system based upon a Hagen-Poiseuille flow regulator and a vertical standpipe. A numerical (CFD) model of the spherical thermistor agreed well with the two experimental calibration procedures over the flow range between 0 and 3 mm/s. The theoretical model - based on the Peclet number - fits the CFD model well between 3 and 100 mm/s, but does not hold true in the buoyancy range below 3 mm/s. For a seepage meter funnel having a bell-to-throat area ratio of 2964, groundwater flow velocities as low as 0.01 μm/s (0.9 mm/day) could be measured using this sensor. © 2009 IEEE.
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© 2009 IEEE