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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 36.8 mW, while monitoring the thermistor's internal temperature TS and TF under still-water and flow conditions respectively. The resultant temperature difference TS-TF 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 mum/s (0.9 mm/day) could be measured using this sensor.