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A null-buoyancy thermal flow sensor is described; it is designed specifically for the measurement of very slow downward fluid flows in a vertical pipe in which a glass-rod thermistor is concentrically located. Sensor power dissipation in this thermistor is adjusted so that the upward thrust of the buoyant thermal plume from the warm thermistor sensor exactly counterbalances the downward bulk fluid velocity. This results in flow stagnation at the sensor tip characterized by a local peak in the sensor's temperature. Experimental results agreed with CFD and engineering models to suggest that the required power level needed to counterbalance the downward velocity at this null-buoyancy point depends upon the square of the velocity over the narrow velocity range between 0.25 mm/s and 2.5 mm/s. The flow sensor has been designed to have the potential to measure the infiltration rate of water into different soil types by applying it to a simpler form of the common disc permeameter (tension-infiltrometer) used to determine the hydraulic conductivity of soils.