Decreasing Measuring Errors of a Thermal Groundwater Flowmeter

Abstract

The probe of a commercial thermal groundwater flowmeter was evaluated in the laboratory to determine the causes of apparent inaccurate results from field experiments conducted under high groundwater velocities in a gravel and coarse sand soil. The test was performed with water velocities from 2.3 to 103 m/day. Errors as large as 40% arose in the magnitude and direction of velocities within the measuring range specified by the manufacturer. The poor cosine response of the thermal sensor axes was found to be a major source of errors, because the accuracy of the velocity vector depends strongly on the angular response of the thermal sensor axes. This was partially corrected with a set of coefficients determined from laboratory tests.