A thermal probe for the in-situ measurement of groundwater flow rates in a borehole was calibrated in a vertical position. The probe is a long slender metal rod having a heat source along its entire length and a temperature sensor at its midpoint. When a constant quantity of heat is applied to the probe, the rise in temperature is inversely related to the rate of water flowing past the probe.

Full scale calibration of the probe was considered necessary because theoretical studies over-simplify the interaction between the heated probe and the horizontal flow of groundwater. The apparatus for calibration consisted of a central sand-filled chamber having a vertical hole lined with a well-screen in its center. The central chamber was hydrologically connected to upstream and downstream water reservoirs that were used to control the rate of water flowing in the central chamber.

Fifty eight calibration runs of the thermal probe were made; most of these tests were used to perfect the experimental techniques of data gathering and the design of the calibration tank. Fourteen of the calibration tests were selected to construct preliminary calibration curves. The selection of specific tests was based on a statistical analysis of the A1 coefficients from a third order polynomial fit of the experimental data. Final calibration curves were constructed on the basis of ten calibration tests. These curves show that if a temperature difference of 0.1 degrees Centigrade can be measured at the end of a two hour test, the probe is capable of distinguishing small differences in specific discharges when the flow exceeds 120 cm/day.

Project A-044-NMEX