Dardo Oscar Guaraglia

Numerical and experimental study of a thermal probe for measuring groundwater velocity

A thermal method for a rapid measurement of groundwater velocity, particularly in aquifers with preferential flow where groundwater velocities over tenths of (m/d) are expected, was studied. Some instruments for measuring groundwater velocity are based on the application of heat. Those consisting of a central heater surrounded by several thermistors seem adequate for the above purpose, but their measuring range lies below 30 md (meters per day) and there are few works about their theory. Based on the diffusion-convection-dispersion equation, an electrical model is proposed for representing this type of instruments and a theoretical study is presented in an attempt to extend their measuring range. The model can be excited with any shape of power or temperature signals, allows the signals to be feedbacked for controlling the heaters excitation, and includes the thermal features of the heater and thermistors. The model was validated through laboratory tests with velocities of up to 100 md and extrapolated up to 1000 md. Numerical results suggest that exciting the heater with constant temperature allows the measuring range of these probes to be extended.

A Dual-Mode Conditioning Circuit for Differential Analog-to-Digital Converters

Several devices such as load cells and pressure sensors, among others, provide differential outputs. Given that present high-resolution analog-to-digital converters (ADCs) have differential inputs, fully differential (F-D) circuits are required to adapt the sensor output to the ADC input. This paper proposes an F-D conditioning circuit that allows adjusting both differential- and common-mode signals to the levels required by the ADC. A design example is presented, and a prototype was built and tested. It transforms a differential input signal of ±25 mV with a common-mode voltage of 5 V to a differential output signal of ±5 and 2.5 V, respectively. It shows an input-referenced peak-to-peak noise of 120 nV, which results in a 112-dB dynamic range (18.7-bit noise-free resolution) for a signal bandwidth of 10 Hz.

A Method to Improve Flow-Velocity Measurements From an Array of Partially Cosine Response Sensors

A groundwater flowmeter whose working principle is based on an array of thermal sensor axes was studied to analyze the errors introduced when the angular response of the sensor axes is partially cosine and varies with flow velocity. It is shown that, although errors due to the partially cosine response can be reduced by increasing the number of sensor axes, the increment in the number of axes introduces some additional errors. A method is proposed to keep the advantages of increasing the number of axes and to minimize the observed additional errors. This method could be extended to other type of arrays whose sensors have a partial cosine response varying with the physical quantity of interest.