Ever stick a wet finger up into a light breeze to determine its direction? You'll find that the colder side of your finger is the upwind side and the direction from which the breeze is coming. Imagine an electronic finger that lets you do the same when placed into ground water. The principle underlying the operation of the VECTOR is really that simple: normal house current continually heats a 2.375" diameter cylinder, placed in direct contact with a saturated lithology, by 20-30 degrees Centigrade. Following the one (and only) calibration step, an integrated array of 30 carefully calibrated temperature sensors, or thermistors, records any temperature variations resulting from ground water flowing, and thus, carrying heat over and away from the surface of the probe. These extremely sensitive thermistors, can measure temperature differences to within +/-0.01 degrees Centigrade. In the presence of flowing ground water, these thermistors will record colder temperatures on the upstream side of the probe, much like that wet finger in the breeze.
The precise geometry and sensitivity of the heater/thermistor array and the very large averaged sampling volume (>1 cubic meter) of the probe allows for unheralded precision and accuracy in observing both horizontal and vertical flow vectors. Recording the compass heading of the probe reference direction then allows the user to determine the direction of the ground water flow to within +/-5 degrees. It is for many of these reasons that the In-Situ Permeable Flow Sensor, as it was originally known, was re-named the VECTOR® (loosely) for Variably Emitting Controlled Thermal Output Recorder. Besides, "VECTOR" just sounded a lot cooler. Of technical importance, the algorithm used to calculate flow velocities assumes several important factors, namely that: 1) thermal and hydraulic properties of the surrounding lithology are homogenous and isotropic and 2) gravitational effects are negligible. Largely because of the former, the probe is not appropriate for use in grossly heterogeneous sediments. The latter dictates that the applied heat must not result in convecting ground water, meaning that the surrounding sediment must allow ground water to flow past the probe, rendering it useless in clays. Published side-by-side tests of the VECTOR® and conventional hydrogeological measuring techniques prove the viability and economy of this technology in many circumstances. There are, to be sure, many steps between installation and data analysis but none beyond the training of todays computer-literate engineering professional.
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