An automated salt-tracing gauge for flow-velocity measurement

This article introduces the SVG (salt-velocity gauge), a novel automated technique for measuring flow velocity by means of salt tracing. SVG allows a high measuring rate (up to one every 2 seconds), short control section length (down to 10 cm), high accuracy (+/-1.5 cm s(-1)), and unbiased calculation of the mean velocity in experimental conditions with turbulent, supercritical How. A few cubic centimetres or saturated salt solution (NaCl) are injected into the flow at regular time intervals using a programmable solenoid valve. The tracer successively passes two conductivity probes placed a short distance downstream. The transformation of the signal between the two probes is modelled as a one-dimensional diffusion wave equation. Model calibration gives an estimation of the mean velocity and the diffusion for each salt plume. Two implementations of the SVG technique are described. The first was an outdoors simulated rainfall experiment in Senegal (conductivity probes at 40 cm apart, 8 Hz measurement rate, salt injections at 10 second intervals). Mean velocity was estimated to range between 0.1 and 0.3 m s(-1). The second was a laboratory-based flume experiment (conductivity probes at 10 cm apart, 32 Hz, salt injections at 2 second intervals). Another SVG with probes at 34 cm apart was used for comparison. An acoustic Doppler velocimeter (ADV) was also used to give an independent assessment of velocity. Using the 10 cm salt gauge, estimated mean velocity ranged from 0.6 to 0.9 m s(-1) with a standard deviation of 1.5 cm s(-1). Comparisons between ADV, 10 cm SVG and 34 cm SVG were consistent and demonstrated that the salt-tracing results were unbiased and independent of distance between probes. Most peaks were modelled with (2) > 90 per cent. The SVG technology offers an alternative to the dye-tracing technique, which has been severely criticized in the literature because of the wide interval of recommended values for the correction factor alpha to be applied to the timings. This article demonstrates that a fixed value of alpha is inappropriate, since the correction factor varies with velocity, diffusion and the length of the control section. Copyright (C) 2005 John Wiley & Sons, Ltd.