Experimenting with a new calibration method for current meters

The necessity of a simple way to control the reliability of laboratory’s current meters brought to develop a method that allows to calibrate the instruments in their usual working conditions, as it happens, for example, in a river or in a sewer, using an equipment simpler than the traditional method: the current meter is still, hung by a support, and the propeller is put in rotation by flow through totally submerged outflows, exploiting the main characteristic of these outflows, that is a uniform velocity profile on all the cross-section. Since current meter is a local velocity gauger, the influence of current meter’s position, respect to the outflow, on velocity measurements reliability, has been investigated. In fact, the current must be linear to obtain satisfying discharge measurements. Two different kinds of submerged outflows has been analyzed: a flow nozzle and an orifice plate. The problem about the orifice plate is to localize with accuracy where the contracted section A_c is located, that is to say where current is linear, and which is the contraction ratio C_c . Usually, the contracted section is considered to be D_OP /2 far from the outflow. On the contrary, the flow nozzle doesn’t cause a contracted section, because of its well-connected entrance; nevertheless, there is a lot of turbulence around it, because of liquid jet expansion, but, near the outflow, current may be considered linear, assuming, as reference piezometric line, that of the axial flow pattern. To confirm the hypothesis at the basis of the method, that is a uniform velocity distribution on all the cross section, velocity profiles along two directions, horizontal and vertical, have been studied, showing results matching with theory. A calibration method consists in putting into relationship the number of propeller’s rates in a fixed time interval with current velocity, determining instrument’s characteristic calibration curve; in this case, two reference current velocities have been adopted for calibration: the one is the ratio between discharge (indicated by an electromagnetic flow meter) circulating in the system and outflow’s area; the other is obtained by the Torricelli’s formula, measuring the difference between upstream and downstream reservoirs’ levels. In particular, the propellers of two current meters, different in dimension and typtology, have been calibrated by the new method. In the end, since a measurement is complete only if associated to its uncertainty, the uncertainty of measurements carried out has been calculated, in particular about Torricelli’s velocity.