Electromagnetic current-meter

   Electromagnetic (e/m) current-meter is an electrical induction-measurement instrument, with no moving parts, mounted in a totally enclosed streamlined probe. The probe can be mounted on rods and held at various depths or suspended on a cable.

The e/m meter has the advantages of being smaller and having a wider measurement range than the propeller meters. It is particularly useful at very low velocities when propeller meters become erratic. Its sensitivity and lower vulnerability to fouling from weeds and debris make it attractive for use in heavily polluted or weedy streams.Each unit is provided with a surface control box with a digital display and dry-cell batteries. A set of stainless steel wading rods is also standard equipment. Latest models have built-in battery-charger circuits.It will be appreciated that since each river is unique, a careful assessment of its width, depth, likely flood velocities, cable-support facilities, availability of bridges, boats, etc. needs to be made before a discharge measurement programme can begin.The discharge at the chosen measuring point is best obtained by plotting each velocity observation on a cross section of the gauging site with an exaggerated vertical scale. Isovels (velocity profiles – contours of equal velocity) are then drawn and the included areas measured by a planimeter. Alternatively, the river may be subdivided vertically into sections and the mean velocity of each section applied to its area. In this method the cross-sectional area of any one section, where measurements, are taken should not exceed 10 per cent of the total cross-sectional area.