2. Classes of Current Meters

Several classes of current meters are used in water measurement.

Most of these will be discussed briefly here. The class that is more commonly used for irrigation and watershed measurements is the anemometer and propeller type; however, the use of electromagnetic velocity meters is very popular among water districts. The discussion in the following sections will mainly describe this class and its use.

(a) Anemometer and Propeller Current Meters

Anemometer and propeller current meters are the most common type used for irrigation and watershed measurements. These meters use anemometer cup wheels or propellers to sense velocity. The Price current meter and the smaller pygmy meter modification are the most common current meters in use. These meters are rated by dragging them through tanks of still water at known speeds. The reliability and accuracy of measurement with these meters are easily assessed by checking mechanical parts for damage and using spin-time tests for excess change of bearing friction. This type current meter does not sense direction of velocity, which may cause problems in complicated flow where backflow might not be readily apparent. For irrigation needs, this problem can be avoided by proper gage station or single measurement site selection.

(b) Electromagnetic Current Meters

Electromagnetic current meters produce voltage proportional to the velocity. The working principle of these meters is the same as the pipeline electromagnetic flowmeter, which is more fully described in chapter 14. One advantage of these current meters is direct analog reading of velocity; counting of revolutions is not necessary. These current meters can also measure crossflow and are directional. Electromagnetic current meters, while still not as reliable as the anemometer type, have improved greatly in recent years. Their use near metallic objects is still a limitation.

(c) Doppler Type Current Meters

Doppler type current meters determine velocity by measuring the change of source light or sound frequency from the frequency of reflections from moving particles such as small sediment and air bubbles. Laser light is used with laser Doppler velocimeters (LDV), and sound is used with acoustic doppler velocimeters (ADV). The working principles for ADV type flowmeters are more fully described in chapter 11. Acoustic Doppler current profilers (ADCP) have also been developed. These instruments measure average velocities of cells of selected size in a vertical series. Thus, they measure vertical current profiles. ADCP measurements are becoming more frequent for deep flow in reservoirs, oceans, and large rivers. Most of the meters in this class are multidimensional or can simultaneously measure more than a single directional component of velocity at a time.

(d) Optical Strobe Velocity Meters

Optical strobe velocity meters developed by the U.S. Geological Survey (USGS) and the California Department of Water Resources use optical methods to determine surface velocities of streams (USGS, 1965). This meter uses the strobe effect. Mirrors are mounted around a polygon drum that can be rotated at precisely controlled speeds. Light coming from the water surface is reflected by the mirrors into a lens system and an eyepiece. The rate of rotation of the mirror drum is varied while viewing the reflected images in the eyepiece. At the proper rotational speed, images become steady and appear as if the surface of the water is still. By reading the rate of rotation of the drum and knowing the distance from the mirrors to the water surface, the velocity of the surface can be determined. The discharge rate of the stream may be estimated by applying the proper coefficient to this surface velocity and multiplying by the cross-sectional area of the flow section.

The meter has several advantages. No parts are immersed in the flowing stream. Moreover, it can be used for high-velocity flows and for flows carrying debris and heavy sediment. The meter can measure large floodflows from bridges. However, the meter measures only the water surface velocity and is very dependent upon the selection of the proper coefficient.