CURRENT METERS

Current Meter

• A current meter is oceanographic device for flow measurement by mechanical, tilt , acoustical or electrical means.

• It is an instrument for measuring the velocity of flow of a fluid (as water) in a stream.

Classes of Current Meters

Several classes of current meters are used in water measurement.

• Electromagnetic current (FLOW) meters

• Anemometer and propeller current meter

• Doppler current meters

• Optical strobe current meters

• 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.

Introduction

• A Electromagnetic flow meter is a volumetric

flow meter which does not have any moving

parts and is ideal for wastewater applications

or any dirty liquid which is conductive or water based.

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 flow meter

One advantage of these current meters is direct analogreading 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.

Working Principle

• The operation of a Electromagnetic flow meter or magmeter is based upon Faraday's Law, which states that:

“The voltage induced across a conductor as it

moves at right angles through the magnetic

field is proportional to the velocity of that

conductor.”

Faraday’s Formula

• E is proportional to V x B x D

where:

• E = The voltage generated in a conductor

• V = The velocity of the conductor

• B = The magnetic field strength

• D = The length of the conductor

(which in this instance is the distance between the electrodes)

• ElectroMagnetic flow meter use Faraday'sLaw of Electromagnetic Induction todetermine the flow of liquid in a pipe. In amagnetic flow meter, a magnetic field isgenerated and channelled into the liquidflowing through the pipe. Following Faraday'sLaw, flow of a conductive liquid through themagnetic field will cause a voltage signal to besensed by electrodes located on the flow tubewalls.

Uses…

• Pipelines...

• Refineries..

Advantages & Disadvantage

Advantages:

• Minimum obstruction in the flow path yields minimum pressure drop.

• It can measure forward as well as reverse flow with equal accuracy.

• Low maintenance cost because of no moving parts.

• corrosive or slurry fluid flow.

Disadvantage:

• Requires electrical conductivity of fluid.

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.

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).

• 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.

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.