1. Current meters measure the velocity of fluid flow using various mechanical, electrical, or optical methods. 2. The most commonly used current meters for irrigation and watershed measurements are anemometer and propeller types, which measure velocity using rotating cups or propellers. 3. However, electromagnetic current meters that produce voltage proportional to flow velocity are also widely used, especially by water districts, as they provide direct analog readings without moving parts.

1. CURRENT METERS

2. 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.

3. 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

4. • 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.

5. 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.

6. 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 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.

7. 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.”

8. 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)

9. • ElectroMagnetic flow meter use Faraday's
Law of Electromagnetic Induction to
determine the flow of liquid in a pipe. In a
magnetic flow meter, a magnetic field is
generated and channelled into the liquid
flowing through the pipe. Following Faraday's
Law, flow of a conductive liquid through the
magnetic field will cause a voltage signal to be
sensed by electrodes located on the flow tube
walls.

12. Uses…
• Pipelines...
• Refineries..

13. 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.

14. 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.

15. 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.

16. 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.