Choosing the right irrigation flow meter

Measuring irrigation flow helps irrigators better manage and
schedule irrigation. Measuring flow also is a tool for estimating irrigation
water use. Here are 2 flow meter options, according to the Louisiana State
University Research and Extension Ag Center.

Propeller flow meters

Propeller flow meters are the most common devices used for
measuring water flow rate. A propeller flow meter measures the velocity inside
a pipe and shows the flow rate reading on a dial. Each of these meters are
designed for a specific pipe size and work best within particular ranges of
flow. With adapters one flow meter can be used to measure several pipe sizes.
Two main types of flow meters are saddle and flanged. Saddle are placed through
a hole in an existing or specifically used pipe. Flanged flow meters are placed
in between an existing flanged joint.

When excessive trash will be in the water, a small propeller
can be installed. Due to their reduced accuracy, these smaller propellers
should not be used for all applications. When meters will be used on more than
one site, irrigators have found it convenient to couple them to a designated
section of aluminum or PVC pipe.

Portable ultrasonic flow meters

A relatively new alternative is the ultrasonic flow meter.
The USFM is a non-invasive device that can be used to measure both flow rate
and volume. Clamp-on transducers eliminate in-line installation, allowing one
meter tobe used at many locations. Exterior installation eliminates pressure
losses and prevents leaking that can be associated with in-line meter
installations. The popularity of ultrasonic flow meters is due in large part to
their portability and ease of use; they can be installed almost anywhere. Due
to their high cost (~$3,000 - $5,000), however, the use of USFMs will likely be
limited to irrigation professionals, technical assistance providers or
irrigators who manage several pumping units and/or farms.

The transmission, or transit-time, ultrasonic flow meter
operates on the principle of phase shift. Two transducers act alternately as
transmitter and receiver as two paths of sonic beams travel back and forth
across the pipe. One beam travels downstream while the other moves upstream.
The motion of the fluid causes a frequency shift in both waves. This shift is
related to the velocity of the fluid. Research has shown that, when installed
properly, USFM accuracy ranges from +/- 1 to +/- 5 percent of full scale.