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.