Ultrasonic Flowmeters

Principle of operation

The basic principle of operation employs the frequency shift (Doppler Effect) of an ultrasonic signal when it is reflected by suspended particles or gas bubbles (discontinuities) in motion. This metering technique utilizes the physical phenomenon of a sound wave that changes frequency when it is reflected by moving discontinuities in a flowing liquid. Ultrasonic sound is transmitted into a pipe with flowing liquids, and the discontinuities reflect the ultrasonic wave with a slightly different frequency that is directly proportional to the rate of flow of the liquid (Figure 1). Current technology requires that the liquid contain at least 100 parts per million (PPM) of 100 micron or larger suspended particles or bubbles.

How to choose the right ultrasonic flow meter?

The key questions which need to be answered before selecting an ultrasonic flow meter or doppler flowmeter are:

• Does the liquid have particulates of 100 ppm of 100 microns in size?
• Do you require a handheld or continuous process monitor?
• Do you require an analog output?
• What is the minimum and maximum flow rate for the flow meter?
• What is the minimum and maximum process temperature?
• What is the minimum and maximum process pressure?
• What is the size of the pipe?
• Is the pipe always full?

Design Variations

Clamp-on ultrasonic flow meters come in either single or dual-sensor versions. In the single-sensor version, the transmit and receive crystals are potted into the same sensor body, which is clamped onto a single point of the pipe surface. A coupling compound is used ultrasonically connect the sensor to the pipe. In the dual sensor version, the transmit crystal is in one sensor body, while the receive crystal is in another. Clamp-on Doppler flowmeters are subject to interference from the pipe wall itself, as well as from any air space between the sensor and the wall. If the pipe wall is made of stainless steel, it might conduct the transmit signal far enough so that the returning echo will be shifted enough to interfere with the reading. There are also built-in acoustic discontinuities in copper, concrete-lined, plastic-lined, and fiberglass-reinforced pipes. These are significant enough to either completely scatter the transmitted signal or attenuate the return signal. This dramatically decreases flowmeter accuracy (to within only ±20%), and, in most cases, clamp-on meters will not work at all if the pipe is lined.

Ultrasonic Flowmeter with Totalization
These ultrasonic flowmeters are for applications with 100 PPM of 100 microns in size particulates or bubbles in the liquid. They are good for pipe sizes 0.25 to 120". These flow meters are used for flow measurement applicatons of 0.1 to 30 FPS. The FD-400 series has an optional display, and 4 to 20 mA output, Relays and 12 V pulse output options. The FD-400 Series ultrasonic flow meter transmitter systems are designed for use where a sensor cannot be installed in the process. For liquids with suspended solids or aeration, standard and enhanced models. Available in non-invasive fixed position style. The FD-400 is an ideal doppler flow measurement solution. They are well suited for full pipes of liquid that contain at least 100 ppm of 100 micron or larger suspended particles or gas bubbles which act as sound "reflectors" and are necessary for operation. Ideal uses for this technology include: flow measurement of wastewater sewage, reactivated sludge, waste activated sludge, primary sludge, slurries, crude oils, lime slurries, phosphates and pulp stock.

Portable Ultrasonic Flow Meter
The FD610 Series ultrasonic flow meter features advanced Trans-Phase measuring technology, providing accurate and reliable flow velocity assessments in closed piping systems. The FD610 Series utilizes a non-invasive clamp-on transducer which is placed on the outside of a pipe. Within seconds, the large 18 mm (0.7") LCD provides stable flowrate readings in feet per second or meters per second. This product operates on metal or plastic pipes containing liquids with more than 100 ppm of 100 micron or larger suspended solids or entrained gases.

Cleaner Liquid Ultrasonic Flow Meter
The FD-7000 is used to measure the flow of liquids containing suspended solids or bubbles, the transducers are mounted on a straight run of pipe in the same manner as other ultrasonic meters. In applications where insufficient "reflectors" exist (a condition indicated by a low reading on the FD-7000's Flow Analyzer bar graph), the transducers are relocated to a point in the piping where non-symmetrical hydraulic turbulence will be present – ideally just downstream of a 90-degree elbow. The FD7000 only requires 30ppm of 25 micron or larger particle in order to make a measurement.