Clamp-on, Ultrasonic Flow Meters

Ultrasonic meters measure flow rate by propagating ultrasound waves into liquid filled pipes and measuring the reflections. There are two technologies to choose from based on the fluid properties; Transit Time and Doppler. 

Transit Time - Clean fluids with small amounts of suspended solids or aeration.

For clean fluids with small amounts of suspended solids or aeration, transit time ultrasonic meters provide the best performance and can be used in a wide range of applications for pipes 1/2 inch and larger.

Transit time flow meters have two transducers, which function as both ultrasonic transmitters and receivers. The flow meters operate by alternately transmitting and receiving a frequency modulated burst of ultrasound energy between the two transducers. The burst is first transmitted in the direction of fluid flow and then against fluid flow. Since ultrasound energy in a moving liquid is carried faster when it travels in the direction of fluid flow (downstream) than it does when traveling against fluid flow (upstream), a differential in the times of flight will occur. The ultrasound's time of flight is accurately measured in both directions, and the difference in time of flight is used to determine the velocity of the fluid.

Doppler Flow Meters - Fluids with suspended solids.

For fluids with significant amount of particles or air bubbles, such as slurries, sewage and plastics, Doppler ultrasonic meters provide the best performance.

Each Doppler flow meter utilizes two separate transducer heads. Ultrasonic waves are transmitted from one transducer and reflected by reflectors suspended within the liquid and then recorded by a receiving transducer.  If the reflectors are moving within the ultrasound transmission path, ultrasound waves will be reflected at a frequency shifted (Doppler shift) from the transmitted frequency.  The difference between the transmitted frequencies and reflected frequencies is directly proportional to the speed of the ultrasonic reflectors.

For more information about clamp-on ultrasonic flow meters, contact Thompson Equipment Company. Call them at 800-528-8997 or visit their web site at https://teco-inc.com.

Water Flow Metering and Measurement

Water flow measurement device comparison (click for larger view)

Water is commonly measured and sold in volumetric measurements, which allows for lower-cost metering options. The specific measurement technology chosen will depend on a number of factors including, but not limited to, current design, budget, accuracy requirements, resolution, minimum flow rate, potable versus non-potable (or at least filtered versus non-filtered water), range of flow rates, and maximum flow rate.

Volumetric water measurement can be broken down into three general operating designs:

• Positive displacement
• Differential pressure
• Velocity

Positive Displacement – Nutating-Disk Flow Meter

Nutating-disk flow meters are the most common meter technology used by water utilities to measure potable-water consumption for service connections up to 3-inch. The nutating-disk flow meter consists of a disk mounted on a spherically shaped head and housed in a measuring chamber. As the fluid flows through the meter passing on either side of the disk, it imparts a rocking or nutating motion to the disk. This motion is then transferred to a shaft mounted perpendicular to the disk. It is this shaft that traces out a circular motion – transferring this action to a register that records flow.

There are a variety of differential pressure devices useful for water metering; two of the more common devices include orifice flow meters and venturi flow meters.

Differential Pressure – Orifice Flow Meter

The orifice element is typically a thin, circular metal disk held between two flanges in the fluid stream. The center of the disk is formed with a specific-size and shape hole, depending on the expected fluid flow parameters (e.g., pressure and flow range). As the fluid flows through the orifice, the restriction creates a pressure differential upstream and downstream of the orifice proportional to the fluid flow rate. This differential pressure is measured and a flow rate calculated based on the differential pressure and fluid properties.

Differential Pressure – Venturi Flow Meter

The venturi flow meter takes advantage of the velocity-pressure relationship when a section of pipe gently converges to a small-diameter area (called a throat) before diverging back to the full pipe diameter. The benefit of the venturi flow meter over the orifice flow meter lies in the reduced pressure loss experienced by the fluid.

The velocity measurement technologies described in this section include the turbine flow meter, vortex-shedding flow meter, and ultrasonic flow meters.

Velocity – Turbine Flow Meter

A multi-blade impellor-like device is located in, and horizontal to, the fluid stream in a turbine flow meter. As the fluid passes through the turbine blades, the impellor rotates at a speed related to the fluid’s velocity. Blade speed can be sensed by a number of techniques including magnetic pick-up, mechanical gears, and photocell. The pulses generated as a result of blade rotation are directly proportional to fluid velocity, and hence flow rate.

Velocity – Vortex-Shedding Flow Meter

A vortex-shedding flow meter senses flow disturbances around a stationary body (called a bluff body) positioned in the middle of the fluid stream. As fluid flows around the bluff body, eddies or vortices are created downstream; the frequencies of these vortices are directly proportional to the fluid velocity.

Velocity – Ultrasonic Flow Meters

There are two different types of ultrasonic flow meters, transit-time and Doppler-effect. The two technologies use ultrasonic signals very differently to determine fluid flow and are best applied to different fluid applications. Transit-time ultrasonic flow meters require the use of two signal transducers. Each transducer includes both a transmitter and a receiver function. As fluid moves through the system, the first transducer sends a signal and the second receives it. The process is then reversed. Upstream and downstream time measurements are compared. With flow, sound will travel faster in the direction of flow and slower against the flow. Transit-time flow meters are designed for use with clean fluids, such as water.

Doppler-effect ultrasonic flow meters use a single transducer. The transducer has both a transmitter and receiver. The high-frequency signal is sent into the fluid. Doppler-effect flow meters use the principal that sound waves will be returned to a transmitter at an altered frequency if reflectors in the liquid are in motion. This frequency shift is in direct proportion to the velocity of the liquid. The echoed sound is precisely measured by the instrument to calculate the fluid flow rate.

Because the ultrasonic signal must pass through the fluid to a receiving transducer, the fluid must not contain a significant concentration of bubbles or solids. Otherwise the high frequency sound will be attenuated and too weak to traverse the distance to the receiver. Doppler-effect ultrasonic flow meters require that the liquid contain impurities, such as gas bubbles or solids, for the Doppler-effect measurement to work. One of the most attractive aspects of ultrasonic flow meters is they are non-intrusive to the fluid flow. An ultrasonic flow meter can be externally mounted to the pipe and can be used for both temporary and permanent metering.

For more information on any flow application, visit http://www.teco-inc.com or call (504) 833-6381.

An Introduction to Ultrasonic Flow Technology

How Ultrasonic energy
is used to measure flow.

Ultrasonic energy flow meters measure, via sound waves, the velocity of liquid flowing through a pipe––however, this pipe includes not just the traditional “pipe” but also mass flow chutes or something with open channels, free surfaces.

There are three different types of ultrasonic energy measuring tools, called flow meters: the first is the Open Channel flow meter which receives its calculations by computing geometrical distance; the second is the Doppler shift flow meter which reflects ultrasonic beams off sonically reflective materials, e.g. air bubbles; the third is the contrapropogating transit-time flow meter or, more recognizably, the transmission flow meter. The transmission flow meter has two versions: the in-line and the clamp-on. The former is “intrusive” whereas the latter is not, an outward device. These 72+ inch tools, using ultrasound technology, have the ability to measure fluids in bulk, all with distinct properties and principles., The use of this technology is most used in the respective oil and nuclear industries, wastewater technologies, pharmaceutical applications, and the food and beverage industry.

For intrusive flow meters, sensors are fitted opposite one another and alternate bouncing ultrasonic signals back and forth in the pipe, in an almost tennis-like format. In an elementary explanation, by increasing the number of sensors, engineers are able to decipher flow proportions through calculations of velocity between sensory transmissions; thereby, the flow volume can be computed.

For unintrusive flow meters, a literal clamp-on flow meter is placed atop the pipe so as not to interrupt flow. One of the most special properties uninstrusive flow meters offer is the ability to bounce ultrasonic sensors through piping up to four meters in diameter; this makes seemingly impossible feats possible, especially in otherwise difficult fields, e.g. hydroelectric.

Although the technology is pervasive, there are disadvantages still as there are advantages. However, the majority of the equipment’s disadvantages are unavoidable, such as costs and apparatus sensitivities. Nonetheless, there is also the threat of low ultrasonic accuracy, or attenuation, dependent on what systems are used and under what circumstances and command. Alternatively, one of the most publicized advantages is that ultrasonic energy flow technology is used for custody transfer of natural gases and petroleum liquids. Custody transfer usually entails following industry, national, and government standards and regulations. Ultrasonic energy flowmeters and analyzers are also relatively low maintenance, e.g. self-diagnosing. The technology has the capability to control and manage high pressures as well as high temperatures, and, being a popular application among engineers, manufacturers and the like, is reliable in its performance and consistency.

For more information on any industrial flow application, contact TECO at 800-528-8997 or visit www.teco-inc.com.