How to Select an Ultrasonic Flow Meter

Functions of an Ultrasonic Flow Meter

An ultrasonic flow meter is a device that measures the speed of the fluid in a pipeline by detecting an echo when the fluid makes contact with a reflector. An ultrasonic frequency is established and reflected linearly depending on the movement’s velocity, and a transmitter processes the signal from the wave to establish a flow rate. A pair of sensors are fitted on opposite sides in the measuring tube. Each sensor can transfer or receive an ultrasonic signal; the transit times of the signals are measured simultaneously. The ultrasonic signals are generated by an electric current that applies voltage; this is usually executed by crystals that create a voltage when an ultrasonic signal impacts the sensor. Having more sensor pairs allows for effective detection and calculation of flow a pipe system. The sensors don’t necessarily have to be fitted inside the pipe wall, clamp-on sensors are fastened directly to the outside of the pipe. With clamp-on sensors, the ultrasonic signal is passed into the fluid directly through the pipe wall. The signal is reflected on the opposite pipe wall and then measured by the second sensor. The clamp design is the most used sensor type; it is unique because flow rates can be measured in very large pipes of up to four meters in diameter, increasing the application area.

Types of Flow Meters

Doppler shift flow meter

A Doppler ultrasonic flow meter uses a transducer to emit an ultrasonic beam into the through the pipe into a stream of fluid. For a Doppler flow meter to operate, there must be solid particles or air bubbles moving through the stream to reflect the ultrasonic beam. The movement of particles shifts the frequency, which is then received by a transducer; this frequency shift is linearly proportional to the rate of flow.

Time transit flow meter

If the application of the flow meter does not have any suspended particles or air bubbles that can be used to get a reading, time transit flow meters become the alternative. This type of flow meter measures the difference in time from when an ultrasonic signal is transmitted from the first transducer until it crosses the pipe and bounces back to be received by the second transducer. When there is an active flow, the sound moves faster if it travels in the same direction as the flow but slower when moving against the flow meters direction. With upstream and downstream measurements, a lack of flow means the travel time will be the same in both directions. The ultrasonic signal must traverse the full length of the pipe to be received by the sensor, liquids cannot contain a significant number of solids or bubbles otherwise the high frequency sound will be too weak to travel across the pipe.

Open channel flow meter

With Open Channel Flow Meters, the sensor is installed on top of a water flow channel such as a flume. An Open Channel Flow Meter uses an ultrasonic level sensor to determine the flow rate and measure the head height of the channel. The level measurement allows the meter to determine a surface point from which it will measure velocity. The meter then focuses its laser beam at this point and analyses the frequency of the reflected light. The difference that results from the original frequency will reveal the direction of flow and its velocity. The ultrasonic sensor is usually equipped with a temperature sensor as well, mainly because the speed of sound in the air is manipulated by the surrounding temperature.

Utilization of Ultrasonic Flow Meters

The other types of flowmeters are mechanical, magnetic, and vortex flow meters. Ultrasonic flow meters can be beneficial in the hydroelectric industry, wastewater streams, power utilities industry, chemical industry, mining industry, along with the food and beverage industry. Hydroelectricity providers will find the open channel flow meter useful because the water container is large compared to other industries that rely on pipelines.

Factors that Affect selecting the Flow Meter

Depth of the channel being measured:

The technology behind the open channel flow meter is generally unaffected by debris and is suitable for low velocity and shallow depths. When water reaches the lasers, a bottom-mounted continuous-wave sensor and pressure transducer take over the flow rate measurement. This makes the open channel flow meter the most effective for readings of fluid velocity aside from pipe systems.

Error tolerance:

With Doppler ultrasonic flow meters, it is important to ensure that the liquids properly reflect the ultrasonic waves. Depending on the design, the reflection displays small bubbles of gas in the liquid, or the presence of water dynamics. Transit time ultrasonic flow meters are unable to affect the flow stream between the transducers when the liquid cannot successfully generate ultrasonic waves. Additionally, ultrasonic waves should laser through the fluid for Doppler Flow Meters to operate successfully. When the fluid is not transparent, such as in wastewater, the failure to penetrate the fluid can cause Doppler Flow Meters to have measurement errors. In such a case, it is preferable to measure velocity closer to the pipe wall.

Installation method:

An analysis of the advantages should be made with the option to choose from clamp-on sensors or internally mounted sensors. The advantage of clamp-on sensors is that they can easily be replaced or repaired in case of damage, sensors that are placed inside a system are difficult to replace as that would require complete dismantlement of the structure. Some meters have difficult installation requirements, which make it very difficult for mounting the sensor; factors like circumference and diameter should be accurately calibrated for the best mounting possible. The process of mounting and installing should also be risk-free of injury to personnel; this can be achieved using a user safety manual and ergonomic tools for placing parts correctly.

Design:

A compact design is usually the preferred option for flow meters, because it is the clamp on type with the added advantage of being lightweight. Factors such as no moving parts, being maintenance-free, and a long-life span should be considered. It is important and worthwhile to find a robust and durable flow meter.

In conclusion, flexible mounting, process safety, and cost-effectiveness are the distinctive factors of opting for the right ultrasonic flow meter. Ultrasonic flow meters have the advantage of being highly accurate, highly sensitive, provide high stability, have a high turn-down ratio, experience lower pressure drops and are temperature independent. The liquids being measured must be relatively free of accumulated gas or solids to minimize signal scattering and absorption

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