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.
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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.
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.
Ultrasonic open channel flow meter is an ideal device for monitoring the flow of channels and rivers. Generally for the measuring range, we have 1m and 2m types. Then how to determine its range and what we should note when installing?Today we will explain in details.
First let's see how many different shapes and types of channels and weirs we have.
Triangle weir Rectangular weir Trapezoidal weir Parshall flume Parabolic weir Channel without weir
No.1. Triangle weir
Choose the open channel flow meter with 1m range. The triangle weir can be made of FRP, PVC or stainless steel.
1. The weir should be smooth, flat without distortion;
2. The triangular lip platform should be straight and smooth;
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No.2. Rectangular weir
Choose the open channel flow meter with 2m range. The rectangular weir can be made of FRP, PVC or stainless steel. Note the following:
1. The weir should be smooth, flat without distortion;
2. The triangular lip platform should be straight and smooth.
No.3 Trapezoidal weir
Choose the open channel flow meter with 2m range.
No.4, Parshall flume
For Parshall flumes 1 to 4, choose the open channel flow meter with 1m range; For Parshall flumes 5 to 25, choose the open channel flow meter with 2m range.
No.5, Parabolic weir
1m or 2m range can be selected.
No.6. Channel without weir
Prerequisites for Measurement of Channels Without Weirs
1. Sufficient slope is required between upstream and downstream. If the water is blocked upstream, the water in the channel will flow away naturally, and there will be no accumulation of water in the channel. Standing water 1 to 2 cm deep is negligible.
2. Downstream water will not flow back.
3. There are 10 times straight channels upstream of the installation point and 5 times straight channels downstream.
4. Stones, aquatic plants, garbage and other debris within the range of 5 times the straight channel upstream and downstream of the installation point should be cleaned up.
For more technical documents and solutions for flow monitoring, contact Holykell.
If you are looking for more details, kindly visit electromagnetic flow transmitter.