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Guangzhou Haoyi Open Channel Flowmeter Principle Description
Haoyi Series Parshall Tank The open channel flowmeter consists of an instrument for measuring liquid level and a water tank made according to documents published by the Institute of Metrology. The liquid level meter directly measures the height of the liquid in the channel or water tank. When used to measure flow in an open channel, a measuring weir should be installed on the open channel. Measuring weirs convert the flow rate in the open channel into liquid level. The instrument measures the water level in the measuring weir and trough, and then calculates the flow rate according to the water level-flow relationship of the corresponding measuring weir and trough.
Principle of ultrasonic liquid level measurement
The ultrasonic pulse emitted by the transmitting ultrasonic transducer propagates to the measured liquid surface through the propagation medium, and then returns to the receiving transducer through the sound transmission medium after reflection. Measure the time it takes for an ultrasonic pulse to propagate in the sound transmission medium from emission to reception. Then according to the speed of sound in the sound transmission medium, the distance from the transducer to the liquid surface can be calculated. This determines the liquid level. Therefore, we can calculate the distance from the probe to the reflecting surface */ (divided by because the sound wave actually goes back and forth from emission to reception, which is the distance, the speed of sound, and the time). The liquid level value can then be obtained by subtraction.
Principle of flow measurement in water measuring weirs
The greater the flow rate in a smooth open channel, the higher the liquid level, the smaller the flow rate, and the lower the liquid level (as shown in the figure). The flow rate can be deduced by measuring the water level. The corresponding relationship between flow rate and water level in an ordinary open channel is affected by the slope ratio of the channel and the roughness of the surface. Installing a water measuring weir and trough in the channel will produce a throttling effect, so that the flow rate in the open channel has a fixed corresponding relationship with the liquid level. This correspondence mainly depends on the structural size of the water measuring weir and trough, so as to minimize the influence of the channel. Small.
Commonly used measuring weirs and troughs include right-angled triangular weirs, rectangular weirs and Parshall troughs (as shown in the picture)
When using a Parshall trough open channel flowmeter, you must know the number of weirs and troughs to be used when installing. Water level-flow correspondence relationship. Once the throat width b of the Parshall trough is known, the corresponding relationship between water level and flow rate can be calculated using the corresponding formula.
The corresponding relationship between water level and flow rate is also calculated using corresponding formulas for right-angled triangular weirs.
There are also corresponding formulas for rectangular weirs. But it is also related to the size of the installed channel. When determining the water level-flow relationship, the rectangular weir is related to the channel width B, the opening width b, and the height of the upstream weir sill.
If you are not familiar with calculating the water level-flow rate of the weir and trough, you can inform our company of the parameters of the weir and trough used and we will help with the calculation. Care should be taken to also provide the parameters mentioned above that are relevant for determining the water level-flow relationship.
3. Water measuring weirs and troughs
When selecting the type of water measuring weirs and troughs, we must consider the size of the flow rate in the channel, the flow pattern of the water in the channel, and whether free flow can be formed. Depending on the large flow rate, we can choose different weirs and grooves.
① It is recommended to use a right-angled triangle weir for large flow rates less than liters/second
② It is recommended to use Parshall troughs for large flow rates greater than liters/second
③ It is recommended to use Parshall troughs for large flow rates greater than liters/second. Rectangular weir.
If conditions permit, it is best to choose the Parschel tank. The water level-discharge relationship of the Parshall tank is calibrated by the laboratory, and the requirements for the upstream channel conditions are weak. The water level-flow relationship of triangular weirs and rectangular weirs is derived from theoretical calculations, which may easily lead to additional errors due to ignoring some usage conditions.
Use fiberglass to make water measuring weirs or troughs. The size of the weir mouth of triangular weirs and rectangular weirs must be accurate, and the surface on the side facing the water inlet must be smooth. The size of the throat part of the Parshall trough must be accurate and the inner surface of the trough must be smooth.
IV. Physical drawings of on-site open channel weir and trough installation
◆Triangular weir installation
◆Rectangular weir installation
◆Parshall trough installation
V. Installation Measuring weir and trough
①. The center line of the water measuring weir and trough should coincide with the center line of the channel, so that the water flow entering the water measuring weir and trough will not cause deviation.
②. After the water measuring weir and trough is filled with water, the flow pattern of the water should be free flowing.
The water level downstream of triangular weirs and rectangular weirs should be lower than the weir, and the submergence degree of the Bashere trough should be less than the critical submersion degree of the "Baschel trough parameter".
③. The upstream end of the water measuring weir and trough should have a straight section larger than twice the width of the channel, so that the water flow can enter the water measuring weir and trough smoothly. The standard is "no waves on the water surface". That is to say, there is no left and right bias flow, and there is no impulse caused by the slope of the channel.
④. The water measuring weir and trough must be installed firmly on the channel. It must be tightly connected to the side wall and bottom of the channel to prevent water leakage. Make all water flows through the metering part of the measuring weir and groove. The measuring part of the water measuring weir plate is the weir mouth and the measuring part of the measuring water tank is the throat section in the tank.
6. Selection instructions:
Dazzle series Parshall trough open channel flowmeter
Code weir trough type
B Parshall trough (indicate the number of troughs)
Triangular weir (indicate the angle)
Rectangular weir (indicate the width of the weir mouth)
Code liquid level probe type
Integrated type
Split type (standard meter cable)
Code liquid level probe material (all explosion-proof)
Ordinary type (polyoxymethylene)
Anti-corrosion type (polytetrafluoroethylene)
Enhanced type (polytetrafluoroethylene + extended functions, such as communication, relay Output, etc.)
Selection example: The highest on-site flow rate range is tons/hour, the common flow rate is tons/hour, the customer wants to choose a Parshall tank, the liquid level probe is split type, the sewage is neutral, normal temperature, no Volatile gases.
Guangzhou Haoyi Automation Equipment Co., Ltd.
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