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I was reading about venturimeters , and this is what I found on net:

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Now my question is , where do we exactly "select" the two points required to put the bernoulli's equation? For example I select two points as follows:

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If I do this , then I will have a horizontal level and there will be no need to include the potential head in the equation.. However the problem is that how will we calculate the pressure at these points? As the pressure will not remain constant throughout the height... Also If i select any two points as follows:

enter image description here

There will be a problem regarding the height difference between the points being considered... so could anyone please help?

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Actually in a Venturi meter we usually make the approximation that height difference within the tube can be neglected. There are two reasons why this is valid:

  1. Venturi tubes are of small diameters (of the order of a few centimetres) and hence height difference can be neglected.
  2. due to the high speed of water through the tube, the pressure is also considerably large and so height difference does not affect it a lot
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