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Regarding an experiment to find the mass flow rate of a fluid (air) using a venturi meter and a pitot tube: What could be the cause of a variance in results obtained using the 2 instruments mentioned? My mass flow rate for pitot tube is 10 times less than the mass flow rate found using the venturi. What limitations could have been the cause of this?

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The reason is that there is no stagnation point in a venturi meter, whereas there is one in a pitot meter. Both rely upon Bernoulli's equation, but use different terms.

The venturi meter takes advantage of a constriction to change the speed of fluid flow, thus changing the dynamic pressure. In doing so, one can measure the relative difference in flow speed between the two sections.

The pitot meter also measures flow speed, but it forces the fluid to reach a stagnation point and abruptly change flow direction. This is the source of the difference in flow speed.

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  • $\begingroup$ I was under the impression that the constriction of the venturi increases the flow speed $\endgroup$ – NeedHelp2015 Jan 6 '15 at 14:35
  • $\begingroup$ Whoops, you are correct. I was thinking about incompressible fluids like water, in which case, the flow would slow. I will fix that typo. It doesn't really change the answer though. $\endgroup$ – honeste_vivere Jan 6 '15 at 15:16
  • $\begingroup$ Hi, do you know why the mass flow rate may be higher in a pitot tube than in a venturi? $\endgroup$ – NeedHelp2015 Jan 6 '15 at 17:28
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    $\begingroup$ @NeedHelp2015 - Don't you have that backwards? The stagnation point in the pitot tube will force the flow rate to be lower than in the venturi tube. Well, if one made the diameter of the pitot tube much much larger than the venturi, one could effectively increase the mass flow rate I suppose. $\endgroup$ – honeste_vivere Jan 6 '15 at 17:35
  • $\begingroup$ SO what you're saying is flow rate in pitot is generally lower than in venturi? Because I thought flow rate should be lower in venturi because only the average velocity can be determined... $\endgroup$ – NeedHelp2015 Jan 6 '15 at 18:00

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