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August 22, 2012, 09:55 
Venturi tube

#1 
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Hi,
If we have a venturi tube we can expect the velocity (kinetic energy) to increase in the convergent section at the expense of pressure (pressure energy). So we measure the pressure before the convergent section of the venturi tube and we measure the pressure at the throat of the tube and use that to calculate the mass flow through the venturi tube. My question is about the measurement of pressure. Do we always use a manometer type of pressure measurement in this kind of setup or are there any alternatives? The reason why I ask is that it seems to me that the decreased pressure in the throat might as well be a suction effect in the manometer due to the increased velocity in the throat (compared to the other side where we have low velocity and hence less suction). So, how can we be certain that we measure the pressure difference and not the suction difference? Any thoughts? 

August 26, 2012, 14:05 

#2 
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Any thoughts? I hope I did not pose the question in a bad way or perhaps this is a trivial matter that everyone knows?
Are the pressure taps of such small diameter that there can't be any significant flow in them? 

August 26, 2012, 14:33 

#3  
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Filippo Maria Denaro
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August 26, 2012, 15:01 

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For this kind of system we use generally piezometric tubes, one before the convergent and one in the convergent, then with the difference between the height of water in the two tubes you obtain the pressure drop between the two sections and applying Bernouilli equation you obtain the velocity. In the conditions for which Bernouilli equation is valid this gives a good estimation of mass flow rate and therefore velocity. Check for basic fluid mechanic books everything is detailled. 

August 26, 2012, 17:11 

#5 
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Hello and thank you for your replies,
My question is rather on how the pressure is measured. As far as I know we both the pitot tube and piezometric tubes introduce a hole in which fluid could potentially move. The static pressure is measured normal to the flow direction and the dynamic pressure is measured head on in case of pitot tube. So my point is; if we introduce a hole in the domain, is it certain that there will be no motion in this hole that, in turn, would remove the possibility to measure static pressure? 

August 26, 2012, 18:16 

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In a piezometric tube, the section is so small compared to the section of the pipe, that there is no movement in it and that you can apply fluid static principle p+rho*g*z= cst which will allow you to determine the pressure drop between the two sections. Then using Bernouilli equation you obtain the mass flow rate and hence the velocity. It s the purpose of a venturi. 

August 27, 2012, 04:42 

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Ok, thank you. I think my confusion comes from the image at the wiki In this figure the holes are clearly large enough to accommodate fluid motion. But as I understand now this is an idealized figure and in reality the pressure taps are much much smaller. So what is the size ratio (pressure tap/flow system) where the flow in the pressure taps becomes of no importance? 

August 27, 2012, 07:23 

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However I can tell you that you don't have to worried about this. It is done in away that there is definetely no motion in the tubes. 

August 27, 2012, 08:09 

#9 
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Filippo Maria Denaro
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I am not an expert of experimental measure but a I can understand from the figure, you measure the difference of the static pressure in two stations, therefore assuming from Bernouilli that the stagnation pressure is conserved, you get a measure of the differential dynamic pressure and then you have a way to measure flow rate.
Microrecirculation in the hole is minimized (if there is) and its contribution is simply disregarded from the evaluation... I don't think that makes sense to consider it, much more relevant are the hypotheses for which you apply the Bernoulli integral... If I remember rightly, the book of White considers an example ... 

August 31, 2012, 02:09 
mass flow measurement

#10  
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Michael Freestone
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If you have two pressure tappings at different locations along the length of the venturi, then a manometer may be used to measure the difference in pressure between them. If the total pressure (i.e. stagnation pressure of the flow) is known, as is the case if the flow through the venturi is being drawn into it from the local ambient atmosphere), then the mass flow through the venturi may be deduced simply from the pressure difference. However, this will involve further assumptions or measurements, for example the temperature of the ambient atmosphere will require to be measured or assumed. Calibration of the venturi is necessary if the mass flow through it is required accurately, and this can be done in various ways, for example by use of an orifice meter. If you are embarking on experimental work, then you should consult some well established reference books, such as Pankhurst and Holder. 

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