# Flow rate - manometer

## Main Question or Discussion Point

Hi ,

Does anyone know , how to calculate the flow rate from a manometer ? there is a pipe attatch to the venturi meter, the inside tube of venturi is 32 mm and the outside 62 mm.
A fan is sucking the air, so flow rate is produced.

Thank You

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First off, is this a gas or another fluid which an be modeled as incompressible (e.g.water)?

If it is a gas you would need the temperature as well to find a mass flow rate. However, if all you needed was a volume flow rate you could get away with just the venturi tube.

Okay, so the mass flow rate of a fluid is as follows:

m_dot=rho*V*A

where V is fluid velocity and A is the area of whatever duct you are using and rho you can find in some reference, for an incompressible fluid we can assume that rho is pretty much constant.

A you can find by measuring and V can be calculated from bernoulli's equation, assuming no major changes in elevation.

V=sqrt(2*(P/rho));

For a gas the density (rho) changes drastically with temperature so one would need to use the ideal gas law to find the correct density which would be a function of pressure and temperature.

It is a gas , assuming air is through the pipe, so i need to find the flow rate of the air.
I need to convert the number that the manometer is giving me to volume flow rate.

thanks a lot