# Mass flow rate using a pitot tube

• Poodge*
In summary, to determine the flow rate using a venturi and compare it to the flow rate from a pitot tube, you will need to calculate the pressure difference between the stagnation pressure and the static pressure, use the Bernoulli equation to find the velocity at each location, and then multiply the velocities by the cross sectional area of the venturi to get the flow rate.
Poodge*
I am majorly stuck, we are doing a report in which we determine the flow rate using a venturi then compare values with the flow rate from a pitot tube.
So basically we have recorded pressure values with a pitot tube, moving the tube 2mm from the centre of the venturi each time and re-recording. I believe these are stagnation pressures and I need to calculate the static pressure to work out the velocity? Which i could then use to work out flow rate
We redid this experiment twice after changing the pressure along the venturi
These are the results obtained from the 30mmH20 reading from the manometer

Distance from center Average pitot pressure
0mm 35.55
2mm 34.85
4mm 34.75
6mm 34.5
8mm 34.1
10mm 33.7
12mm 33.15
14mm 31
16mm 25.55

density = 1.247600752 (kg/m^3)
air pressure = 759 mmHg
temperature = 290K
Large diameter of venturi = 108.10 mm
Small diameter of venturi = 29.00 mm

I have all these values and I am so lost as to what to do with them.

The first step is to calculate the pressure difference between the stagnation pressure and the static pressure. To do this, you will need to subtract the atmospheric pressure (759 mmHg) from the stagnation pressure readings for each distance from the center. This will give you the pressure difference at each distance. Next, you will need to calculate the velocity of the air at each location. This can be done using the Bernoulli equation and the pressure difference you just calculated. The equation is v = (2(P1-P2)/ρ)^0.5, where P1 is the stagnation pressure, P2 is the static pressure, and ρ is the density of the air.Once you have the velocities for each location, you can calculate the flow rate by multiplying the velocities by the cross sectional area of the venturi. The cross sectional area of the venturi is (π/4)*(D2^2 - D1^2), where D1 is the smaller diameter of the venturi and D2 is the larger diameter. Finally, you can compare the flow rate calculated with the venturi to the flow rate calculated with the pitot tube.

## 1. What is a pitot tube and what is its purpose?

A pitot tube is a device used to measure fluid flow velocity in a pipe or duct. It consists of a small tube inserted into the flow, with one end pointed into the flow and the other end connected to a pressure gauge. The purpose of a pitot tube is to measure the total pressure of the fluid flow, which includes both the static pressure and the dynamic pressure.

## 2. How does a pitot tube measure mass flow rate?

A pitot tube measures mass flow rate by using the Bernoulli's equation, which states that the total pressure of a fluid is constant along a streamline. By measuring the total pressure at the tip of the pitot tube and comparing it to the static pressure, the velocity of the fluid can be calculated. This velocity is then used in conjunction with the cross-sectional area of the pipe or duct to determine the mass flow rate.

## 3. What are the benefits of using a pitot tube for measuring mass flow rate?

One of the main benefits of using a pitot tube is its accuracy. It is a direct measurement method that does not rely on assumptions or calculations. Additionally, it can be used to measure the mass flow rate of both gases and liquids, making it a versatile tool for various applications. Pitot tubes are also relatively inexpensive and easy to install, making them a practical choice for many industries.

## 4. What are the limitations of using a pitot tube for measuring mass flow rate?

One limitation of using a pitot tube is that it can only measure the mass flow rate in one specific location within the pipe or duct. This means that the measurement may not be representative of the entire flow field. Additionally, the accuracy of the measurement can be affected by factors such as turbulence and variations in the fluid properties. Proper calibration and placement of the pitot tube can help mitigate these limitations.

## 5. Are there any safety considerations when using a pitot tube for measuring mass flow rate?

Yes, safety should always be a top priority when using a pitot tube. When installing the pitot tube, it is important to ensure that it is securely attached and will not become dislodged during operation. It is also essential to follow proper safety protocols when working with pressurized fluids and to use appropriate personal protective equipment. Regular maintenance and cleaning of the pitot tube are also necessary to ensure accurate and safe operation.

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