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MECH 220 MECHANICAL ENGINEERING LABORATORY I

Experiment B

Pressure Measurement

OBJECTIVE

The purpose of this experiment is to demonstrate different pressure measurement methods. Pressure measurement operations will be conducted by using U-tube and inclined tube manometers. Bourdon tube manometer will be calibrated by using dead weight piston gauge. THEORY

Pressure is a very important characteristic of a fluid. It is represented as a force per unit area. There are numerous devices and techniques that are used in pressure measurement. The pressure at a point within a fluid mass will be designated as either an absolute pressure or a gage pressure. Figure 1 illustrates the absolute and gage pressures. Absolute pressure refers to the absolute value of the force per unit area exerted on the containing wall by a fluid. Gage pressure represents the difference between the absolute pressure and the local atmospheric pressure. A gage pressure of zero corresponds to a pressure that is equal to the local atmospheric pressure. It is resulted as, absolute pressures are always positive but gage pressures can be either positive or negative. Vacuum represents the amount by which the atmospheric pressure exceeds the absolute pressure. A negative gage pressure is also referred to as vacuum pressure.

Figure 1. Graphical representation of absolute and gage pressure.

The standard SI unit for pressure is the Newton per square meter (N/m2) or Pascal (Pa). In the imperial system, pressure is usually expressed in pounds per square inch (psi). Pressure can also be expressed as the height of a column of liquid. Then, the units will refer

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to the height of the column and also the liquid in the column must be noted. At standard atmospheric pressure, the height is 760 mm of mercury (760 mmHg). Most commonly used units for pressure are,

1 = 1.01325 105

2= 760

1

2= 1

1 = 105

2= 100

1 = 133.322

2= 133.322

One type of a manometer which is widely used consists of a tube formed into the

shape of a U. The difference between the unknown pressure and the atmospheric pressure is determined as a function of the differential height . Figure 2 shows a schematic of U tube manometer.

Figure 2. U-tube manometer.

The pressure can be written in terms of the column heights as,

= (1) Manometers can be oriented in an inclined position to lengthen the scale and to

improve readability. These types of manometers are used to measure small pressure changes. Note that, in inclined manometers, vertical distances between the fluid levels must be considered to measure pressure difference.

Bourdon pressure gage is one of the most commonly used devices. It operates on a principle that when pressure acts on an elastic structure, the structure will deform, and this deformation can be related to the magnitude of the pressure. The essential mechanical element in this gage is the hollow, elastic curved tube which is connected to the pressure source. As the pressure within the tube increases the tube tends to straighten, this deformation translated into the motion of a pointer on a dial. A zero reading on the gage indicates that the measured pressure is equal to the atmospheric pressure.

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EXPERIMENTATION Test Set-up

The experimental set-up is composed of two units for two sets of experiments. The first unit is for measuring pressure by using manometers which is shown in Figure 3. It has two manometers one of which is inclined. The inclination angle is given as 60o. The fluid in the U-tube manometer is water, and in the inclined U-tube manometer is water mixed with ink. Second unit is also shown in Figure 3 which is used to calibration by using load weights.

First unit 1. U-tube manometer 2. Dial manometer 3. Inclined manometer Second unit 4. Bourdon tube manometer 5. Piston 6. Dead weights

Figure 3. Schematic of the experimental setup.

Table 1 shows some important parameters of the test set-up that are needed in the analysis.

Table 1. Some parameters about the test set-up.

Parameter Value

Density of the fluid in manometers Density of water Inclination angle of the inclined manometer 60o Diameter of the piston 12 mm Weight of the carrier 519 g Uncertainty in electronic scale 0.05 Uncertainty in Bourdon gage 0.025

Procedure For the first unit, 1. Check if the U-tube manometer is open to the atmosphere through the blue hose (i.e. the

level of the fluid in the manometer is the same for both sides). 2. Connect the syringe to the small white hose which is connected to one side of the U-tube

manometer by the blue hose. 3. Apply a certain pressure with the help of the syringe. For the first step, this certain

pressure should be equal to 5 ml difference on the syringe. 4. Read the level of the fluid in the U-tube manometer. 5. Repeat steps 3 and 4 for 6 times.

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6. Change the applied pressure to 2 ml difference on the syringe and repeat the same procedure.

7. Check if the inclined U-tube manometer is also open to the atmosphere through a blue hose.

8. Remove the small white hose from the U-tube manometer and place it through the blue hose which is connected to the inclined U-tube manometer.

9. Repeat steps 3 to 6.

For the second unit, 10. Rotate the piston arm to balance the carrier on the piston. 11. Read the corresponding value from the Bourdon gage. 12. Measure the masses of the load weights by using an electronic scale provided to you. 13. Place the smallest weight onto the piston and read the corresponding value from the

Bourdon gage. 14. Start placing the three of the larger weights one by one on the piston and read every

change from the Bourdon gage.

Analysis 1. Estimate the mean values and associated uncertainties of your readings for both

manometers. 2. Compare the results from U-tube and inclined U-tube manometers for the same

pressures. 3. Calculate the pressures caused by the weights by using the equation for pressure with the

associated uncertainties. 4. Draw the measured pressure vs calculated pressure for the Bourdon gage. Show the

uncertainties as error bars on the graph.

CAUTION!

The experiment will be conducted in the Heat Transfer Laboratory in the Lab Building.

You will have only 40 minutes to complete the experiment.

Name: Group No: Date:

MECH 220 MECHANICAL ENGINEERING LABORATORY I EXPERIMENT B

PRESSURE MEASUREMENT Data Sheet

Table 1. Fluid levels in U-tube manometer.

Fluid Level (mm) (5 ml difference)

Fluid Level (mm) (5 ml difference)

Fluid Level (mm) (2 ml difference)

Fluid Level (mm) (2 ml difference)

1

2

3

4

5

6

Table 2. Fluid levels in inclined U-tube manometer.

Fluid Level (mm) (5 ml difference)

Fluid Level (mm) (5 ml difference)

Fluid Level (mm) (2 ml difference)

Fluid Level (mm) (2 ml difference)

1

2

3

4

5

6

Table 3. Pressure and mass values from the second unit of the experiment.

Pressure (bar) (Bourdon Gage Reading)

Mass (g)

1

2

3

4

5

6