Download - Lab 1 Hydraulic

Lab 1 Dead Weight Pressure Gauge Calibrator | Group 6

1.0 Introduction

The Bourdon pressure gauge, patented by the French engineer Eugene Bourdon in 1849,

remains one of the most widely used gauges for measuring pressure in liquids and gases of

many different types. It is a type of aneroid pressure gauge consisting of a flattened curved

tube attached to a pointer that moves around a dial. The Bourdon pressure gauge as shown in

figure below has three primary components which is a fluid that transmits the pressure, a

weight and piston used to apply the pressure, and also an attachment point for the gauge to be

calibrated. The weight applies a force over a precisely known area, therefore applying a

known pressure to the fluid. The fluid is water that is essentially incompressible. Since a dead

weight tester is relatively compact the effect of elevation changes on the pressure are

negligible. The pressure at the piston face, therefore, is equal to the pressure throughout the

water in the tester. A spill-pipe is fitted into the barrel which prevents the piston being ejected

by excess pressure.

Figure 1: The Bourdon pressure gauge

Since pressure is derived from force divided by area (F/A), the pressure generated by

a dead weight tester is calculated by multiplying the mass by the acceleration due to gravity

to determine the applied force, and then dividing this by the surface area of the piston

cylinder. A piston cylinder assembly is mounted vertically to the top of the base and

connected via manifolds and pipe work to the screw press or regulator. There are two

1


markings on the piston cylinder which indicate the region in which the weights must be

floating and spinning before checking the reading of the device being calibrated. Some

deadweight testers come with piston cylinders of different sizes which extend the range of the

instrument to higher or lower set of pressures.

2.0 Objective of Experiment

To calibrate a Bourdon type pressure gauge using a dead weight pressure gauge

calibrator

To study the importance of relative height between the dead weight tester and the

gauge in calibration

2


3.0 Theoretical Background

Bourdon pressure gauge is used to measure medium to very high pressures. The pressure to

be measured is connected to the fixed open end of the bourdon tube. The applied pressure

acts on the inner walls of the bourdon tube. Due to the applied pressure, the bourdon tube

tends to change in cross – section from elliptical to circular. This tends to straighten the

bourdon tube causing a displacement of the free end of the bourdon tube.

To calibrate the gauge, we can add weights to a platform on a dead weight tester. The weights

put a known force on to a piston. The piston has a known area, so we can calculate the

pressure. A flexible tube containing water transfers the pressure on the piston to the Bourdon

tube. By adding the weights in increments, we can record pressure readings from the gauge at

each increment. They then remove the weights and record gauge readings. By working out

theoretical results they can work out gauge error and discuss possible causes.

This theory expressed as simply as force acting upon a known area. The pressure produced by

pump is distributed by the manifold, to the base of precision machined piston and to a device

being calibrated or checked. The preselected weights is loaded onto the piston platform are

acted upon by gravity and develop a force that is to be equally opposed by the fluid pressure

from the pump.

Pressure in exerted by water in the cylinder is

P= FA

Where P=Pressure exerted∈the cylinder

F=Force of piston ( pistonmass × gravitational force , 9.81 )

A=Area of piston

3


4.0 Experiment Setup

1. Hydraulic bench2. Dead weight pressure gauge calibrator3. Bourdon pressure gauge4. Weighs5. Water container

Figure: Dead Weight Pressure Gauge Calibrator

4


5.0 Methodology

1. The pressure gauge was placed on the bench top and it was connected inlet tube to the

isolating cock on the gauge manifold.

2. A length of tube may be attached to the calibrator drain and laid into the channel to

prevent spillage of water on to the bench top.

3. The calibrator is leveled by the adjusting feet whilst observing the spirit level.

4. The piston was removed and its mass was determined accurately. The masses of the

calibration weights were determined accurately.

5. The control valves were closed on the bench. Both cocks operate pump starter were

opened.

6. The valve was opened and water admitted to the cylinder. When full, close the valve

on the bench, the pump was switched off.

7. Isolating cock was closed.

8. The piston was inserted and the piston was spin to minimize the effects of friction.

Whilst the piston is spinning, the gauge reading was noted.

9. Procedure was repeated using different masses.

5


6.0 Flow Chart

6

Set up the apparatus

Attach a tube from the calibrator

Check the spirit level whether the calibrator is stable

Determine the mass of piston and masses of the calibration weights

Close control valves and open pump starter

Fill the cylinder with water until full

Close the valve, the isolating cock and the pump

Insert the piston and weights and spin it

Record the gauge reading whilst the piston is spinning

End


7.0 Result and Calculation

Piston

Mass

(kg)

Area of

Piston (m2

)

x10−6

Pressure in

Cylinder

(kN/m2)

Gauge

Reading

(kN/m2)

Absolute

Gauge

Reading

% Gauge Error =

Absolute gauge ErrorActual Pressure

0.5 244.8 20.037 20.000 0.037 0.185

1.0 244.8 40.074 39.000 1.074 2.754

1.5 244.8 60.110 58.000 2.110 3.638

2.0 244.8 80.147 75.000 5.147 6.863

2.5 244.8 100.184 98.000 2.184 2.229

3.0 244.8 120.221 118.000 2.221 1.882

3.5 244.8 140.257 140.000 0.257 0.184

4.0 244.8 160.294 160.000 0.294 0.184

4.5 244.8 180.331 182.000 2.331 1.281

Table 1: Experimental Result

Pressure in Cylinder (kN/

m2)

Gauge Reading

(kN/m2)

Accuracy

(%)

20.037 20.000 99.815

40.074 39.000 97.246

60.110 58.000 96.362

80.147 75.000 93.137

100.184 98.000 97.771

120.221 118.000 98.118

140.257 140.000 99.816

160.294 160.000 99.816

180.331 182.000 98.719

Table 2: Accuracy of the Gauge

7


Calculation:

Pressure in cylinder = (piston mass ×9.81) / area of piston

= (0.5 9.81) / 244.8×10−6

= 20.037 kN/m2

Absolute gauge reading = pressure in cylinder – gauge reading

= 20.037 – 20.000

= 0.037 kN/m2

Percentage gauge error = (absolute gauge error / actual pressure) 100%

= (0.037/ 20.037) 100%

= 0.185 %

Accuracy/Efficiency = (gauge reading / pressure in cylinder) x 100%

= (200 / 200.037) X100%

= 99.815%

*Calculation for the remaining values is carry out by using the same steps as shown at above.

8


0.037 1.074 2.11 5.147 2.184 2.221 0.257 0.294 2.3310

20406080

100120140160180200

Graph of Gauge Reading against Absolute Gauge Error

Absolute Gauge Error (kN/m2)

Gaug

e Re

adin

g (k

N/m

2)

Figure: Graph of gauge reading against absolute gauge error.

0.185 2.754 3.638 6.863 2.229 1.882 0.184 0.184 1.2810

20406080

100120140160180200

Graph of Gauge Reading against Percentage Gauge Error

Percentage Gauge Error (%)

Gaug

e Re

adin

g (k

N/m

2)

Figure: Graph of gauge reading against percentage gauge error.

9


8.0 Discussion

1. Accoding to the plotted graphs of gauge reading against absolute gauge error, the

pressure values obtained through calculations, it shows the deviation of the gauge

readings of 0.037 kN/m2. In the other hand, the graph of gauge reading against

percentage error has a little scattered trend compare to the previous graph. This may

cause by errors and inaccuracies. The average gauge error is 0.185 %.

2. During the experiment, to reduce the effects of friction we spun the piston before

pressure reading is taken. Due to the design of the apparatus, there is only a small gap

between the cylindrical wall and the piston. If the cylindrical walls touch the piston,

friction will be induced and hence the frictional force denoted will lower the force

exerted on the liquid.

3. During the experiment, the thermal expansion will cause the size of the cylinder or

piston to increase because there is a dissipation of energy in the fluid escaping

between the piston and cylinder. Therefore, this will affect the effective area and the

measured pressure will be different. Eventually will affect also the deviation of the

experiment result from the theoretical result.

4. The dynamic viscosity of water at 25°C is lower than oil. This shows tat water has a

lower lubricating property compare to the oil. The irregularities on the surface of the

piston and cylinder will produce a corkscrew effect when spun. This rotation is rised

to a certain velocity to avoid any friction. But anyhow, friction will still exist and this

will cause our reading of pressure to be inaccurate.

5. The Bourdon gauge has a glass cover plate so that the internal mechanisms can be

seen. The pressure element is a curved, hollow metallic tube closed at on end, while

the other end is connected to the pressure to be measured. The tube straightens out a

small amount, pulling on a linkage to which is attached a pointer that moves across a

metallic scale, this happen when the internal pressure is increased. The dial shows

zero when the inside and outside of the bent tube are at the same pressure.

10


6. Another error may causes the changes in the result is the parallax error. Therefore, we

have at least two observers to see the reading. Moreover, the piston may not be

perfectly smooth and frictionless. So, it will contribute the discrepancies throughout

the experiment.

7. Impurities, dirt or corroded is also another factor that causes error which will affect

the result. In order the avoid this, the apparatus must be always leveled.

8. From the equation,

Pressure, P = Force(F)

Area of Piston (A )

The value of F is the product of mass of the calibration weigh with the acceleration

due to gravity (9.81ms-2). The acceleration in Malaysia is less than 9.81 ms-2 because

Malaysia is located at the equator and is described by the equation

Gat equator=9.81m/s2 - Rw2

where, R=Radius of Earth, w=angular velocity of Earth.

11


9.0 Conclusion

According to the plotted graphs of gauge reading against absolute gauge error, the pressure

values obtained through calculations, it shows the deviation of the gauge readings of 0.037

kN/m2. In the other hand, the graph of gauge reading against percentage error has a little

scattered trend compare to the previous graph. This may cause by errors and inaccuracies.

The average gauge error is 0.185 %. %. This can all be due to the errors as discussed in the

discussion section such as value of acceleration of gravity, friction, thermal expansion of

equipment, parallax errors etc. Due to the increasing number of usage, the gauge readings

will slowly deviate due to its elastic properties. Therefore, it is important that the dead weight

tester is used to calibrate the gauge equipment. Hence, the gauge can be quite accurate

provided precaution steps are taken to eliminate some ‘removable errors’ as will be discussed

below.

10.0 Recommendation

1. Before the experiment, make sure that there is no bubble in the pipe to avoid

inaccurate readings.

2. Before the experiment, make sure that the tube is connected and must not be any

leakage to avoid inaccuracy when taking down the readings of the gauge.

3. Before the experiment, make sure that the piston and cylinder is clean to avoid

extra weight on the piston when the experiment is carried out and this will

minimize the friction induced from the surface irregularities.

4. Take reading only when the piston and its calibration weight is in static

equilibrium with to the force up thrust. This is due to the liquid pressure and force

that caused by the down thrust of the liquid weight.

11.0 References

1. B.R. Munson, D.F. Young, and T.H. Okiishi. 2002. Fundamentals of Fluid

Mechanics, 4th ed. John Wiley and Sons, Inc., New York

2. Robert L. Mott. 2006. Applied Fluid Mechanics, 6th ed in SI units. Prentice Hall.

12