Calibration of pressure gauge using dead weight tester

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Aim: On completion of this unit you will be able to check the accuracy of a Bourdon tube pressure gauge using dead weight tester.

Theory: Many types of gauges are available for measurement of pressure. The Bourdon gauge (named after its inventor Bourdon) uses the deflection of a tube of oval cross-section to cause a pointer to move over a scale. Because of its simplicity and low cost, and the large selection of pressure ranges, which are available, the Bourdon gauge is widely used engineering practice.
All pressure gauges, of whatever type, needs to be calibrated. As the calibration may change over a period, repeat calibrations may well be needed from time to time. The normal calibration procedure is to load the gauge with known pressures, using a dead weight tested using oil. The present experiment, however, works satisfactorily with oil.
In this experiment we check the accuracy of the Bourdon Tube pressure gauge. The procedure will be to calibrate the gauge by applying weights of known magnitude to the piston of known cross-sectional area and hence create a known (calibration) pressure. The experiment will explore gauge error and prompt backlash scale marking.
The actual hydro-static pressure(p) in the system due to a mass of M kg (including the piston mass) applied to the piston is given by:
$P1 = \frac{M(in kg)}{A}$ Kg/cm²
Where A is the piston area in cm²
% Gauge error = $ \frac{Gauge Reading(P2)-Calculated pressure(P1)}{Full scale deflection} *100 $ In our setup piston area = A = $\frac{\pi d^2}{4}$ (Where d= diameter of piston i.e. bore of cylinder = 1.6 cm)
A = 2cm².
Per kg put in the pan,should produce pressure equal to half of in kg/cm² i.e. for 10kg weight, pressure = 5kg/cm².
Equipment needed:

• Dead weight tested setup.
• Weights: 1kg, 2kg, 5kg, etc.
• Oil hydraulic 68 no.

About the setup: Dead weight tester (DWT) setup consists of 4X3 aluminum profile rack, two numbers of double acting cylinders of 16mm bore for adjusting piston & for vertical piston. The vertical piston hold the weighting pan for weight, pressure gauge of 0-25 kg/cm², oil reservoir, 6 way manifold, shutoff valve etc., All the about components are mounted on PCV composite sheet and all the piping of the set up is made using SS tube.
To test external gauge, a quick release coupling (QRC) has been QRC connector cable without air bubble. One port from manifold is provided for external connection of sensor to measure pressure electronically for signal conditioning circuit.
Method of flotation: The piston has been made leak proof by using rubber 'O' rings inside the cylinder. This 'O' ring offers friction to movement of piston under small/delta addition in the weights e.g. 1kg, 2kg, etc., observed while taking the readings. you overcome the friction by

1. Either press down the weight pan manually & release, the bourdon pressure gauge then will show the correct pressure. the pointer will move up & fall back when you release your hand/manual pressure. Now that is the correct pressure proportional to the weight.
2. Method of floatation: There is another way to doing this. Press down adjusting piston by rotating the handle clockwise until the pointer on pressure gauge shows a slight jump & it settles to pressure proportional to weight having overcome the friction of the rubber 'O' rings inside.

Procedure:

1. Cross sectional area of piston should be noted i.e. = 2 cm².
2. Rotate the handle provided on adjusting piston to CCW so that the rod of adjusting piston fully comes out.
3. Pour the oil into the reservoir provided on rack up to its half level.
4. Open the shut off valve connected inline to oil reservoir.
5. Gently lift the piston of vertical cylinder manually to its maximum position so that the oil from reservoir enters in to the setup filling it completely without leaving any air bubbles.
6. Close the shutoff valve.
7. Now the setup is ready for experimentation.
8. Spin the vertical cylinder piston to insure it is floating freely.

Operation:

• Now add the different masses/weights gradually & note down the pressure gauge reading. Fill up observation table. After addition of each weight into the pan press the pan downwards & release to overcome friction of piston (piston sticking) due to 'O' rings used inside OR press down adjusting piston by rotating the handle clockwise until the pointer on pressure gauge shows a slight jump & settles to pressure proportional to weight having overcome the friction of the rubber 'O' rings inside.
• Reverse the procedure while taking readings as the masses/weights are removed in steps.

Note: Do not open the shut off valve in between the readings as the oil inside the cylinder will come out in reservoir with pressure and may spill over when under pressure due to weight.
Observation table
Pressure gauge reading with empty pan P_z= 0.9 kg/cm²(This is gauge initial error)

Sno	M	P1	Gauge reading				Gauge error %
			Increasing		Decreasing		Increasing	Decreasing
			PG	P2	PG	P2
1	2
2	4
3	5
4	8
5	10

Where M = Mass added to piston(kg).
P1 = Actual pressure calculated using formula. $P1 = \frac{M}{A}$kg/cm².
P_G = Gauge indicated pressure.
P2= P_GP_z
Plot following graphs.

• Gauge pressure versus calculated pressure.
• Gauge error versus calculated pressure.

Conclusion The graph plotted shows that the increasing pressure is similar with the decreasing pressure where the slope plotted is almost overlapped to each other.
However, the point plotted is not smoothly linear or form a straight line. Furthermore, we can see that the gauge error occurred inconsistently where the line plotted is going upward and downward. There are several factors that may influenced the results of this experiment such as:

1. There are bubbles or air trapped inside the tube when the experiment is carried out.
2. Pressure is exerted on the piston during it is placed into the cylinder where it can affected the reading of gauge pressure taken for entire experiment.
3. The piston is not totally in stable condition during the value of gauge pressure is taken.
4. Parallac error is occurred where the observer's eyes are not perpendicular to the scale of the Bourdon gauge during the value of gauge pressure is observed.
5. theobjective to determine the accuracy in Bourdon gauge readings and calibration requirements is achieved.