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Air Pressure Measurement System

Gaurav Rajendra Deshmukh      Rahul Prasad Prajapati     Nitin Thakur    Rasika Yenorkar    

Department Of Instrumentation and Control Engineering

College Of Engineering, Pune

Abstract— In this paper, we have explained how the air pressure measurement system works. The maximum air pressure to be measured is 10kg/sq.cm. The operating range is 6kg/sq.cm to 8 kg/sq.cm. The pressure sensor we have used is diaphragm along with strain gauge. Our main purpose is to measure the air pressure within the operating range.  

Keywords—diaphragm; deflection;gauge factor; strain

I. INTRODUCTION

Pressure sensors can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the type of pressure they measure.

A pressure sensor is a device which senses pressure and converts it into an analog electric signal whose magnitude depends upon the pressure applied. It is used to convert a certain value of pressure into its corresponding mechanical or electrical output.

 The types of pressure sensors are differentiated according to the amount of pressure they are able to measure. For low differential pressure measurement Liquid Column Manometers are used. Elastic type pressure gauges are also used for pressure measurement up to 700 MPa.

Pressure metrology is the technology of transuding pressure into an electrical quantity. Normally, a diaphragm construction is used with strain gauges either bonded to, or diffused into it, acting as resistive elements. Under the pressure-induced strain, the resistive values change.

Diaphragm are specialists in the process industry when it comes to critical measuring tasks such as with highly corrosive or viscous media or when it comes to overpressure. Diaphragm pressure gauges are suitable for gauge, absolute and differential pressure.

Sensors used for Air Pressure measurement are:

1) Diaphragm

 Shape: Flat

  Corrugated

  Flattened tube

   Capsule

2) Bellow                           3) Manometer                           4) Bourdon Tube                                         5) vacuum gauge

II. WORKING PRINCIPLE

Diaphragm pressure gauges :

Diaphragm pressure gauge use is cast iron flat circular diaphragms.

A diaphragm will be loaded with the process pressure on one side. When pressure is applied on one side of diaphragm, it get deform. Deformation is function of applied pressure, the deformation is measured using a suitable device for measurement of displacement. Such device work on an electrical principle like change in resistance.

The mathematical relation between pressure and central deflection for a flat circular diaphragm is given by

              

To have a linear pressure deflection relation, the second and later terms should be small.

p = applied pressure

t = thickness

a = radius

yc = central deflection

E = Young’s modulus

μ = Poisson’s ratio  lateral strain

                              Axial strain

As seen from the equation the pressure deflection relation is not linear. If a non-linearity of 5% is acceptable the deflection must be less than 1/3 of thickness. Neglecting the higher order terms within brackets,

 

Where K is the sensitivity of the device.

Strain Gauge :

Strain gauge: it is an electrical conductor whose resistance changes as it strained. When strain is applied to a thin metallic wire, its dimension changes, thus changing the resistance of the wire. The factors responsible for the change in resistance is Gage Factor.

The gauge factor {\\displaystyle GF} is defined as:

 

The strain gauge is tightly bonded to a measuring object so that the sensing element may elongate or contract according to the strain borne by the measuring object. When bearing mechanical elongation or contraction, most metals undergo a change in electric resistance. The strain gauge applies this principle to strain measurement through the resistance change.

A typical arrangement of strain gauges on a diaphragm is shown in figure. Application of pressurized air displaces the diaphragm. This displacement is measured by the stain gauges in terms of radial and/or lateral strains. These strain gauges are connected to form the arms of a Wheatstone bridge.

 

III. PROBLEM STATEMENT

Measurement of header pressure of the pneumatic line and limiting the header pressure at 10 kg/sq.cm.Designing the primary sensor i.e. diaphragm, designing the casing for the transducer, mounting the secondary sensor with its signal conditioning unit & calculating the sensitivity of the bridge circuit and verifying it theoretically simulation based and practically. The operating range is to be maintained within the range of 6 kg/sq.cm to 8 kg/sq.cm.   

IV. BLOCK DIAGRAM

1) Pumping unit: - It’s a storage tank which will act as a source of air supply and provide an air pressure within the operating range of 6kg/cm2-8kg/cm2.

Its consist of a pressure gauge which will help to know the exact air pressure at that instant.

2)Diaphragm:-The diaphragm which is very much sensitive to the pressurized air  is made up of cast iron and produces the detectable displacement when its lower side is subjected to the pressure. As a result of applied pressure, deformation of diaphragm occurs which is proportional to the magnitude of the air supply.

3) Strain gauge:-It’s a secondary transducer that converts displacement into electrical quantity. The electrical resistance varies in proportion to the amount of strain on the diaphragm. This strain gauge consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid is bonded to a thin backing called the carrier, which is attached directly to the test specimen. Therefore, the strain experienced by the test specimen is transferred directly to the strain gauge, which responds with a linear change in electrical resistance. A fundamental parameter of the strain gauge is its sensitivity to strain, expressed quantitatively as the gauge factor (GF). GF is the ratio of the fractional change in electrical resistance to the fractional change in length, or strain.

4) Signal conditioning Bridge:-Here, Wheatstone   bridge is used as it can measure the static as well as dynamic change in resistance. It consist of four arms of which, one arm is connected to the leads of strain gauge.  The output voltage of the Wheatstone bridge is expressed in millivolts output per volt input. The output depends upon the parameters connected in bridge which helps to balance the bridge.

5)Operational amplifier:- IC 741 can be used to amplify the signal produced at the output of the Wheatstone’s bridge .It is an 8pin IC of which pin no.2,3 and 6 are most important pins from operation point of view where pin 2 is inverting input, pin 3 is non-inverting and output is taken from pin no.6. In this case, it will produces the output within the range of 0-10volts.

The output produced in terms of voltage at this stage is proportional to the pressure applied on the diaphragm.

V. EQUATIONS

Radial Strain:

Bridge Output:

Vo = (Vs/4)* Gf*strain

Where Gf-gauge factor

Vs-bridge excitation voltage

Vo-bridge output voltage

VI. RESULTS

Fig. 1.

Fig. 1. Shows the graph of change in deflection to the applied pressure.It can be seen from the graph that for per 0.2 kg/sq.cm of air pressure, the deflection of the diaphragm is 0.272638658 mm.Therefore, the relationship seems to be linear for the overall applied pressure range. The sensitivity in terms of mm per kg/sq.cm is 1.36319329.

Fig. 2.

Fig. 2. Shows the graph of radial strain versus the applied pressure. It can be seen from the graph that for per 0.2 kg/sq.cm of air pressure, the radial strain developed along the strain gauge is 2585.018382 microns. Therefore, the sensitivity of diaphragm in terms of micron per kg/sq.cm is 12925.0919.

Fig. 3.  

Fig. 3. Shows the graph of change in resistance versus deflection. It can be seen from the graph that for per 0.272638658 mm of deflection of the diaphragm, the resistance change observed within the strain gauge is 1.420467601 ohms for 350 ohm strain gauge and 0.487017463 ohms for 120 ohms strain gauge. Therefore, the sensitivity of the strain gauge in terms of change in resistance to diaphragm deflection is 5.21 ohms per mm for 350 ohms strain gauge and 1.78 ohms per mm for 120 ohms strain gauge.

Fig. 4.

Fig. 4. Shows the graph of change in resistance versus the applied pressure. It can be seen from the graph that for per 0.2 kg/sq.cm air pressure, the change in resistance observed for 350 ohms is 1.420467601 ohms and that for 120 ohms strain gauge is 0.487017463 ohms.

Fig. 5.

Fig. 5. Shows the graph of sensitivity of bridge versus the applied pressure. It can be seen from the graph that for per 0.2 kg/sq.cm of air pressure, the sensitivity of the bridge changes 1.014619715 mv/v. The overall bridge sensitivity i.e. the average of the individual sensitivities is 25.85 mV/V that closely matches to the theoretically calculated e0 (sensitivity) of the bridge circuit.

Fig. 6.

Fig. 6. Shows the final sensitivity graph i.e. the graph of bridge output voltage versus the input pressure. The final graph shows that for every 0.2 kg/sq.cm of pressure change, the bridge output increases by 10.14 mV.

Fig. 7.

Fig. 7. Shows the graph of amplifier output voltage versus the change in deflection. It can be seen from the graph that for every 0.272638658 mm of diaphragm deflection, the amplifier output increases by 0.193 V.

Fig. 8.

Fig. 8. Shows the final sensitivity graph i.e. the graph of amplifier output voltage versus the input pressure. The final graph shows that for every 0.2 kg/sq.cm of pressure change, the amplifier output increases by 0.1927 Volts.

VII. CONCLUSION

Based on the theoretical calculations, we have observed that there exists linear relationship between various parameters as discussed in the results.

And also we have observed that the calculated value of the bridge sensitivity and the average of the bridge sensitivities at various pressure levels is approximately equal i.e. 25.36 mV/V.

The whole assembly is calibrated to display the pressure range from 0 kg/sq.cm to 10 kg/sq.cm to give output within the range 0-10 Volts.

VIII. REFERENCES

[1] Ambarish G. Mohapatra / International Journal of Engineering Science and Technology (IJEST) Design and Implementation of Diaphragm Type Pressure Sensor in a Direct Tire Pressure Monitoring System (TPMS) for Automotive Safety Applications ISSN : 0975-5462 Vol. 3 No. 8 August 2011.

[2] https://en.wikipedia.org/wiki/Pressure_sensor

[3] http://www.vishaypg.com/docs/11060/tn5101tn.pdf “Design considerations for diaphragm pressure transducers”.

[4] http://www.dynisco.com/stuff/contentmgr/files/1/2604e6ab8ea5dedf1e46af7a3d771c37/pdf/straingage.pdf “Strain gauge pressure transducers design”.

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