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Changing Static Pressure Feasibility Test – Applying a Vacuum

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Vacuum Feasibility Testing

Author: Greg Penoyer

I.INTRODUCTION

B

rainstorming and initial feasibility researchyielded that there are multiplefeasible methods to eliminate air bubbles from water that should be tested. All feasible elimination concepts must undergo feasibility testing gain a better understanding of their capabilities.

One of the considered concepts to remove the bubbles is to reduce the static pressure of the water. After a design of experiment was constructed, fixturing fabricated and materials gathered; an experiment was conducted. Testing was conducted at a Bausch & Lomb Product Development Laboratory on Saturday January 29, 2006. This experiment used the same water cells, contact lenses, and inspection equipmentand was performed under the same constraints as used in all other feasibility experiments unless other feasibility concepts involved changing these conditions.

II.Governing Theory

To estimate change in bubble behavior that would result from a change in static pressure, the bubble was treated as a simple air pocket within a body of water with a fixed boundary. Fixed in this case assumes that all air that is in the bubble must remain in the bubble and no air is able to enter the bubble.

The equations that govern this theory are derived from the ideal gas law. This equation states that when a contained body of gas undergoes a thermodynamic change, the relation between the Volume in which it’s contained, the Pressure within the gas, and the Temperature of the gas contained must all remain the same. More specifically: ((PV)/T)=constant. To analyze how a volume of gas reacts to a thermodynamic process the following equation can be used.

Where the subscript i and f denote properties, at initial and final states respectively.

The pressure of the fluid around the bubble is directly related to the atmospheric pressure above the liquid by the following equation:

Using this equation we can assume that when temperature shall remain constant, if static pressure is reduced, the volume must increase.

We can also assume that this increase in volume will cause an increase in the buoyancy force of the bubble. Buoyancy force is defined as the force lifting a volume suspended in a fluid. This force is a result of the pressure acting on all surface areas of a liquid. This buoyancy force acting on the suspended volume is equally to the weight of the displaced liquid.

In this case, the liquid is de-ionized water and the object is the bubble suspended in the water. This theoretical reasoning yields adequate justification to conduct feasibility testing for this method of bubble removal.

In summary, based on the theoretical information, the buoyancy force can be increased by decreasing the pressure of the bubble. From the following equation,

(patm=atmospheric pressure about liquid, d=depth of liquid, g=gravity)

it can also be concluded that the buoyancy force will be increased by when the atmospheric pressure above the liquid surface.

III.Materials Used/Set up

Materials used are as follows:

  1. 1 HP Vacuum Unit
  2. .25” OD Air Hose
  3. Vacuum Head
  4. 30 Contact Lenses
  5. Digital Camera
  6. BK7 Optical Water Cell
  7. Tweezers
  8. De-ionized Water
  9. Vacuum Pressure Gage
  10. Open/close valve for vacuum lines

Vacuum equipment was connected with the air hose as shown in illustration:

Prior to placing the vacuum head onto water cell and conducting the test, the pressure was set. To set the pressure, the vacuum unit was switched on, the vacuum head was placed on a flat surface and turned on and adjusted to the proper pressures based on each of the testing scenarios.

IV.Procedure

Once properly set up, 12ml of de-ionized water was place in a water cell along with one contact lens. At this time a picture was taken of the contact lens using the inspection camera on the one up station. This picture is used as the ‘before’ picture. Once the picture was taken the vacuum head was placed on top of the water cell as shown in illustration:

The head was checked to make sure that it was centered on the water cell. The air valve was then opened to apply the vacuum to the water cell. After the desired time elapsed the valve was then closed.

After the vacuum was applied and released, the vacuum head was removed from the water cell. An ‘after’ picture was then taken and compared to the before picture

V.Testing Scenarios

Three lenses were tested at each of the following pressures for the different lengths of time.

Pressure was applied for 4 seconds for each of the following gage pressures kPag (atmg): -23.7 (-.234), -47.4 (-.468), -67.7 (-.668), -84.6 (-.836).

Pressure was then applied at -91.4 kPag (-.902 atmg) for 8, and 20 seconds.

Ambient air pressure in lab was approximately 98.2 kpa (.969 atm).

VI.Results

Tabulated Results and pictures are attached in Appendix

4 seconds:

When various pressures were applied for 4 seconds there were little to no results seen in the pictures for any of the pressures. One of the -84.6kPag trials displayed a slight reduction in bubbles.

*It should be noted that two trials of the -23.7 kPag for 4 seconds were repeated because the vacuum head was not centered correctly on the water cell. This misalignment caused a wind vortex within the water cell causing a great deal of bubbles and disordered. This test error should be considered as an area of future concern in design.

-91.4 kPag for 8 seconds:

Trial 1 removed nearly all of the bubbles in the water cell. Some bubbles that remained after the process may have been impurities in the water.

Trial 2 removed only a few of the large bubbles. Nearly 80-90% of the bubbles remained after.

Trial 3 did not show any bubbles removed after the vacuum process.

-91.4kPag for 20 seconds:

All 3 trials showed the removal of 90-95% of the bubbles.

VII.Conclusion

The success of this test varied, when pressure was applied for the known cycle time of 4 seconds; the bubbles could not be removed at any pressure. However, the success of the highest vacuum applied for 20 seconds inspired the idea of degassing the de-ionized water prior to inserting the lens. Since the results of this test are mixed and have yielded some positive results, it can be determined that applying vacuum pressure is a method that should be further researched and developed.

In parallel to a stand alone bubble removal process, degassing the water should also be researched and developed to help prevent the formation of bubbles.

Appendix

All Pressures in kPa gage (atm gage)

Pressure Applied for 4 seconds
Pressure / Trial 1 / Trial 2 / Trial 3
-23.7
(-.234) / No change / No change / No change
-47.4
(-.468) / No change / Little to no change / No change
-67.7
(-.668) / No change / No change / No change
-84.6
(-.836) / No change / No change/ Greatly moved the lens / Removed some bubbles
Pressure Applied for 8 seconds
Pressure / Trial 1 / Trial 2 / Trial 3
-91.4
(-.902) / Removed nearly all bubbles / Removed some large bubbles, less than 20% removed / Little to no change
Pressure Applied for 20 seconds
Pressure / Trial 1 / Trial 2 / Trial 3
-91.4
(-.902) / Removed almost all bubbles, >90% / Removed almost all bubbles, >90% / Removed almost all bubbles, >95%

Table 1. Summary of DOE

Report submitted for review February 10, 2006. This work was supported in part by Rochester Institute of Technology Multidisciplinary Senior Design Project, group T06219. All financial support was provided by Bausch & Lomb co.