Axial magnetic field regulated low temperature plasma material surface processing device

Abstract

This invention belongs to the field of plasma applications and relates to a material surface treatment device for axial magnetic field-controlled low-temperature plasma. The device includes an outer cylinder, a high-voltage tip electrode, a flat ground electrode, an electromagnetic coil, and a magnetic core. One end of the high-voltage tip electrode penetrates the left cover plate and extends into the discharge cavity, while the other end is connected to a high-voltage power supply. The flat ground electrode is disposed inside the outer cylinder and opposite to the high-voltage tip electrode. A placement platform for placing the material to be treated is pre-fabricated on the side of the flat ground electrode closest to the high-voltage tip electrode. An electromagnetic coil is laterally arranged around the outer cylinder and connected to a magnetic control unit. A magnetic core is disposed on the inner wall of the outer cylinder. An air inlet is opened on the left cover plate, and an air outlet is opened on the right cover plate. A large-area diffuse distribution of atmospheric pressure glow discharge plasma can be obtained between the high-voltage tip electrode and the flat ground electrode. The size of the distribution space can be precisely adjusted by the magnitude of the magnetic field strength, thereby expanding the distribution space of atmospheric pressure glow discharge diffuse low-temperature plasma.

Description

Technical Field

[0001] This invention belongs to the field of plasma applications, and specifically relates to a material surface treatment device for axial magnetic field-controlled low-temperature plasma. Background Technology

[0002] Plasma is a fourth-state substance rich in electrons, ions, neutral particles, and active free radicals. Low-temperature plasma, due to its low ionization density and minimal thermal effects from high-energy electrons and other active particles, is widely used in biomedicine, material modification, and surface treatment applications such as thin film and coating deposition. Atmospheric pressure glow discharge is an effective and low-cost method for obtaining low-temperature plasma without the need for vacuum equipment. In most applications that do not require a special gas medium, low-temperature plasma with high electron density and abundant active free radicals can be directly generated in open air. Therefore, atmospheric pressure glow discharge plasma has broad industrial application prospects.

[0003] Glow discharges are easily generated under low pressure. However, due to the large number of gas molecules per unit space at atmospheric pressure, the mean free path of electrons during the development of gas discharge is significantly reduced compared to low pressure. The high collisional ionization frequency leads to rapid electron avalanche development, making it difficult to obtain stable diffuse atmospheric pressure glow discharges. In many low-temperature plasma applications, especially in material surface treatment, plasma must be diffusely distributed over a large area; otherwise, excessive local energy concentration can damage biological tissues or material surfaces.

[0004] Currently, atmospheric pressure glow discharge plasma with a length exceeding 10 mm can be generated in the gap between the tip plates using a DC source combined with a large resistance ballast, a nanosecond pulse power supply, and a resonant constant current source. However, the structural characteristics of the tip plate gap cause the plasma to be distributed in a contracted filamentary region, and the energy is still too concentrated. At present, the low-temperature plasma sources used for material surface treatment are basically in the form of dielectric barrier discharge. The glow discharge region is too small. The diffuse plasma distribution formed by dielectric barrier discharge is actually composed of a large number of microfluidic column discharge channels that are randomly distributed in space and time, which may cause damage to the material surface. Summary of the Invention

[0005] The purpose of this invention is to provide a material surface treatment device for axial magnetic field-controlled low-temperature plasma, which solves the problem that the current glow discharge region is too small and may cause damage to the material surface.

[0006] This invention is achieved through the following technical solution:

[0007] A material surface treatment device for axial magnetic field-controlled low-temperature plasma includes an outer cylinder, a high-voltage pointed electrode, a flat ground electrode, an electromagnetic coil, and a magnetic core.

[0008] The outer cylinder has a left cover plate on one side and a right cover plate on the other side. The cavity sealed by the outer cylinder, the left cover plate, and the right cover plate is the discharge cavity.

[0009] One end of the high-voltage tip electrode passes through the left cover plate and extends into the discharge cavity, while the other end is connected to the high-voltage power supply.

[0010] The flat ground electrode is set inside the outer cylinder and is positioned opposite to the high-voltage tip electrode. A placement platform for placing the material to be processed is prefabricated on the side of the flat ground electrode that is close to the high-voltage tip electrode.

[0011] An electromagnetic coil is horizontally arranged around the outer cylinder, and the electromagnetic coil is connected to the magnetic control unit.

[0012] A magnetic core is provided on the inner wall of the outer cylinder;

[0013] An air inlet is provided on the left cover plate and an air outlet is provided on the right cover plate. The air inlet and outlet provide channels for the discharge gas medium to enter and exit.

[0014] Furthermore, the magnetic core has a ring-shaped structure.

[0015] Furthermore, the air inlet includes an upper air inlet located on the upper side of the left cover plate and a lower air inlet located on the lower side of the left cover plate, and the air outlet includes an upper air outlet located on the upper side of the right cover plate and a lower air outlet located on the lower side of the right cover plate.

[0016] When the molar mass of the target gas medium is greater than or equal to that of air, the upper air inlet and lower air outlet should be selected.

[0017] When the molar mass of the target gas medium is less than or equal to that of air, the lower inlet and upper outlet should be selected.

[0018] Furthermore, both the left and right cover plates are insulated cover plates.

[0019] Furthermore, the flat ground electrode is connected to a support rod, the outer end of which extends out of the right cover plate. The support rod is connected to the ground wire to provide zero potential.

[0020] Furthermore, the high-voltage tip electrode, the flat ground electrode, and the support rod are made of brass.

[0021] The insulating cover is made of epoxy board or polytetrafluoroethylene;

[0022] The outer cylinder is made of copper, aluminum, or aluminum alloy.

[0023] Furthermore, the diameter of the outer cylinder is selected according to the size of the material to be processed in order to reduce the spatial attenuation of the applied magnetic field strength.

[0024] Furthermore, the magnetic core is made of non-metallic magnetic core material.

[0025] Furthermore, both the air inlet and outlet are connected to air valves. The air valve connected to the outlet is connected to a pressure gauge via a three-way air connector to monitor the air pressure inside the cavity.

[0026] Furthermore, the gap between the high-voltage tip electrode and the flat ground electrode is the diffuse plasma distribution region. The magnetic field lines formed by the magnetic core and the electromagnetic coil are parallel to the axial direction of the electrode gap. The electric field lines formed between the high-voltage tip electrode and the flat ground electrode have both vertical and horizontal components in the axial direction. The vertical component provides rotational driving force for the plasma channel, while the horizontal component provides acceleration and binding force for the axial transmission of charged particles in the gap.

[0027] By adjusting the intensity of the external magnetic field, a diffusely distributed atmospheric pressure glow discharge low-temperature plasma is formed between the high-voltage tip electrode and the flat ground electrode, which is used to control the distribution space of the discharge plasma and the contact area on the flat ground electrode.

[0028] Compared with the prior art, the present invention has the following beneficial technical effects:

[0029] This invention discloses a material surface treatment device for axial magnetic field-controlled low-temperature plasma, comprising an outer cylinder, a high-voltage tip electrode, a flat ground electrode, an electromagnetic coil, and a magnetic core. The high-voltage tip electrode provides a strong electric field for discharge in the gas gap between the electrode tip and the ground electrode. The electromagnetic coil and the magnetic core constitute an electromagnet unit that provides an external magnetic field for the discharge. The strength of the magnetic field lines can be adjusted according to actual needs. By controlling the external axial magnetic field, a large-area diffuse distribution of atmospheric pressure glow discharge plasma can be obtained between the high-voltage tip electrode and the flat ground electrode. The size of the distribution space can be precisely adjusted by the magnitude of the magnetic field strength, thereby ensuring the flexibility and controllability of the material surface treatment area and expanding the distribution space of atmospheric pressure glow discharge diffuse low-temperature plasma. This provides a technical foundation for the industrial application of atmospheric pressure glow discharge plasma in the field of material surface treatment. The plasma density, rich in high-energy active particles, can be flexibly adjusted by the output parameters of the power supply, thereby ensuring the flexibility and controllability of the material surface treatment intensity and efficiency.

[0030] This device can flexibly design different planar ground electrode structures according to the size and structure of different materials to be processed to meet different placement requirements, thus realizing the diversification of application scenarios. At the same time, according to the different requirements of the materials to be processed, the composition of high-energy active particles can be changed by adjusting the gas composition. For example, increasing the proportion of oxygen in the gas can increase the oxidation degree of carbon fiber or other organic materials, thereby introducing carboxyl functional groups on the material surface; increasing the proportion of nitrogen in the gas can introduce polar functional groups on the surface of metals or polymers, increasing surface roughness and hydrophilicity; mixing a certain proportion of inert gas components such as argon and helium into the gas can improve plasma acquisition efficiency through the Penning effect.

[0031] Furthermore, the magnetic core is a ring-shaped structure placed inside the cavity, which can ensure insulation between the electrodes and the outer cylinder while enhancing the axial magnetic field strength of the space where the electrode gap is located. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the material surface treatment device for axial magnetic field-controlled low-temperature plasma provided by the present invention;

[0033] Figure 2 This is a schematic diagram of the material surface treatment device for axial magnetic field-controlled low-temperature plasma in use according to the present invention.

[0034] Figure 3 This is a schematic diagram of the electric and magnetic field distribution in the electrode gap of the present invention;

[0035] Figure 4 This is a schematic diagram of obtaining diffuse plasma by regulating atmospheric pressure air glow discharge according to the present invention.

[0036] in:

[0037] 1. Upper air inlet; 2. Left cover plate; 3. Nut; 4. Lower air inlet; 5. High-voltage pointed electrode; 6. Magnetic core; 7. Flat ground electrode; 8. Support rod; 9. Right cover plate; 10. Ground wire fixing screw; 11. Upper air outlet; 12. Lower air outlet; 13. Outer cylinder; 14. Electromagnetic coil; 15. Material to be processed; 16. Air valve; 17. Voltage monitoring instrument; 18. Current monitoring instrument; 19. Magnetic field lines; 20. Electric field lines; 21. Dispersed plasma. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the present invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0039] The components described and illustrated in the accompanying drawings and embodiments of this invention can be arranged and designed in various different configurations. Therefore, the detailed description of the embodiments of the invention provided in the following drawings is not intended to limit the scope of the claimed invention, but merely to illustrate one selected embodiment of the invention. All other embodiments obtained by those skilled in the art based on the accompanying drawings and embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0040] It should be noted that the terms "comprising," "including," or any other variations are intended to cover non-exclusive inclusion, such that a process, element, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to the process, element, method, article, or apparatus. Furthermore, the terms "horizontal" and "vertical" are based on the orientation and positional relationship of the devices or components shown in the accompanying drawings and are used only for better description of the invention, not to require that the shown devices, components, or apparatus must have that specific orientation, and therefore should not be construed as limiting the invention.

[0041] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0042] like Figure 1 As shown, this invention discloses a material surface treatment device for axial magnetic field-controlled low-temperature plasma, including an outer cylinder 13, a high-voltage tip electrode 5, a flat ground electrode 7, an electromagnetic coil 14, and a magnetic core 6. A left cover plate 2 is provided on one side of the outer cylinder 13, and a right cover plate 9 is provided on the other side. The cavity sealed by the outer cylinder 13, the left cover plate 2, and the right cover plate 9 is a discharge cavity. One end of the high-voltage tip electrode 5 passes through the left cover plate 2 and extends into the discharge cavity, while the other end is connected to a high-voltage power supply. The flat ground electrode 7 is disposed inside the outer cylinder 13 and is positioned opposite to the high-voltage tip electrode 5. A placement platform for placing the material to be treated 15 is prefabricated on the side of the flat ground electrode 7 closest to the high-voltage tip electrode 5. An electromagnetic coil 14 is laterally arranged around the outer cylinder 13 and is connected to a magnetic control unit. A magnetic core 6 is provided on the inner wall of the outer cylinder 13. An air inlet is opened on the left cover plate 2, and an air outlet is opened on the right cover plate 9. The air inlet and outlet provide channels for the discharge gas medium to enter and exit.

[0043] like Figure 2 As shown, the high-voltage tip electrode 5 is connected to the high-voltage output terminal wire of the high-voltage power supply, providing a strong electric field for the gas gap discharge between the electrode tip and the ground electrode. The voltage applied to the high-voltage tip electrode 5 is adjusted by the high-voltage power supply according to the reading of the voltage monitoring instrument 17.

[0044] Specifically, the high-voltage tip electrode 5 is threaded for fastening the nut 3 of the high-voltage wire and for adjusting and fixing its position on the left cover plate 2.

[0045] like Figure 2 As shown, the flat ground electrode 7 is securely connected to the power grounding terminal and the ground wire through the ground wire fixing screw 10 on the support rod 8, providing zero potential. At the same time, the material to be processed 15 is placed on the side of the flat ground electrode 7 near the high voltage tip electrode 5.

[0046] The high-voltage tip electrode 5, the flat ground electrode 7, and the support rod 8 are made of brass, which has good electrical and mechanical properties and is easy to process.

[0047] Both the left cover plate 2 and the right cover plate 9 are insulating cover plates. The cover plates have screw holes for fixing the air hole bolts, the high voltage tip electrode 5, and the support rod 8, which also serve to seal the cavity.

[0048] The insulating cover is made of epoxy board or polytetrafluoroethylene, which meets the requirements for insulation and mechanical strength and has a low cost.

[0049] The air inlet includes an upper air inlet 1 located on the upper side of the left cover plate 2 and a lower air inlet 4 located on the lower side of the left cover plate 2. The air outlet includes an upper air outlet 11 located on the upper side of the right cover plate 9 and a lower air outlet 12 located on the lower side of the right cover plate 9. These serve as the inlet and outlet of the glow discharge gas medium in the cavity. When the molar mass of the medium gas is greater than or equal to that of air, the upper air inlet 1 and the lower air outlet 12 are selected. When the molar mass of the medium gas is less than or equal to that of air, the lower air inlet 4 and the upper air outlet 11 are selected. The air inlet and outlet are equipped with quick-connect rotary straight-through connectors for the air pipes, which are threaded into the insulating cover plate to ensure airtightness and provide convenience for the use of the gas circuit.

[0050] The outer cylinder 13 is not only the main component of the discharge cavity, but also the main support component of the entire device. It is used to support the two cover plates inside the cylinder, the magnetic core 6, and the external electromagnetic coil 14. The outer cylinder 13 is made of copper, aluminum, or aluminum alloy. It has excellent mechanical strength and does not shield the magnetic field. The outer cylinder 13 with a smaller inner and outer diameter should be selected as much as possible according to the size of the material to be processed 15, so as to reduce the spatial attenuation of the external magnetic field strength.

[0051] The electromagnetic coil 14 and the magnetic core 6 constitute an electromagnet unit that provides an external magnetic field for discharge. The strength of the magnetic field line 19 can be adjusted according to actual needs.

[0052] The parameters of the electromagnetic coil 14 need to be calculated based on the size of the outer cylinder 13 and the magnetic field strength requirements. Enamelled wire is selected for close winding. The coil is wound on an insulating slot frame to keep it insulated from the outer cylinder 13 and to facilitate disassembly and replacement.

[0053] The magnetic core 6 is made of a non-metallic magnetic core material with high magnetic permeability. The magnetic core 6 is a ring structure placed inside the cavity to ensure insulation between the electrode and the outer cylinder 13 while enhancing the axial magnetic field strength of the space where the electrode gap is located.

[0054] like Figure 2 As shown, the low-voltage power supply provides current to the electromagnetic coil 14. Preferably, it is a DC power supply that can sensitively adjust the voltage and current. Before the device is used, the relationship between the magnetic field strength inside the discharge cavity and the voltage and current of the power supply needs to be calibrated using a gaussmeter. During discharge, it is only necessary to adjust the current monitoring instrument 18 as needed to achieve precise control of the transverse electromagnetic field strength of the discharge gap.

[0055] Both the air inlet and the air outlet are connected to air valves 16. The air valve 16 connected to the air outlet is connected to a pressure gauge through a three-way air connector to monitor the air pressure inside the cavity.

[0056] The connections of the cavity are sealed with PTFE tape and sealant. When the device discharges, the gas pressure is less than or equal to but basically equal to atmospheric pressure, so no vacuum device is needed. Before discharge, the medium gas enters through the air inlet and exits through the air outlet for a period of time. The specific time is determined by the size of the cavity.

[0057] Reference Figure 2 The specific implementation process of the material surface treatment device for axial magnetic field-controlled low-temperature plasma provided by the present invention is as follows:

[0058] Step 1: Select the appropriate size of the flat ground electrode 7 for placing the material 15 according to the actual needs of the material to be processed;

[0059] Adjust the relative positions of the high-voltage tip electrode 5 and the flat ground electrode 7 according to actual needs. Connect the high-voltage power supply and the high-voltage tip electrode 5 with wires. Connect the high-voltage power supply output ground electrode and the support rod 8 to the grounding wire.

[0060] Step 2: After calibrating the relationship between the voltage, current, and magnetic field strength of the low-voltage power supply, proceed according to... Figure 2 It is reliably connected to the electromagnetic coil 14 as shown;

[0061] Step 3: Select the inlet and outlet ports based on the gas density to be introduced into the discharge unit. Figure 2 In this embodiment, an upper air inlet 1 and a lower air outlet 12 are selected. The air inlet and the gas cylinder are connected by a gas pipe. A pressure gauge is connected between the air outlet and the gas valve 16 through a three-way gas connector to monitor the internal air pressure of the cavity. The valves of the unused air inlet and air outlet are closed.

[0062] Step 4: Keep the air valves 16 of the lower air inlet 4 and the upper air outlet 11 closed, open the air valves 16 of the upper air inlet 1 and the lower air outlet 12, and open the gas cylinder valve 16. Estimate the gas flow rate and time according to the cavity size to purge the inside of the cavity. After purging, close the gas cylinder and the air valves 16 of the lower air inlet 4 and the upper air outlet 11.

[0063] Step 5: According to actual needs, adjust the voltage of the high-voltage power supply to the required value according to the reading of the voltage monitoring instrument 17 connected to the high-voltage tip electrode 5. After confirming that the discharge has stabilized, adjust the low-voltage power supply to provide an axial magnetic field of a certain intensity to the electrode gap. By adjusting the output parameters of the high-voltage power supply and the magnetic field strength, the distribution of atmospheric pressure glow discharge plasma between the high-voltage tip electrode 5 and the flat ground electrode 7 can be flexibly controlled.

[0064] This device can flexibly design different flat ground electrode 7 structures according to the size and structure of different materials 15 to meet different placement requirements, thus realizing the diversification of application scenarios. At the same time, according to the different requirements of the materials 15 to be treated, the composition of high-energy active particles can be changed by adjusting the gas composition. For example, increasing the oxygen ratio in the gas can make the surface oxidation degree of carbon fiber or other organic materials higher, thereby introducing carboxyl functional groups on the material surface. Increasing the nitrogen ratio in the gas can introduce polar functional groups on the surface of metals or polymers, increasing surface roughness and hydrophilicity. Mixing a certain proportion of inert gas components such as argon and helium into the gas can improve plasma acquisition efficiency through the Penning effect.

[0065] like Figure 3 As shown, the magnetic field lines 19 are parallel to the axial direction of the electrode gap. The electric field lines 20 between the high-voltage tip electrode 5 and the flat ground electrode 7 have both vertical and horizontal components in the axial direction. The vertical component provides rotational driving force for the plasma channel, while the horizontal component provides acceleration and binding force for the axial transport of charged particles in the gap. By adjusting the intensity of the externally applied magnetic field, the distribution space of the discharge plasma and the contact area on the flat ground electrode 7 can be precisely controlled.

[0066] like Figure 4 As shown, by controlling the external axial magnetic field, a very well distributed atmospheric pressure glow discharge low-temperature diffused plasma 21 is formed between the high-voltage tip electrode 5 and the flat ground electrode 7.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A material surface treatment device for axial magnetic field-controlled low-temperature plasma, characterized in that, It includes an outer cylinder (13), a high-voltage tip electrode (5), a flat ground electrode (7), an electromagnetic coil (14), and a magnetic core (6). The outer cylinder (13) has a left cover plate (2) on one side and a right cover plate (9) on the other side. The cavity sealed by the outer cylinder (13) with the left cover plate (2) and the right cover plate (9) is a discharge cavity. One end of the high-voltage tip electrode (5) passes through the left cover plate (2) and extends into the discharge cavity, while the other end is connected to the high-voltage power supply. The flat ground electrode (7) is set inside the outer cylinder (13) and is positioned opposite to the high voltage tip electrode (5). A placement platform for placing the material to be processed (15) is prefabricated on the side of the flat ground electrode (7) that is close to the high voltage tip electrode (5). An electromagnetic coil (14) is provided horizontally around the outer cylinder (13), and the electromagnetic coil (14) is connected to the magnetic control unit; A magnetic core (6) is provided on the inner wall of the outer cylinder (13); An air inlet is provided on the left cover plate (2), and an air outlet is provided on the right cover plate (9). The air inlet and air outlet provide channels for the discharge gas medium to enter and exit. The gap between the high-voltage tip electrode (5) and the flat ground electrode (7) is the distribution area of ​​diffuse plasma (21). The magnetic field lines (19) formed by the magnetic core (6) and the electromagnetic coil (14) are parallel to the axial direction of the electrode gap. The electric field lines (20) formed between the high-voltage tip electrode (5) and the flat ground electrode (7) have both vertical and horizontal components in the axial direction. The vertical component provides rotational driving force for the plasma channel, and the horizontal component provides acceleration and binding force for the axial transmission of charged particles in the gap. By adjusting the intensity of the external magnetic field, a diffusely distributed atmospheric pressure glow discharge low-temperature plasma is formed between the high-voltage tip electrode (5) and the flat ground electrode (7) to control the distribution space of the discharge plasma and the contact area on the flat ground electrode (7).

2. The material surface treatment device for axial magnetic field-controlled low-temperature plasma according to claim 1, characterized in that, The magnetic core (6) has a ring structure.

3. The material surface treatment device for axial magnetic field-controlled low-temperature plasma according to claim 1, characterized in that, The air inlet includes an upper air inlet (1) located on the upper side of the left cover plate (2) and a lower air inlet (4) located on the lower side of the left cover plate (2). The air outlet includes an upper air outlet (11) located on the upper side of the right cover plate (9) and a lower air outlet (12) located on the lower side of the right cover plate (9). When the molar mass of the target gas medium is greater than or equal to that of air, the upper air inlet (1) and the lower air outlet (12) are selected. When the molar mass of the target gas medium is less than or equal to that of air, the lower inlet (4) and the upper outlet (11) are selected.

4. The material surface treatment device for axial magnetic field-controlled low-temperature plasma according to claim 1, characterized in that, Both the left cover plate (2) and the right cover plate (9) are insulating covers.

5. The material surface treatment device for axial magnetic field-controlled low-temperature plasma according to claim 1, characterized in that, The flat ground electrode (7) is connected to a support rod (8), the outer end of which extends out of the right cover plate (9). The support rod (8) is connected to the ground wire to provide zero potential.

6. The material surface treatment device for axial magnetic field-controlled low-temperature plasma according to claim 5, characterized in that, The high-voltage pointed electrode (5), the flat ground electrode (7), and the support rod (8) are made of brass. The insulating cover is made of epoxy board or polytetrafluoroethylene; The outer cylinder (13) is made of copper, aluminum or aluminum alloy.

7. The material surface treatment device for axial magnetic field-controlled low-temperature plasma according to claim 1, characterized in that, The diameter of the outer cylinder (13) is selected according to the size of the material to be processed (15) in order to reduce the spatial attenuation of the applied magnetic field strength.

8. The material surface treatment device for axial magnetic field-controlled low-temperature plasma according to claim 1, characterized in that, The magnetic core (6) is made of non-metallic magnetic core material.

9. The material surface treatment device for axial magnetic field-controlled low-temperature plasma according to claim 1, characterized in that, Both the air inlet and the air outlet are connected to air valves (16). The air valve (16) connected to the air outlet is connected to a pressure gauge through a three-way air connector to monitor the air pressure inside the cavity.

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