EP2972121A1

EP2972121A1 - Rotary needle and expanding bellows pressure indicator

Info

Publication number: EP2972121A1
Application number: EP14770082.7A
Authority: EP
Inventors: David Troy ROWE; John C. VICTOR; Jeffrey Vitullo; Rodney Wilmer DENLINGER
Original assignee: Teleflex Medical Inc
Current assignee: Teleflex Medical Inc
Priority date: 2013-03-14
Filing date: 2014-03-14
Publication date: 2016-01-20
Also published as: WO2014152807A1; US20140271292A1; WO2014152807A8; CA2906087A1; CN105190252A; AU2014236571A1; JP2016513807A; EP2972121A4

Abstract

A fluid pressure indicating device is disclosed. The fluid pressure indicating device includes a base defining an aperture therethrough, a plate separated from the base by a variable distance, the plate defining a hole therethrough, a shaft including a helical surface disposed within the hole of the plate, a first bellows extending between the base and the plate, and a second bellows extending between the base and the plate, the second bellows disposed around the first bellows, the first bellows and the second bellows defining a first chamber therebetween. The first bellows and the second bellows expand or contract in response to a difference between a pressure inside the first chamber and a pressure outside the first chamber. The variable distance between the base and the plate varies upon expansion or contraction of the first bellows and the second bellows, such that the shaft rotates relative to the plate.

Description

ROTARY NEEDLE AND EXPANDING BELLOWS PRESSURE INDICATOR

FIELD OF THE INVENTION

[0001] The present invention relates generally to pressure indicators. More particularly, the present invention relates to a pressure indicator suitable for an inflatable cuff of an endotracheal tube.

BACKGROUND OF THE INVENTION

[0002] Inflatable cuffs have been proposed for applying occlusive pressure to a flow lumen of a patient's body, or for sealing between a medical instrument and a flow lumen of a patient's body. For example, an endotracheal tube may include an inflatable cuff that conforms to the contours of a patient's trachea when inserted into the trachea and inflated to seal a gap between the endotracheal tube and the trachea. Further, pressure indicators have been proposed for monitoring a fluid pressure within an inflatable cuff.

[0003] U.S. Patent No. 4,727,887 (hereinafter "the '887 patent") describes an artificial sphincter with an occlusion cuff for applying occlusive pressure to a flow lumen of a patient's body (e.g., a urethra). The '887 patent further proposes a combination hypodermic manometer including a piston assembly which is adapted for reciprocal movement through a fluid-filled sleeve. As the piston assembly of the '887 patent is moved through the sleeve, a bellows is compressed, thereby changing a height of fluid within a manometer fluidly coupled to the bellows. However, accuracy of the hypodermic manometer of the '887 patent may depend upon orientation of the manometer with respect to gravity, making it difficult to use with accuracy in practice. Moreover, a rupture or leak of the bellows poses the risk of mixing the manometer fluid with the fluid in contact with the occlusive cuff.

[0004] Pressure indicators including a bellows within a hollow housing made of a transparent material, such that an indicator mark on the bellows is visible through the housing, are known for use with pressure cuff devices. However, such devices may have long axial lengths to provide sufficient indication precision, and may not work with opaque housing materials.

[0005] Accordingly, methods and apparatus for indicating pressure in an inflatable cuff are desired that provide sufficient precision with a small and inexpensive indicator, and provide accuracy that is insensitive to its spatial orientation.

SUMMARY OF THE INVENTION

[0006] The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect an apparatus is provided that in some embodiments includes a plate defining a hole therethrough, and a shaft rotatable about an axis, the shaft including a helical surface disposed within the hole through the plate, wherein a portion of the plate is in fluid communication with a fluid whose pressure is to be measured.

[0007] One aspect of the present invention advantageously provides a fluid pressure indicating device including a base defining a base aperture therethrough; a plate separated from the base by a variable distance, the plate defining a hole therethrough; a shaft rotatable about a first axis, the shaft including a helical surface disposed within the hole of the plate; a first bellows extending between the base and the plate; and a second bellows extending between the base and the plate, the second bellows disposed around the first bellows, the first bellows and the second bellows defining a first chamber therebetween. The first bellows and the second bellows expand or contract in response to a difference between a pressure inside the first chamber and a pressure outside the first chamber, and the variable distance between the base and the plate varies upon expansion or contraction of the first bellows and the second bellows, such that the shaft rotates relative to the plate. [0008] Another aspect of the present invention advantageously provides an expandable cuff system including a tube defining a lumen therein; an expandable bladder in fluid communication with the lumen; and a fluid pressure indicator. The fluid pressure indicator includes a plate defining a hole therethrough, and a shaft rotatable about a first axis, the shaft including a helical surface disposed within the hole through the plate. A portion of the plate is in fluid communication with the expandable bladder through the lumen.

[0009] Yet another aspect of the present invention advantageously provides a fluid pressure indicating device including a base defining a base aperture therethrough; a plate separated from the base by a variable distance, the plate defining a hole therethrough; a shaft rotatable about a first axis, the shaft including a helical surface disposed within the hole of the plate; first means for defining a variable volume extending between the base and the plate; and second means for defining the variable volume extending between the base and the plate, the second means for defining the variable volume disposed around the first means for defining the variable volume. The first means for defining the variable volume and the second means for defining the variable volume define a first chamber therebetween. A volume of the first chamber expands or contracts in response to a difference between a pressure inside the first chamber and a pressure outside the first chamber, and the variable distance between the base and the plate varies upon expansion or contraction of the volume of the first chamber, such that the shaft rotates relative to the plate.

[0010] There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. [001 1] In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, the phraseology and terminology employed herein, as well as the Abstract, are for the purpose of description and should not be regarded as limiting.

[0012] As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention.

Therefore, the claims shall be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 presents a schematic view of a pressure cuff system according to an embodiment of the present invention.

[0014] FIG. 2 presents a schematic view of a pressure cuff system according to another embodiment of the present invention.

[0015] FIG. 3 presents a schematic view of an endotracheal tube system according an embodiment of the present invention.

[0016] FIG. 4 shows a cross section of the endotracheal tube system illustrated in FIG. 3 along section 3-3.

[0017] FIG. 5 presents a perspective view of a pressure indicator according to an embodiment of the present invention.

[0018] FIG. 6 shows an exploded view of the pressure indicator in FIG. 5. [0019] FIG. 7 shows an exploded view of a variant of the pressure indicator according to another embodiment of the present invention.

[0020] FIG. 8 shows a top view of the pressure indicator in FIG. 5.

[0021] FIG. 9 shows a cross sectional view of the pressure indicator in FIG. 8 along section 9—9.

[0022] FIG. 10 shows an upper perspective view of a bellows assembly according to an embodiment of the present invention.

[0023] FIG. 11 shows a lower perspective view of the bellows assembly illustrated in FIG. 10.

DETAILED DESCRIPTION

[0024] The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. An embodiment in accordance with the present invention provides a pressure indicator for detecting an inflation pressure applied to an inflatable cuff.

[0025] FIG. 1 presents a schematic view of a pressure cuff system 10 according to an embodiment of the present invention. The pressure cuff system 10 includes an inflatable cuff 12 in fluid communication with a pressurized fluid source 14 through a supply channel 16. The pressure cuff system 10 further includes a pressure indicator 18 in fluid communication with the supply channel 16, such that a flow of pressurizing fluid from the pressurized fluid source 14 flows through the pressure indicator 18 en route to the inflatable cuff 12. The pressurizing fluid may include any liquid fluid, such as water, or any gaseous fluid, such as air, for example.

[0026] The pressure cuff system 10 may include a first valve 20 disposed in the supply channel 16, and a second valve 24 branching off from the supply channel 16 and in fluid communication therewith. In one embodiment of the present invention, the first valve 20 is a check valve oriented such that the first valve 20 permits flow only in a direction from the pressurized fluid source 14 toward the inflatable cuff 12. In another embodiment of the present invention, the first valve 20 is a normally-closed, spring-loaded isolation valve that is configured to be opened by a syringe luer engaging the pressure cuff system 10 as the pressurized fluid source 14. In yet another embodiment of the present invention, the valve 24 is a manually operated bleed valve. In still yet another embodiment of the present invention, the valve 24 is a spring-loaded pressure relief valve that is configured to vent pressure from the inflatable cuff 12 at a threshold high pressure.

[0027] The pressurized fluid source 14 could include any pump, such as, for example, a syringe, an elastic bulb, a rotary pump, or a positive displacement pump; a plenum pressurized with a fluid; combinations thereof; or any other means for delivering a pressurized fluid known to persons with ordinary skill in the art. The inflatable cuff 12 could include an elastic membrane or other structure that expands upon pressurization known to persons with ordinary skill in the art.

[0028] FIG. 2 presents a schematic view of a pressure cuff system 10 according to an alternate embodiment of the present invention. In FIG. 2, the pressure indicator 18 is in fluid communication with the supply channel 16 through a branch channel 25. Here, the pressure indicator 18 has an inlet in fluid communication with the supply channel 16 but no outlet.

[0029] Referring now to FIGs. 3 and 4, it will be appreciated that FIG. 3 presents a schematic view of an endotracheal tube system 26 according an embodiment of the present invention; and FIG. 4 shows a cross section of the endotracheal tube system 26 illustrated in FIG. 3 along section 3—3. The endotracheal tube system 26 includes an elongated tube 28, defining a main lumen 30 (see FIG. 4) therein. The inflatable cuff 12 is disposed around an outer periphery 32 of the elongated tube 28. In one embodiment, the inflatable cuff 12 defines an annular volume therein, which surrounds the elongated tube 28.

[0030] The elongated tube 28 may further define an inflation lumen 34 (see FIG. 4), which is fluidly coupled to a volume within the inflatable cuff 12. The supply channel 16 may be fluidly coupled to the inflation lumen 34, thereby effecting fluid communication between the inflatable cuff 12 and the pressurized fluid source 14.

[0031] FIG. 5 presents a perspective view of a pressure indicator 18 according to an embodiment of the present invention. The pressure indicator 18 includes a housing 36 that defines an inlet port 38 and may define an exit port 40. The pressure indicator 18 may be fluidly coupled to the supply channel 16 (see FIG. 1) through both the inlet port 38 and the exit port 40, or coupled to the supply channel 16 (see FIG. 2) through only the inlet port 38 when there is no exit port 40. The pressure indicator 18 may include a pointer 42 and pressure indicia 44.

[0032] FIG. 6 shows an exploded view of the pressure indicator 18 in FIG. 5. An internal surface 46 of the housing 36 further defines a cavity 48 therein. The cavity 48 of the housing 36 receives a shaft 50 having a helical surface 52, and a bellows assembly 54. The bellows assembly 54 includes at least one bellows 56 a plate 58. The plate 58 defines a hole 60 therethrough, where the hole 60 receives the helical surface 52 of the shaft 50.

[0033] The pressure indicator 18 may include a housing top 62 disposed on the housing 36. In one embodiment of the present invention, the pressure indicia 44 are disposed on the housing top 62. Alternatively, the pressure indicia 44 may be disposed on the plate 58. The housing top 62 may define a hole 64 therethrough, such that the hole 64 receives the shaft 50. A pointer 42 may be coupled to a first end 68 of the shaft 50, such that the pointer 42 rotates in unison with the shaft 50. The pressure indicator 18 may further include a window 70 disposed on the housing top 62, such that the pointer 42 is disposed between the housing top 62 and the window 70.

[0034] At least a portion of the window 70 is permeable to visible light such that a user of the pressure indicator 18 can see at least a portion of the housing top 62, the pointer 42, or combinations thereof. Further, all of the window 70 may be substantially transparent or translucent. In one embodiment, translucent pressure indicia 71 may be disposed on the window 70, such that the pointer 42 is visible through the translucent pressure indicia 71 and the window 70.

[0035] FIG. 7 shows an exploded view of a variant of the pressure indicator 18 according to another embodiment of the present invention. In FIG. 7, a disk 72 is coupled to the shaft 50, such that the disk 72 rotates in unison with the shaft 50. Pressure indicia 44 may be disposed on the disk 72 and an indicating mark 74 may be disposed on the window 70, such that the indicating mark 74 does not move relative to the housing 36. It will be appreciated that the non-limiting embodiment shown in FIG. 7 may include any of the features shown in FIG. 6, either individually or in combination.

[0036] Referring now to FIGS. 8 and 9, it will be appreciated that FIG. 8 shows a top view of the pressure indicator 18 in FIG. 5, and FIG. 9 shows a cross sectional view of the pressure indicator 18 along section line 9-9 in FIG. 8. As shown in FIG 9., the housing 36 further includes a base 76 and a wall 78 extending from the base 76. The internal surface 46 of the housing 36 may be a surface of the wall 78 that faces the bellows assembly 54.

[0037] The bellows assembly 54 includes an outer bellows 80 and an inner bellows 82, such that the outer bellows 80 is disposed around the inner bellows 82. Both the outer bellows 80 and the inner bellows 82 extend between the base 76 of the housing 36 and the plate 58. Further, both the outer bellows 80 and the inner bellows 82 are attached to the base 76 and the plate 58 by sealed connections at longitudinal ends of each bellows. The sealed connections between the bellows 80, 82 and the base 76 and the plate 58 may include welds, adhesives, fasteners such as screws or rivets, combinations thereof, or other fasteners known to persons of ordinary skill in the art.

[0038] The housing 36 further defines a flow channel 84 therethrough. In one embodiment, the flow channel 84 is in fluid communication with both the inlet port 38 and the exit port 40. In an alternate embodiment, where the pressure indicator 18 is connected to a branch channel 25 off of the supply channel 16 of a pressure cuff system 10 (See FIG. 2), the flow channel 84 is only in fluid communication with an inlet port 38. The outer bellows 80, inner bellows 82, base 76, and the plate 58 define a first chamber 86 therein. The first chamber 86 may be in fluid communication with the flow channel 84 via one or more base apertures 88 extending through the base 76.

[0039] The housing 36, the outer bellows 80, the inner bellows 82, and the plate 58 may define a second chamber 90. The second chamber 90 may be sealed from an ambient environment 92 of the pressure indicator 18. Alternatively, the housing 36 may define an aperture 94 therethrough, such that the second chamber 90 may be in fluid communication with the ambient environment 92 through the aperture 94.

[0040] The outer bellows 80 and the inner bellows 82 may include a flexible membrane having a substantially cylindrical shape. The bellows 80, 82 may be fabricated from a polymer, metal foil, textiles, papers, combinations thereof, or other similar materials known to persons with ordinary skill in the art. In one embodiment of the present invention, the bellows 80, 82 include a corrugated structure, which contributes to the resiliency of the bellows 80, 82. In another embodiment of the present invention, the bellows 80, 82 are free from corrugations, yet derive resilience from elastic properties of the material forming the membrane. [0041] The plate 58 is separated from the base 76 by a variable distance 96, depending upon a pressure difference between the first chamber 86 and the second chamber 90. Resilience of the outer bellows 80 or the inner bellows 82 may bias the plate 58 toward the base 76 of the housing 36. Moreover, resilience of the outer bellows 80 may bias the location of the plate 58 toward a position that minimizes the volume within the first chamber 86 and maximizes the volume within the second chamber 90. As pressure is increased in the flow channel 84, a pressure difference between the first chamber 86 and the second chamber 90 also increases, thereby extending the bellows 80, 82 in an axial direction of the shaft 50. In turn, the variable distance 96 between the plate 58 and the base 76 increases with increasing pressure within the flow channel 84. Further, a volume within the first chamber 86 increases with increasing pressure within the flow channel 84.

[0042] Movement of the plate 58 in a direction of the axis 97 of the shaft 50 rotates the shaft 50 through engagement of the helical surface 52 of the shaft 50 with the hole 60 through the plate 58. The base 76 may include a rotational bearing 98 that engages a second end 100 of the shaft 50, thereby allowing the shaft 50 to rotate about an axis 97 of the shaft 50. Further, the housing top 62 may include a rotational bearing 102 that engages the shaft 50. Accordingly, the rotational position of the shaft 50 relative to the housing 36, and therefore also the rotational position of the pointer 42, are functionally related to the pressure difference between the first chamber 86 and the second chamber 90. As a result, the pressure indicia 44 may be designed to either quantitatively or qualitatively indicate a difference in pressure between the first chamber 86 and the second chamber 90 in cooperation with the pressure indicia 44, 71 on the housing 36.

[0043] In one embodiment of the present invention, the pressure indicia 44 quantitatively delineate pressure magnitudes sensed. In another embodiment of the present invention, the pressure indicia 44 qualitatively indicate a first range 104 where the pressure is too low, a second range 106 where the pressure is acceptable, and a third range 108 where the pressure is too high (see FIG. 8). Further, unique colors may be associated with each of the first range 104, the second range 106, and the third range 108. For example, the first range 104, the second range 106, and the third range 108 could be associated with the colors yellow, green, and red, respectively. Persons of skill in the art will appreciate that other similar pressure indicia 44 may be applied to the pressure indicator 18 based on the particular application. Further, it will be appreciated that the examples for pressure indicia 44 also apply to the translucent pressure indicia 71 (see FIG. 6) previously discussed.

[0044] The relationship between change in the variable distance 96 between the plate 58 and the base 76, and the corresponding change in shaft 50 rotational position is determined by the thread pitch of the helical portion 52. As a result, the thread pitch of the helical portion 52 of the shaft 50 could be selected such that fairly small changes in the variable distance 96 could result in noticeably large changes in the rotational position of the shaft 50 and pointer 42. Accordingly, an overall dimension of the pressure indicator 18 may be minimized by utilizing small displacements in the variable distance 96 to indicate pressure via the pointer 42, when such displacements of the variable distance 96 may not otherwise be apparent without the corresponding rotation of the shaft 50 and pointer 42.

[0045] Referring now to FIGS. 10 and 1 1, it will be appreciated that FIG. 10 shows an upper perspective view of a bellows assembly 54 according to an embodiment of the present invention; and FIG. 11 shows a lower perspective view of the bellows assembly 54 illustrated in FIG. 10. The bellows assembly 54 has an inner bellows 82 and an outer bellows 80. In one embodiment, the inner bellows extends in a substantially longitudinal direction 1 10 from a first annular edge 1 12 toward an end cap 114. An intersection of the inner bellows 82 and the end cap 1 14 may define a second annular edge 1 16. [0046] The end cap 1 14 may extend in a substantially radial direction 118 from the second annular edge 1 16 toward the outer bellows 80. An intersection of the end cap 1 14 and the outer bellows 80 may define a third annular edge 120. The outer bellows 80 extends away from the end cap 1 14 in a substantially longitudinal direction alongside the inner bellows 82. In another embodiment, the outer bellows 80 terminates at a fourth annular edge

[0047] In one embodiment of the present invention, the bellows assembly 54 may be fabricated from a single, continuous piece of material. In another embodiment of the present invention, the inner bellows 82 and the end cap 114 may be fabricated from separate pieces of material and then be joined by welding, adhesive, or other fastening method known to persons with ordinary skill in the art. In yet another embodiment of the present invention, the outer bellows 80 and the end cap 114 may be fabricated from separate pieces of material and then be joined by welding, adhesive, or other fastening method known to persons with ordinary skill in the art.

[0048] Although the pressure indicator 18 is useful to indicate a fluid pressure within a pressure cuff system 10, the pressure indicator 18 can also be used to measure fluid pressure in other systems that could benefit from either quantitative or qualitative indication of a fluid pressure.

[0049] The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims

What is claimed is:

1. A fluid pressure indicating device, comprising:

a base defining a base aperture therethrough;

a plate separated from the base by a variable distance, the plate defining a hole therethrough;

a shaft rotatable about a first axis, the shaft including a helical surface disposed within the hole of the plate;

a first bellows extending between the base and the plate; and

a second bellows extending between the base and the plate, the second bellows disposed around the first bellows, the first bellows and the second bellows defining a first chamber therebetween,

wherein the first bellows and the second bellows expand or contract in response to a difference between a pressure inside the first chamber and a pressure outside the first chamber, and

wherein the variable distance between the base and the plate varies upon expansion or contraction of the first bellows and the second bellows, such that the shaft rotates relative to the plate.

2. The fluid pressure indicating device of Claim 1, further comprising a flow channel in fluid communication with the first chamber through the base aperture.

3. The fluid pressure indicating device of any one of Claims 1 and 2, further comprising a pointer disposed on the shaft.

4. The fluid pressure indicating device of Claim 3, further comprising pressure indicia disposed on a surface of the plate facing the pointer.

5. The fluid pressure indicating device of any one of Claims 3, further comprising: a housing surrounding the second bellows; and

pressure indicia disposed on a surface of the housing.

6. The fluid pressure indicating device of Claim 5, wherein a portion of the housing adjacent the pressure indicia is permeable to visible light.

7. The fluid pressure indicating device of Claim 5, wherein the pointer is disposed between the plate and the housing, and the pointer is visible through the housing and the pressure indicia.

8. The fluid pressure indicating device of any one of Claims 1-7, further comprising an housing surrounding the second bellows.

9. The fluid pressure indicating device of Claim 8, wherein the housing and the second bellows define a second chamber therebetween, and

wherein the housing defines a housing aperture therethrough, such that the second chamber is in fluid communication with an ambient environment around the fluid pressure indicating device through the housing aperture.

10. The fluid pressure indicating device of Claim 9, wherein the first bellows and the second bellows expand or contract in response to a difference between the pressure inside the first chamber and a pressure inside the second chamber.

1 1. The fluid pressure indicating device of any one of Claims 1-10, wherein the first bellows and the second bellows each includes a plurality of corrugations.

12. The fluid pressure indicating device of any one of Claims 1-11, wherein the first bellows and the second bellows each has a substantially cylindrical shape.

13. The fluid pressure indicating device of any one of Claims 1-12, further comprising a bearing fixed to the base and rotatably coupled to the shaft.

14. The fluid pressure indicating device of any one of Claims 8-10, further comprising a bearing fixed to the housing and rotatably coupled to the shaft.

15. An expandable cuff system, comprising:

a tube defining a lumen therein;

an expandable bladder in fluid communication with the lumen; and

a fluid pressure indicator including

a plate defining a hole therethrough, and

a shaft rotatable about a first axis, the shaft including a helical surface disposed within the hole, wherein a portion of the plate is in fluid communication with the expandable bladder through the lumen.

16. The expandable cuff system of Claim 15, wherein the fluid pressure indicator further includes a base defining a base aperture therethrough, the base separated from the plate by a variable distance;

a first bellows extending between the base and the plate; and

a second bellows extending between the base and the plate, the second bellows disposed around the first bellows, the first bellows and the second bellows defining a first chamber therebetween.

17. The expandable cuff system of Claim 16, wherein the first bellows and the second bellows expand or contract in response to a difference between a pressure inside the first chamber and a pressure outside the first chamber, and

18. A fluid pressure indicating device, comprising:

a base defining a base aperture therethrough;

first means for defining a variable volume extending between the base and the plate; and

second means for defining the variable volume extending between the base and the plate, the second means for defining the variable volume disposed around the first means for defining the variable volume, the first means for defining the variable volume and the second means for defining the variable volume defining a first chamber therebetween,

wherein a volume of the first chamber expands or contracts in response to a difference between a pressure inside the first chamber and a pressure outside the first chamber, and wherein the variable distance between the base and the plate varies upon expansion or contraction of the volume of the first chamber, such that the shaft rotates relative to the plate.

19. The fluid pressure indicating device of Claim 18, further comprising a resilient member that biases the plate toward a position that minimizes a volume of the first chamber.

20. The fluid pressure indicating device of any one of Claims 18-19, further comprising a resilient member that biases the plate toward a position that maximizes a volume of the second chamber.