A ril 7, 1970 w. c. EDWARDS AIR FLOW CONTROL Filed Oct. 21, 1965 L ZO B TiL FIG. I.
INVENTOR WILLIAM C. EDWARDS ATTORN EYS United States Patent 3,504,663 AIR FLOW CONTROL William C. Edwards, Plandome Manor, N.Y., assignor to Smith Kline & French Laboratories, Philadelphia, Pa., a corporation of Pennsylvania Filed Oct. 21, 1965, Ser. No. 500,431 Int. Cl. A61b /02 U.S. Cl. 128-2.05 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to air flow controllers, and particularly to controllers of a type suitable for bleeding air or other gases from a chamber at a constant rate so that a constant rate of decrease of pressure is produced therein. The invention is particularly applicable to air bleed devices forming part of a blood pressure measuring apparatus, and for purposes of illustration will be described with reference to such an apparatus.
In the measurement of blood pressure, where an infiatable cuff is used to constrict an artery in the arm, blood pressure is conventionally determined by inflating the cuif, and by decreasing the pressure in the cuff slowly, and noting the pressure at which characteristic Korotkoif sounds commence and by noting the pressure at which the sounds cease or become muffled. There is international agreement amongst medical authorities that it is desirable to reduce the pressure in the cuif at a constant rate.
In conventional air bleed devices used both with the manual and automatic blood pressure measuring apparatuses, air is released through a simple or manually adjustable orifice. Pressure decreases more and more slowly as more air is bled out, and, in fact, the pressure follows an exponential function with time.
An object of the present invention, therefore, is to provide an air bleed device which produces a constant rate of pressure drop in an air chamber over a wide range of pressures.
A further difficulty is encountered in air bleed devices involving small orifices in the matter of clogging by minute particles of lint or dust. Such clogging is particularly prevalent in devices involving tapered needle valves provided for the purpose of adjusting flow rate. Accordingly, a further object of the present invention is to provide an air bleed device which is self-cleaning.
Other objects Will become apparent from the following description when read in conjunction with the accompanying drawings in which:
FIGURE 1 is a vertical section showing an air bleed device in accordance with the present invention; and
FIGURE 2 is a section showing details of the construction of part of the apparatus of FIGURE 1.
Referring to FIGURE 1, a housing 4 is shown having a threaded opening 6 at its lower end in which is inserted an air hose connector 8 having a central passage 10 leading to the interior 12 of the housing 4. Within passage 10 there is provided a check valve comprising seat 14 and ball 16, and a spider 18 permitting the passage of air through passage 10 but preventing the entrance of ball 16 into the interior of the housing. A similar air hose connector 20 is threaded to the upper end of hous- 3,504,663 Patented Apr. 7, 1970 ing 4, and a passage 22 communicates with interior 12 of the housing. Connector 20 has no check valve.
A rod 24 which is slidable in a bushing 26 in the housing is provided at one end with a push button 28 held away from the housing by a helical spring 30, and at its other end with a disc 32 to which is bonded an annulus of packing material 34. Packing 34 closes a large bleed passage 36 leading from interior 12 to the exterior. When push button 28 is depressed, air is allowed to escape quickly from the interior through passage 36. The provision of passage 36 and push button 28 has particular utility in blood pressure measurement since it facilitates quick removal of the cuff after diastolic pressure has been determined.
The opposite side of housing 4 is closed off by a metal diaphragm 38 which is clamped against annular packing 40 by a member 42. Member 42 is provided with a plurality of passages 44 leading to the exterior, and a threaded passage 46 in which is threaded an adjusting screw 48. Within screw 48 and along the axis thereof there is fastened a cylindrical pin 50 which extends from the end of screw 48 and part way through a circular aperture 52 provided in the center of diaphragm 38.
It will be apparent that diaphragm 38 will be normally flat when the pressure on both sides is the same, but it is shown here deformed by a pressure differential for the purpose of illustration.
Typically, connector 8 is connected to a pressure source such as a squeeze-bulb 8A by a line 8B, and connector 20 is connected to a hose 20A leading to a pressure cuff 20B of the type used for determining blood pressure. The part of the interior of the housing to the right of diaphragm 38 as viewed in the drawing, passage 22, hose 20A and pressure cuff 20B constitute on enclosure.
The theory of operation will become clear from reference to FIGURE 2, which shows in detail the relationship between diaphragm 38 and cylindrical pin 50. An annular passage 54 is formed between diaphragm 38 and pin 50, and its length L is variable according to the degree of deformation of diaphragm 38 and the adjustment of the position of the pin by adjusting screw 48.
For any gas, the velocity of flow of the gas through an orifice having an annular cross section of constant area can be expressed to a good approximation by:
V is the velocity,
P is the pressure difference, L is the length of the passage, K is a constant.
It will be apparent that diaphragm 38 will be normally 38 will be proportional to the difference of pressure on either side of the diaphagm, and therefore length L is proportional to the pressure difference P. It follows that the velocity of the air flowing through passage 54 is constant. Thus, the rate of change in pressure in the chamber to which passage 22 leads is also constant.
In order to realize the further advantage of the present invention discussed above, it is necessary that adjusting screw 48 be adjusted so that when the pressure differential is just below the range of pressures of interest and above zero, the diaphragm should be in such a position that it just clears the end of pin 50 so that the length L of passage 54 is zero. When the pin clears aperture 52, air will rush out through passage 52 from the interior 12 of the housing, and will remove any lint or dust that might have accumulated. When the invention is used in conjunction with a blood pressure measuring apparatus, adjusting screw 48 might be adjusted so that aperture 52 clears pin 50 when the pressure decreases to a value below the lowest expected diastolic pressure.
It will be apparent that pin 50 and aperture 52 should he of uniform cross-sectional area so that the cross-sectional area of passage 54 is constant in order to obtain a constant velocity of flow through passage 54. It is not necessary to use a tapered pin, and undesirable clogging, which is an accompanying characteristic of valves involving tapered pins and tapered openings is avoided. An arrangement involving a tapered pin and a tapered hole would ordinarily involve the simultaneous variation of passage length and cross-sectional area. Since the velocity of flow varies with diameter as well as with length, it would consequently be difiicult to produce a constant air flow velocity without a more complicated arrangement of parts.
It will be apparent that the invention is not limited to bleed devices for use in conjunction with blood pressure measuring apparatuses, and that numerous other uses may be made of the apparatus in accordance with the present invention wherever it is desired to produce a constant rate of pressure drop in a fluid.
What is claimed is:
1. An apparatus for producing a constant rate of decrease of the pressure of a gas within an enclosure comprising an enclosure, a flexible diaphragm forming part of the boundary of said enclosure and movable in re sponse to pressure changes within said enclosure, a cylindrical aperture in said diaphragm, a cylindrical pin having a smaller cross-section than said aperature, means fastening said cylindrical pin to a part of said enclosure to which the aperture is relatively movable and holding said pin with its axis in parallel relation to the axis of said cylindrical aperture and so that it extends part way through said aperture from the side of the diaphragm outside the enclosure to form a passage having a substantially constant cross-section between the pin and the wall of said aperture when the pressure in said enclosure is at least greater by a predetermined amount than the pressure on the opposite sides of said diaphragm.
2. A sphygmomanometer comprising an inflatable cuff, means communicating with said cuff, indicating means connected to the interior of said cuff for indicating the pressure of gas within said cuff, a chamber, a passage providing communication between said chamber and said cuff, a flexible diaphragm forming part of the boundary of said chamber and movable in response to pressure changes within said cuff, a cylindrical aperture in said diaphragm, a cylindrical in having a smaller cross-section than said aperture, means fastening said cylindrical pin to a part of said chamber to which the aperture is relatively movable and holding said pin with its axis in parallel relation to the axis of said cylindrical aperture and .4 so that it extends part way through said aperture from the side of the diaphragm outside said chamber to form a passage having a substantially constant cross-section between the pin and the wall of said aperture when the ressure in said chamber is at least greater by a predetermined amount than the pressure on the opposite side of said diaphragm.
3. A sphygmomanometer according to claim 2 including a second passage providing communication between the interior and exterior of said chamber, and controllable valve means normally closing said second passage for quickly releasing gas from said chamber through said second passage when said valve means is opened.
4. A sphymomanometer in accordance with claim 2 including means connecting said means for supplying a gas under pressure to the cufl through said chamber, and a check valve at the inlet to said chamber from said means for supplying a gas to prevent deflation of the culi through the gas supply means.
5. A sphygmomanometer comprising an inflatable cuflF, means communicating with said cufi' for supplying a gas under pressure to the cuff, indicating means connected to the interior of the cult for indicating the pressure of gas within the cult, a chamber, a passage providing communication between said chamber and said cutf, means defining a cylindrical aperture providing communication between the interior and exterior of said chamber, a cylindrical pin having a smaller cross-sectional area than said aperture, means mounting said pin with respect to said aperture so that the axes of said pin and aperture are parallel and so that said pin can extend into said aperture to form a restricted passage, having a substantially constant transverse cross-section, between the pin and the wall of said aperture, and means responsive to the difference between the pressure inside said chamber and the pressure outside said chamber for producing relative movement between said pin and said aperture to decrease the length of said restricted passage linearly with a decrease in the pressure difference between the inside and outside of said chamber.
References Cited UNITED STATES PATENTS 3,105,477 10/1963 Lowther 138-45 XR 3,164,141 1/1965 Jones 13845 XR RICHARD A. GAUDET, Primary Examiner K. L. HOWELL, Assistant Examiner US. Cl. X.R. 138-46