Exhaust valve having a constant bleed rate

Disclosed is an exhaust valve having an approximately constant rate of pressure loss per unit time which is particularly useful with an automatic sphygmomanometer. A pair of abutting plates are provided in a fluid flow path. The plates are pressurized into contact with each other at least in part by a pressurized fluid to be exhausted through the valve. At least one of the plates is formed of a resilient material and a gap-forming member is disposed between the abutting plates to define an exhaust passage which has a cross section which varies as the pressure of the pressurized fluid varies.

BACKGROUND OF THE INVENTION 
This invention relates to an exhaust valve and, in particular, to a slow 
release valve for use with an automatic sphygmomanometer which measures 
human blood pressure. 
An automatic sphygmomanometer which uses an exhaust valve is shown in FIG. 
1. A cuff and a pressurizing pump for feeding air into the cuff, indicated 
by reference numerals 21 and 22, respectively, are provided, between which 
an exhaust valve 1 for slow exhaust and another exhaust valve 23 for rapid 
exhaust are interposed. Valves 1 and 23 are fluidically connected with 
each other as well as with the cuff and the pump, the cuff 21 being also 
connected to a pressure detecting means 24 composed of a bellows and a 
differential transformer. Reference numeral 25 indicates a microphone for 
sound detection, 26 an amplifier, 27 a low-pass filter, 28 a band-pass 
filter, 29 a level detecting circuit, 30 a Korotkoff's sound 
discriminator, 31 an operation control circuit, 32 a converter, 33 a pulse 
frequency computation circuit, 34 a memory circuit for storing maximum and 
minimum blood pressure and pulse frequency values, and 35 a display for 
displaying these values. 
During operation of the FIG. 1 apparatus, air pressure in the cuff is 
increased to exceed the maximum blood pressure value, and then is 
gradually reduced by bleeding air through the slow release exhaust valve 
1. After maximum and minimum blood pressure measurements and a pulse 
frequency measurement are taken, the air in the cuff 21 is rapidly 
exhausted through the rapid exhaust valve 23. 
It is generally known that when measuring blood pressure, discrimination of 
Korotkoff's sounds using an automatic sphygmomanometer according to the 
Riva-Rocci-Korotkoff method is most easily done when the slow release 
exhaust rate, e.g. the release rate through exhaust valve 1 in FIG. 1, is 
about 2 to 3 mmHg/sec. However, it is impossible to maintain an optimum 
exhaust rate under a wide range of pressure conditions because 
conventional slow release exhaust valves in general have a device such as 
a plate in which a small exhaust hole is bored which causes a rapid 
exhaust when the air pressure is high, but a slower exhaust when the air 
pressure is low, as shown in FIG. 5. 
SUMMARY OF THE INVENTION 
An object of the invention is the provision of an exhaust valve having a 
configuration which enables it to bleed air at an approximately constant 
rate of pressure loss per unit time regardless of the air pressure. 
An additional object of the invention is the provision of an exhaust valve 
which is highly resistant to damage due to external shocks caused, for 
example, by dropping or vibration of the valve. 
These objects and advantages of the invention and others will be clearly 
seen from the following description of the invention which is provided in 
connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 2 and 3 show a first embodiment of an exhaust valve constructed in 
accordance with the teachings of this invention. Reference numeral 10 
indicates a casing composed of a pair of cylindrical bodies 11 and 12, 
cylindrical body 11 being provided with an opening 13 for fluid connection 
with the cuff 21 (FIG. 1), and another opening 14 for fluid connection 
with the exhaust valve 23 and the pressurizing pump 22 (FIG. 1). The 
cylindrical body 12 is provided with an exhaust port 15. One end of the 
cylindrical body 12 is inserted into the cylindrical body 11 and is 
provided with a recess 41 having an open end which is sealed with an 
O-ring 19. O-ring 19 engages with and seals a space between cylindrical 
bodies 11 and 12. An exhaust pipe 16, which communicates with the exhaust 
port 15, is provided within cylindrical body 12 and projects into recess 
41. A valve unit is formed by a pair of resilient plates 2 and 3 and a 
gap-forming member 4 is interposed between plates 2 and 3. One resilient 
plate 3 of the two is provided with a sealing part 8 formed integrally 
with the plate with a thin-walled part 9 connecting them. Resilient plate 
3 is secured to the exterior of the exhaust pipe 16 so that the sealing 
part 8 is in tight contact therewith. The other plate 2 is placed over 
plate 3 and is biassed toward plate 3 by means of a spring 18 and air 
pressure within a passage way 43 of cylindrical body 11 which act on a 
support 20. Support 20 pushes plate 2 into tight contact with the plate 3. 
The gap-forming member 4 is interposed between the resilient plates 2 and 
3 and forms a gap which serves as an exhaust route therebetween from the 
recess 41 through an exhaust passage 45 to the exhaust pipe 16. It is 
composed of a needle-like pin of about 0.15 mm diameter which is provided 
with an annular stopper 5 into which the exhaust pipe 16 is inserted. The 
gap-forming member 4 is positioned within a slit 6 (FIG. 3) provided on 
the upper end of the exhaust pipe 16. 
When the air pressure applied to the upper surface of the valve support 20 
is high, the pressure applied to plates 2 and 3 by support 20 is likewise 
high and the volume deformation of the resilient plates 2 and 3 is large 
causing the cross sectional area of the gap produced by the presence of 
the gap-forming member 4 to be small, as shown in FIG. 4. With a decrease 
in pressure, the volume deformation of the resilient plates is reduced and 
the cross sectional area of the gap increases. As a result, an 
approximately constant rate of pressure loss, as shown in FIG. 6, is 
maintained through plates 2 and 3 regardless of variations in the air 
pressure which is being exhausted. 
If the gap-forming member 4 has a large degree of movement freedom, any 
impact on the FIG. 2 valve construction could cause the gap-forming member 
4 to be ejected from between the resilient plates 2 and 3. It may be 
displaced in one direction until a first end thereof is stopped by the 
inner wall of the casing 10, or it may be displaced in an opposite 
direction until the other end thereof is caught between the plates 2 and 
3. 
To prevent this from occurring, stopper 5 is provided in the exhaust 
passage 45 between the resilient plates 2 and 3, so that the contact of 
the inner surface of the annular stopper 5 with the exhaust pipe 16 and 
the contact of the outer surface of the stopper 5 with the inner surfaces 
of the resilient plates 2 and 3 controls the lengthwise movement of the 
gap-forming member 4, restricting it according to the radii of the 
exterior of the exhaust pipe 16 and the interior of the exhaust passage 45 
between resilient plates 2 and 3. Slit 6 is also provided at the end of 
exhaust pipe 16 through which the gap-forming member passes. The 
positioning of the gap-forming member 4 in the slit 6 also prevents the 
displacement of the gap-forming member 4 in the horizontal direction 
perpendicular to the length thereof beyond the point where the fixed 
transverse length l.sub.1 along the contacting surface between the 
resilient plates 2 and 3 as shown in FIG. 8(b) is changed to a transverse 
length l.sub.2 as shown in FIG. 8(a). 
FIG. 7 illustrates a modified construction having a recess 17 on the inner 
surface of the casing 10 for receiving the tip of the gap-forming member 
4. 
As noted above, the resilient plate 3 is provided with a sealing part 8 
which contacts with the outer periphery of exhaust pipe 16 to prevent air 
leakage from recess 41 to the exhaust passage 45, however, when strain 
caused during the fitting of the sealing part 8 on exhaust pipe 16 
develops it will cause irregularities in the contacting of the surface of 
the plate 3 with that of the plate 2, causing fluctuations in the exhaust 
rate to occur. The thin-walled part 9 between the main part and the 
sealing part 8 of plate 3 absorbs the strain described above and helps 
maintain a uniform contact between resilient plates 2 and 3. 
A projection 7 is preferably provided on the resilient plate 2, extends 
toward the exhaust passage between the resilient plates 2 and 3, and is 
positioned in the end of the opening of the exhaust pipe 16 to prevent the 
gap-forming member 4 from jumping out of the slit 6 should the exhaust 
valve receive an external impact. 
FIGS. 9(a) and 9(b) show another embodiment of this invention, in which one 
end of the linear gap-forming member 4 is provided with a spherical 
stopper 5a which prevents gap-forming member 4 from being displaced 
longitudinally beyond a range permitted by the displacement of the stopper 
5a between the exterior of the exhaust pipe 16 and the interior of the 
resilient plates 2 and 3. This purpose may also be fulfilled by bending 
one end of the linear gap-forming member 4 at a right angle so that it can 
be inserted into the exhaust pipe 16 through the slit 6 as shown in FIG. 
10. In this case, the bent end of the gap-forming member 4 serves as a 
stopper 5b constrained to move within the interior of the exhaust pipe 16. 
As another variant shown in FIG. 11, a clamping part ay be used as a 
stopper 5c, the clamping part being formed integrally with the resilient 
plate 3 for the insertion and positive retention of one end of the 
gap-forming member. 
FIG. 12 shows another embodiment employing an annular stopper 5d which 
surrounds exhaust pipe 16. Stopper 5d is made of a thin plate integrally 
with the gap-forming member 4 and has the advantages of allowing for less 
play while using a simple structure. Displacement of gap-forming member 4 
is limited by the space between the exterior of exhaust pipe 16 and the 
interior of stopper 5d. 
In the above described embodiments, the pair of resilient plates 2 and 3 
are made of a resilient material such as silicone rubber or NBR, however, 
the objects and advantages of this invention can be achieved even when one 
of the two plates is made of rigid material and the other of a resilient 
material. 
As has been described heretofore, this invention provides a structure in 
which a gap-forming member 4 interposed between a pair of abutting plates 
which are brought into close contact with each other at least in part by 
air fed into the valve, provides an exhaust route between the plates. The 
gap thus formed varies in cross sectional area in response to fluctuations 
in air pressure so as to keep the rate of pressure loss approximately 
constant. A stopper, provided in the exhaust passage, prevents the 
gap-forming member from being displaced beyond a fixed range, thereby 
ensuring a constant rate of pressure loss while restricting the movement 
of the gap-forming member which might cause fluctuations in the cross 
sectional area of the gap. 
Although various embodiments of the invention have been shown and 
described, the invention is not limited by the foregoing description as 
many modifications may be made without departing from the spirit and scope 
of the invention.