Abstrict A positive displacement piston flow meter for measuring air flow
comprising a hollow flowtube, preferably arranged in a vertical
orientation, having a movable piston disposed in the flowtube for
movement between a starting position, preferably at the bottom end
of the piston stroke, and an elevated position at the top end of
the piston stroke. The piston is reciprocated by means of a pressure
relief control valve assembly which controls the ascent and descent
of the piston. The piston is physically connected to the pressure
relief control valve assembly by means of a flexible string, which
causes the piston to automatically close the valve assembly for
automatic operation on reaching the bottom end of the piston stroke
and to reverse direction.
Claims What is claimed:
1. A positive piston flow meter for use in measuring air flow comprising:
(a) a hollow flowtube vertically oriented to form a top end and
a bottom end, and having a movable piston disposed therein for movement
between said top and bottom end, respectively;
(b) inlet means for connecting one end of said flowtube to an external
pump, with said flowtube having ingress and egress to the ambient
atmosphere from at least one end thereof;
(c) means for detecting said piston as it moves past at least two
predetermined positions along said flowtube from which air flow
through said flowtube can be measured; and
(d) a pressure relief control assembly for controlling the ascent
and descent of the piston in said flowtube, said pressure relief
control assembly comprising: a housing, a chamber within the interior
of said housing, and a poppet valve for interconnecting said said
flowtube to said chamber, with said poppet valve having a first
position for providing a first flow path to said inlet means from
said top end of said flowtube to cause said piston to rise, and
a second position for providing a second flow path from said chamber
to said inlet means to cause said piston to fall; and with said
poppet valve, including a valve stem extending into said flowtube
for engaging said piston when said piston reaches a fixed position
adjacent the top end of the flowtube, whereby said poppet valve
is automatically switched into said second position to permit said
piston to descend by gravity from the top end of said flowtube;
latching means for holding said poppet valve in a fixed position
after said piston and said valve stem have disengaged, and means
for resetting the poppet valve into said first position.
2. A positive piston flow meter, as defined in claim 1 wherein
said means for resetting the poppet valve is a manually operated
switch.
3. A positive piston flow meter, as defined in claim 1 wherein
said flow meter is automatically operated, with said means for resetting
said poppet valve comprising a flexible string, physically connecting
said poppet valve and said piston.
4. A positive piston flow meter, as defined in claim 3 wherein
said latch means comprises a permanent magnet mounted in said valve
stem, and means for holding a steel member in a stationary position
relative to said magnet.
5. A positive piston flow meter as defined in claim 4 wherein
the position of said means for holding said steel member is adjustable
to vary the position of said steel member relative to said magnet.
6. A positive piston flow meter as defined in claim 4 wherein
said flexible string is connected at one end to a member mounted
in a compression spring extending from said valve stem such that
said flexible string is suspended from said compression spring.
7. A positive piston flow meter as defined in claim 6 wherein said
compression spring has a free end extending into said flowtube with
the position of said member in said compression spring being adjustable
relative to said free end.
8. A positive piston flow meter, as defined in claim 6 wherein
said poppet valve further comprises a flexible valve head, a valve
body, and a passageway extending through said valve body for providing
access through said poppet valve to said chamber.
Description BACKGROUND OF THE INVENTION
The accurate measurement of ambient fluid (air) flow is becoming
increasingly more important in the application and control of many
processes, as well as in the research laboratory. One of the major
applications is in the field of air sampling, where an accurate
knowledge of the sampled air quality determines the exposure level
to various contaminants. The most widely accepted primary standard
method for a gaseous fluid is the bubble flow meter. In the basic
form of the bubble flow meter, a soap film is generated from a soap
solution, and is propelled by the gas flow under measurement from
one end of the flow meter to the other. By timing the rise of the
soap film between calibrated volume marks, the volume flow is obtained.
Since for all practical purposes, the soap film is massless, it
requires almost no force to accelerate the bubble. Furthermore,
a seal is always insured by the presence of the bubble. The very
nature of the bubble eliminates the friction which is associated
with a piston type flow meter. The soap film flow meter is essentially
transparent to the flow being measured, having a no-load effect.
Accordingly, the soap film flow meter comes closest to meeting the
unique requirements of the ideal calibrator.
The measurement of air flow using a positive displacement reciprocating
piston flow meter is susceptible to errors based on the following
requirements:
1. Initial breakaway friction;
2. Acceleration and deceleration of the piston after breakaway
(until equilibrium is reached);
3. Running friction; and
4. Fixed pressure loading determined by the mass of the piston.
The resolution of the above conditions presents a load to the air
flow system being measured. The arrangement of the present invention
minimizes the initial breakaway friction and acceleration forces
of the piston on reversing its direction at the bottom of the piston
stroke and, if desired, permits automatic operation without an external
power source. An additional advantage of the positive displacement
piston flow meter of the present invention is its simplicity in
design for reversing the direction of the piston on both the upstroke
and downstroke of the piston.
SUMMARY OF THE INVENTION
An improved positive displacement piston flow meter for measuring
air flow has been developed in accordance with the present invention,
preferably using a vertically oriented flow meter assembly having
a movable piston disposed within a precision bore flowtube, for
movement from a starting position at or near the bottom end of the
flowtube to an elevated position by the flow of air under measurement.
The piston is automatically reset to its starting position on reaching
the upper end of its vertical stroke by means of a pressure relief
control assembly responsive to the piston at the elevated position.
The piston is physically connected to the pressure relief control
assembly by means of a flexible string.
The positive displacement piston flow meter of the present invention
comprises:
(a) a hollow flowtube vertically oriented to form a top end and
a bottom end, and having a movable piston disposed therein for movement
between said top and bottom end, respectively;
(b) inlet means for connecting one end of said flowtube to an external
pump, with the flowtube having ingress and egress to the ambient
atmosphere from at least one end thereof;
(c) means for detecting said piston as it moves past two or more
predetermined positions along said flowtube from which air flow
through said flowtube can be measured; and
(d) a pressure relief control assembly for controlling the ascent
and descent of the piston in said flowtube, said pressure relief
control assembly comprising: a housing, a chamber within the interior
of said housing, and a poppet valve for interconnecting said top
end of said flowtube to said chamber, with said poppet valve having
a first position for providing a first flow path to said inlet means
from said top end of said flowtube to cause said piston to rise,
and a second position for providing a se said poppet valve, including
a valve stem extending into said flowtube for engaging said piston
when said piston reaches a fixed position adjacent to the top end
of the flowtube, whereby said poppet valve is automatically switched
into said second position, so as to permit said piston to descend
from the top end of said flowtube by gravity; magnetic latching
means for holding said poppet valve in a fixed position after said
piston and said valve stem have disengaged, and means for resetting
the poppet valve into said first position, without the use of an
external power source.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the present invention will become
apparent from the following detailed description of the invention,
when read in conjunction with the accompany drawings, of which:
FIG. 1 is a view in vertical section of one embodiment of the flow
meter apparatus of the present invention;
FIG. 2 is a sectional view of the flow meter of FIG. 1 with the
poppet valve in the pressure relief control assembly of FIG. 1 shown
in the open position;
FIG. 3 is a view similar to FIG. 2 with the flow meter rotated
ninety degrees (90.degree.);
FIG. 4 is a view of an alternate embodiment of the piston and LED
arrangement of FIG. 2;
FIG. 5 is a sectional view taken along the lines 5--5 of FIG. 4;
FIG. 6 is a view in vertical section of an alternate embodiment
of the flow meter apparatus of the present invention;
FIG. 6A is an exploded view of the pressure relief control assembly
of FIG. 6 with the pressure relief control valve shown in the valve-closed
position;
FIG. 7 is a view similar to FIG. 6 with the flow meter apparatus
rotated ninety degrees (90.degree.), and with the pressure relief
control valve shown in the valve-open position; and
FIG. 8 is a view similar to FIG. 6 in the rotated position of FIG.
7 with the pressure relief control valve closed, and with the piston
shown advancing toward the top end of the piston stroke.
DETAILED DESCRIPTION OF THE INVENTION
The positive displacement flow meter apparatus of the present invention
is identified by the reference numeral "10" in the embodiment
of FIGS. 1-3 and in the alternate embodiment of FIGS. 6-8 with
the same reference numbers used in each of the figures to identify
corresponding parts. The flow meter (10), shown in FIGS. 1 to 3
comprises a hollow cylindrical open-ended flowtube (12), which is
a precision bore, glass tube, with a lightweight, smooth, surface
piston (14) fitted therein to a tight tolerance so as to provide
substantially frictionless piston movement. The flowtube (12) is
supported in a substantially vertical position, with its bottom
end (16) mounted on a base platform (18) and sealed by an O-ring
(19). The base platform (18) has an inlet opening (20) for providing
access to the atmosphere. An air filter (21) is used to filter air
entering through the inlet opening (20). The air filter (21) is
held in place by a retaining plate (23) connected to the base platform
(18) and sealed by an O-ring (22).
A pressure relief control assembly (25) is mounted on the top end
(24) of the flowtube (12) for controlling the directional movement
of the piston (14) within the flowtube (12). The pressure relief
control assembly (25) includes a manually operated push button switch
(30), a poppet valve (32), a poppet valve actuator (33), and a latching
member (34). The manually operated push button switch (30) is affixed
to a cover plate (35), and includes a contactor (36) which engages
the poppet valve actuator (33) to close the poppet valve (32) upon
depressing the switch (30), and a spring (37) for resetting the
push button switch (30) on its release. The cover plate (35) is
affixed to the body (38) of the control assembly (25) by any conventional
means, such as screws (not shown). A flexible diaphragm (40) is
supported by a retaining plate (41) connected between the cover
plate (35) and the body (38). The diaphragm (40) attenuates pulsations
in air flow, as will be explained hereafter in greater detail.
The poppet valve (32) comprises a valve head (42), a stem (43),
a valve seat (44), a valve seat plate (45), a leaf spring (46),
and a steel support plate (47). The leaf spring (46) is supported
by an anchor pivot (48) on one side thereof, and a spring-loaded
pivot (50) on the opposite side. The spring-loaded pivot (50) may
be manually adjusted by a pivot spring (51) through a spring-adjusting
screw (52). The valve seat plate (45) is affixed through screws
(not shown) to the body (38) of the control assembly (25), and is
configured to provide a depending extension (53), over which the
top end (24) of the flowtube (12) is press-fitted and sealed with
an O-ring (54).
The poppet valve actuator (33) is moutned on a movable diaphragm
(55) supported between the retaining plate (41) and the body (38),
and includes a stem (56) which extends through the latching member
(34) into contact with the steel support plate (47). The latching
member (34) is a permanent magnet.
The control assembly (25) further includes an outlet fitting (58)
for attachment to the suction side of a conventional pump to draw
an airstream through the control assembly (25), for raising the
movable piston (14) in the flowtube (12) when depressing the push
button switch (30). The outlet fitting (58) is sealed through an
O-ring (59). The air filter (60), sealed with an O-ring (61), is
supported within the air passageway (62) in the body (38) of the
control assembly (25) to filter the airstream. The passageway (62)
communicates with the diaphragm chamber (63) which, in turn, communicates
through a passageway (64), as shown in FIG. 3 and through the opening
(65) into the open space (67) between the piston (14) and the top
end (24) of the flowtube (12).
Two sets of conventional optical LED photoelectrical sensor elements
(70) and (72) are mounted in a block (74) surrounding the flowtube
(12). The photoelectric sensor elements (70) and (72) are spaced
a fixed distance apart along the flowtube (12) and operate to measure
the displaced transmit time of the leading or trailing edge of the
piston (14) as it moves between the two sensor locations. The operation
of the photoelectric elements (70) and (72) and the method of calculating
the transmit time between sensor locations is conventional. An alternate
embodiment for arranging the LED elements to solicit data as the
piston (14) moves is shown in FIGS. 4 and 5 and will be discussed
hereafter.
In the operation of the embodiment of FIGS. 1-3 the push button
switch (30) is momentarily depressed, which causes the stem (56)
of the poppet valve actuator (33) to press down on the steel support
plate (47). This, in turn, causes the leaf spring (46) to snap into
the stable position shown in FIG. 1 with the valve head (42) engaging
the valve seat (44). In this position, the poppet valve (32) is
closed and air drawn from the open space (67) into the flowtube
(12) will cause the piston (14) to rise from its at-rest position
at the bottom end (16) of the flowtube (12). The piston (14) will
advance at a speed proportional to the flow of air through the control
assembly (25). On contacting the stem (43) of the valve head (42),
the poppet valve (32) is forced open and the leaf spring (46) is
caused to snap into the latched position, as shown in FIG. 2.
The tension of the leaf spring (46) may be manually adjusted by
adjustment of the screw (52). The permanent magnet latching member
(34) functions to assist the leaf spring (46) in moving to the latched
position of FIG. 2 and to maintain the leaf spring (46) in this
position until the push button switch (30) is actuated. With the
poppet valve (32) open, air flow through the fitting (58) communicates,
as shown in FIG. 3 through an air filter (75), cavity (76), and
through the valve opening (80) to the open space (67) at the top
end (24) of the flowtube (12).
As indicated heretofore, the open space (67) is in direct communication
with the diaphragm chamber (63) through the opening (65) and passageway
(64). Thus, in the poppet valve (32) open position, the piston (14)
will fall freely under gravity from the top end (24) to the bottom
end (16), and will remain there until the push button switch (30)
is again activated. Any pulsations or sudden changes in the rate
of air flow through the diaphragm chamber (63) will deflect the
diaphragm (40) to attenuate the pulsation.
In FIGS. 4 and 5 the piston (82), which corresponds to piston
(14) in FIGS. 1-3 has a plurality of reflective bands or grooves
(84) formed about its circumference. The photoelectric devices (86)
and (87) represent an infrared transmitter and receiver, respectively,
and are arranged in close proximity to sense the presence of a band
or groove (84) by detection of the reflected infrared signal. The
measurement of the flow is calculated in the same way as for the
arrangement of FIGS. 1-3. This feature may be added to the embodiment
previously described in FIGS. 1-3.
An alternative embodiment of the present invention, preferably
intended to automatically reciprocate the piston (14) in the flowtube
(12), is shown in FIGS. 6 6A, 7 and 8 respectively, with the
same reference numbers used in the above figures when identifying
parts corresponding to those identified in FIGS. 1-3. The piston
(14) is arranged in the flowtube (12) in a manner identical to that
shown in FIG. 1 to be reciprocated from a starting position adjacent
the bottom end (16) of the flowtube (12) to an elevated position
adjacent the top end (24), by means of a pressure relief control
assembly (90).
The pressure relief control assembly (90) comprises a housing (91)
having an annular member (92) securely mounted to the top end (24)
of the flowtube (12), a cover (94) fitted over the annual member
(92) to form a chamber (95), and a poppet valve (100) for interconnecting
the chamber (95) and the flowtube (12), based on whether the poppet
valve is open or closed. The cover (94) is removably sealed to the
annular member (92) by an O-ring (93). Additional O-rings (96) and
(97) form seals between the poppet valve (100) and the annular member
(92), and between the annular member (92) and the flowtube (12).
A flexible diaphragm (98) is located in the cover (94) above the
chamber (95).
The poppet valve (100) comprises a valve body (101) having a movable
valve stem (102), slidably mounted in a cylindrical sleeve (103)
fitted within a bore (104) formed substantially in the center of
the valve body (101). The movable valve stem (102) has a flexible
valve head (105) which, in the position shown in FIG. 6 covers
a passageway or channel (106) extending through the body (101),
leading into the upper piston chamber (107) of the flowtube (12).
A compression spring (108) is mounted around the sleeve (103) within
the bore (104), between the flexible head (105) and the valve body
(101).
The movable valve stem (102) is an elongated, tubular member having
an oval slot (110) at its distal end (111), in which a permanent
magnet (112) is inserted in a fixed position. A mounting bracket
(115) of cylindrical geometry extends from the distal end (111)
of the rod (102) into the upper piston chamber (107) of the flowtube
(12). A flexible string (116) is secured at each opposite end thereof
to corresponding members (117) and (118), with the member (117)
supported in a fixed position within a compression spring (120)
which is mounted over the bracket (115) and with the member (118)
affixed to the piston (14). The ends of the flexible string (116)
is embedded in the members (117) and (118), thereby physically connecting
the piston (14) to the poppet valve (100). The member (118) may
be press-fitted in an opening (119) of the piston (14) or otherwise
physically connected thereto. The compression spring (120) has a
free end (145) extending into the piston chamber (107).
A cylindrical collar (122) is mounted over a depending section
of the valve body (101) surrounding the permanent magnet (112).
The collar (122) has a steel pin (124) which extends through the
oval slot (110) adjacent to the permanent magnet (112) to form a
magnetic latch for holding the valve stem (102) in the valve-closed
position, as shown in FIGS. 6 6A, and 8 with the flexible valve
head (105) covering the channel (106) against the force of the compression
spring (108). In the valve-closed position, the piston (14) is held
suspended from the spring (116) at a location adjacent the bottom
end of the flowtube (12). This represents the starting position
of the piston (14), which is preferably slightly above the base
support (130) for the flowtube (12). The base support (130) corresponds
to the base platform (18) of FIG. 1 with the flowtube sealed at
the bottom end (16) by an O-ring (131).
As is more clearly shown in FIG. 7 the base support (130) has
an access passageway (132) extending therethrough, to provide direct
access between the lower piston chamber (135) and the ambient atmosphere
through an air filter (136). The air filter (136) is used, in the
embodiment of FIG. 7 to filter air from the common inlet (137)
through the vertical standpipe (138) to the access passageway (132),
as well as to the chamber (95) in the housing (91). A boss (99)
extends from the annular member (92) of the housing (91) to function
as an inlet coupling for attachment to an external pump (not shown),
preferably on its suction side. The boss (99) has an internal passageway
(139) communicating with the upper piston chamber (107). A manually
operated vent valve (140) is mounted on the boss (99) to permit
manual activation of the unit (10) of FIGS. 6-8.
The optical LED photoelectric sets of sensor elements (70) and
(72) are mounted in fixed positions adjacent to surrounding the
flowtube (12), and operate as explained earlier in connection with
FIGS. 1-3 to measure the displaced transit time of the piston (14)
as it moves between the fixed sensor positions along the flowtube.
The unit (10) of FIGS. 6-8 operates in a substantially similar
fashion to the corresponding flow meter (10) of FIG. 1 for controlling
the ascent of the piston (14) in the flowtube (12). The vent valve
(140) is depressed and air is suctioned by the external pump (not
shown) from the upper piston chamber (107) through the boss (99).
As the air evacuates the upper piston chamber (107), the piston
(14) rises and air is drawn from the ambient atmosphere through
the inlet air filter (136), down the vertical standpipe (138), and
into the lower piston chamber (135) via the access passageway (132)
in the base support (130). Thus, only filtered air enters the flowtube
(12). The piston (14) continues to rise until it physically makes
contact with the spring (120). The spring (120) is forced to compress
until the compression force overcomes the magnetic latching force
between the steel pin (124) and the permanent magnet (112). At this
point, the valve stem (102) rises, opening up the poppet valve assembly
(100) to the valve-open position, as shown in FIG. 7. The compression
spring (108) surrounding the rod (102) will keep the valve (100)
fully opened until the valve (100) is forcibly closed. In the valve-open
position, the ambient air drawn through the air inlet filter (136)
passes through the chamber (95) and through the open channel (106)
into the upper piston chamber (107), from whence it is withdrawn
by the external pump (not shown) through the passageway (139) in
the boss (99). The piston (14) will now begin to drop by gravity,
due to its own weight, and continue to fall, until the weight of
the piston (14) tenses the string (116) attached to the valve assembly
(100). This automatically pulls the valve stem (102) to the valve-closed
position, which relatches the magnet (112) to the steel pin (124)
for holding the valve (100) in the valve-closed position.
Of key importance to the invention is that the tension on the string
(116), when the piston (14) hits bottom, causes the piston (14)
to rebound to its upward position. This action overcomes the initial
breakaway friction, which would otherwise be encountered during
acceleration of the piston, to achieve a state of constant velocity
during ascent of the piston (14). Once constant velocity is achieved,
the only losses are those associated with the mass and diameter
of the piston (14).
The ease with which the piston (14) reverses its direction and
moves upward without external assistance (i.e., without loading
down the external pump) is based upon the ability of the piston
(14) to rebound when it reaches the end of its downstroke. This
is adjustable by positioning the ball member (117) relative to the
free end (145) of the spring (120). By fine tuning the position
of the ball member (117) little, if any, acceleration of the piston
(14) is required to initiate the upstroke cycle. Without the need
for acceleration of the piston, a no-load condition on the pump
is approximated. The placement of the ball member (117) relative
to the spring free end (145) controls the acceleration/deceleration
characteristics of the piston (14).
The latching of the magnet (112) is adjustable by changing the
vertical position of the collar (122) on the valve body (101). The
position of the collar (122) controls the position of the steel
pin (124) relative to the magnet (112) for both the valve open and
closed positions. If the collar (122) is moved downwardly, less
force is necessary to open the valve from the closed position. Conversely,
if the collar (122) is adjusted upward, less force is necessary
to close the valve from the valve open position. |