Abstrict A reciprocating piston flow meter for measuring gas flow through
the flow meter comprising a hollow precision bore flowtube in a
vertical orientation, with a movable piston containing a valve assembly
for movement in concert with the piston between one position at
or near the bottom end of the flowtube, and an elevated position
at or near the upper end of the flowtube. The valve assembly is
mechanically activated at each opposite end of the piston stroke
to permit the piston to reciprocate between each opposite end in
response to the presence of a gas flow.
Claims What is claimed is:
1. A reciprocating piston flow meter for use in measuring air flow
through the flow meter comprising:
(a) a hollow flowtube vertically oriented to form an open top end
and an open bottom end, with one end connected to the atmosphere;
(b) means for connecting the opposite end of said flowtube to an
external pump for directing a flow of air through said flowtube;
(c) a piston disposed in said flowtube for movement between a bottom
position adjacent said bottom end and an elevated position relative
to said top end;
(d) a valve assembly contained in said piston for movement in concert
therewith, with said valve assembly comprising: a valve body, valve
seat(s), a valve head having a valve-open and a valve-closed position,
and a valve shuttle connected to said valve head for shifting said
valve head into the valve-open and valve-closed position, respectively,
in response to the relative position of said piston;
(e) means disposed at each opposite end of said flowtube for moving
said valve shuttle in said valve assembly, such that when said piston
reaches said elevated position, said valve head is shifted into
said valve-open position for causing said piston to reverse direction
and descend by gravity to said bottom position, and upon reaching
said bottom position, said valve head is shifted into said valve-closed
position for causing said piston to ascend in response to the presence
of said air flow; and
(f) photoelectric sensor means arranged along said flowtube for
detecting the passage of said piston between two predetermined locations.
2. A reciprocating piston flow meter, as defined in claim 1 wherein
said piston includes an annulus of predetermined configuration,
into which said valve shuttle extends when said valve head is in
the valve-closed position.
3. A reciprocating piston flow meter, as defined in claim 2 wherein
said valve assembly further comprises a control member surrounding
one end of said valve shuttle, for engaging said activating means
at said elevated position to open said valve assembly.
4. A reciprocating piston flow meter, as defined in claim 3 wherein
said annulus has at least one section with a flared geometry.
5. A reciprocating piston flow meter, as defined in claim 4 wherein
said activating means comprises a spring.
Description FIELD OF THE INVENTION
This invention relates to air flow measuring devices using a positive
displacement piston flow meter and, more particularly, to a reciprocating
piston flow meter for measuring gas flow on a continuous basis.
BACKGROUND OF THE INVENTION
The measurement of gas flow is becoming increasingly more important
in the application and control of many processes, as well as in
the research laboratory. To assure accuracy in the measurement of
gas flow, the flow meter is calibrated against a higher level flow
standard. One of the accepted primary standards for gas flow measurement
and calibration is the use of a vertically oriented precision bore
flowtube and mercury-wetted piston. Although this device is quite
precise, it is also very expensive and very large. Another standard
for gas flow measurement is the bubble flow meter. In the basic
form of the bubble flow meter, a soap film is generated from a soap
solution which 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.
Although it is generally agreed that the bubble flow meter accuracy
may be affected by changes in ambient conditions such as, humidity
and temperature and is dependent upon the gas flow rate, it is understood
that these factors can be readily corrected or compensated for in
a laboratory setting. This is not, however, as easily done in a
field setting or in a commercial process environment. Moreover,
the bubble flow meter is a cumbersome and generally unwieldy instrument
to use as compared to a reciprocating piston flow meter, particularly
under field conditions. A flow meter, which uses a positive displacement
piston comparable in operation to the mercury-wetted piston, but
substantially smaller in size, weight, and cost, would have substantial
utility for field use.
SUMMARY OF THE INVENTION
An improved reciprocating piston flow meter for measuring fluid
flow has been developed, in accordance with the present invention,
using a vertically oriented flow meter assembly having a movable
piston disposed within a precision bore flowtube for reciprocating
movement between one position at or near the bottom end of the flowtube
and an elevated position near the upper end of the flowtube. The
piston contains a valve assembly located within the body of the
piston for movement in concert with the piston. The valve assembly
in the piston is opened upon reaching the end of the piston stroke
corresponding to the elevated position, and is closed upon reaching
the opposite end of the piston stroke corresponding to the bottom
position. The valve assembly is mechanically activated at each opposite
end of the piston stroke to cause the piston to reciprocate in a
continuous fashion, or to cause the piston to move from only one
end of the piston stroke to another, in response to the presence
of a gas flow. Optical detectors are arranged at predetermined positions
along the flowtube for detecting the rate of movement of the piston
between the predetermined positions.
The reciprocating piston flow meter of the present invention broadly
comprises:
(a) a hollow flowtube vertically oriented to form a top and a bottom
end;
(b) a piston disposed in said flowtube for movement between a bottom
position adjacent said bottom end and an elevated position relative
to said top end;
(c) a valve assembly contained in said piston for movement in concert
therewith, with said valve assembly having a valve-open and valve-closed
position, comprising: a valve body, valve seat(s) for providing
fluid access through said piston in the valve-open position, a valve
head, and a valve shuttle connected to said valve head for shifting
said valve head into the valve-open and valve-closed position, respectively,
in response to the relative position of said piston;
(d) means for activating said valve assembly at each opposite end
of the piston stroke within said flowtube, such that upon reaching
said elevated position, said piston is caused to reverse direction
and descend by gravity to said bottom position, and upon reaching
said bottom position, is caused to ascend in response to the presence
of a fluid flow;
(e) means through which fluid may be introduced into said flow
meter, at a flow rate to be measured by said flow meter; and
(f) photoelectric sensor means arranged along said flowtube at
said bottom position and elevated position, respectively, for detecting
the presence of said piston at each such position, respectively.
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 accompanying drawings of which:
FIG. 1 is a view in vertical section of the preferred embodiment
of the piston flow meter of the present invention, with the piston
shown in its ascending mode of operation;
FIG. 2 is a view similar to FIG. 1 with the piston shown in its
descending mode of operation;
FIG. 3 is an enlarged view of the lower end of the flow meter of
FIG. 1 showing the piston at rest at the bottom end of the piston
stroke with the piston valve closed;
FIG. 4 is an enlarged view of the piston and valve assembly, as
shown in FIG. 1 with the valve in the closed position;
FIG. 5 is an enlarged view, similar to FIG. 4 with the valve shown
in the open position;
FIG. 6 is an enlarged sectional view of the upper end of the flow
meter of FIG. 1 with the piston shown at the top end of the piston
stroke and the piston valve in the closed position;
FIG. 7 is a view similar to FIG. 6 with the piston valve shown
opened; and
FIG. 8 is a view in vertical section of an alternate embodiment
of the piston flow meter of the present invention in which the flow
meter can be operated by negative or positive pressure.
DETAILED DESCRIPTION OF THE INVENTION
The flow meter of the present invention is identified by the reference
numeral (10) and is represented in each of the FIGS. (1) through
(8), respectively, with its corresponding parts identified by the
same reference numbers. The flow meter (10) comprises a hollow cylindrical
open-ended precision bore flowtube (12) having a lightweight, smooth
surface piston (14) fitted therein to a tight tolerance to provide
substantially leakproof and frictionless movement. The piston (14)
is composed of a solid material, such as graphite, and contains
a valve assembly (15) disposed therein which moves in concert with
the piston (14) and reciprocates from the bottom end of the flowtube
(12) to an elevated position corresponding to the upper end of the
piston stroke and back.
The flowtube (12) is supported in a substantially vertical position
with its bottom end (16) mounted upon a platform (18) and sealed
by an O-ring (19). The 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) and is secured
by an O-ring (22) against leakage. A cover plate (23) is press fitted
into the top end (24) of the flowtube (12) and sealed by an O-ring
(25). An outlet fitting (27) extends from the cover plate (23) to
an external pump (not shown) for drawing air from the piston flow
meter (10). The outlet fitting (27) provides a fluid passageway
(28), which communicates through the passageway (29) to the chamber
(30) formed between the piston (14) and the top end (24) of the
flowtube (12). The passageway (29) extends to a manually controlled
switch or poppet valve (32), which normally operates in the closed
position, as shown, to permit continuous flow meter operation. In
its open position, the passageway (28) is open to the atmosphere
through valve (32) for by-passing the flow meter (10). The valve
(32) may also be momentarily depressed for a single stroke operation
of the piston (14). A solid shaft (34) extends through the cover
plate (23) into the area (30), and includes a compression spring
(36) mounted over its free end (37) for stopping the piston (14)
at the upper end of the piston stroke, and for activating the valve
assembly (15), as will be explained in detail hereafter. Likewise,
the platform (18) has an upright, hollow member (38) which supports
a compression spring (40) for stopping the piston (14) at the lower
end of the piston stroke.
The shaft (34) is supported in the cover plate (23) by a retaining
plug (41) and an O-ring (42). The shaft is also mechanically connected
through the arms (43) and (44) to an upper set of photoelectric
sensor elements (D1) and (D2), respectively, which are positioned
adjacent to the flowtube (12) to detect the piston (14) when it
reaches a height corresponding to the position of the sensor elements
(D1) and (D2).
A second set of photoelectric sensor elements (D3) and (D4) are
positioned adjacent to the bottom end of the flowtube (12), and
are spaced a predetermined distance from the upper set of sensor
elements (D1) and (D2). Each set of photoelectric sensor elements
may consist of an IR transmitter and receiver, with the position
of each set along the flowtube (12) corresponding to each opposite
end of the piston stroke. The displaced transit time of the piston
(14) between the two sets of sensor elements is used in a conventional
manner to calculate fluid flow. All of the photoelectric sensor
elements operate in a conventional manner, and are preferably connected
to an external electronic control system (not shown) for automatically
calculating and recording flow rate. The spacing between the two
sets of sensor elements may be mechanically adjusted by depressing
or extending the shaft (34) to correspond to different ranges of
fluid flow.
The valve assembly (15), as is more specifically shown in FIG.
3 includes a valve body (46) having a cylindrical sleeve (47),
one or more openings (48) and (49) formed around the cylindrical
sleeve (47), valve seats (50) and (51), an elastomeric valve head
(52), a valve shuttle (53) extending from the valve head (52) through
the sleeve (47), and a compression spring (56) mounted upon the
valve shuttle (53) in engagement with the valve head (52). The valve
body (46) is press fitted into a bore (54) in the body of the piston
(14), and is secured against leakage by an O-ring (55). Alternatively,
the valve body (46) may be formed as an integral part of the piston
(14). The valve shuttle (53) is longer than the sleeve (47), exposing
a free end (53) upon which is mounted a control member (60). The
body of the piston (14) includes a cylindrical annulus (61) adjacent
the chamber (30), and a tapered annulus (62), which flares outwardly
from the cylindrical annulus (61) and has a frusto-conical geometry
in cross-section. The control member (60) has a diameter which is
substantially equal to the diameter of the annulus (61) for providing
a controlled amount of leakage when the control member (60) resides
in the annulus (61). The combination of the dual annulus arrangement
and the control member (60) functions as a second valve to control
the pressure differential across the control member (60) in the
ascending mode of operation, which causes the valve shuttle (53)
to shift from the valve-closed position, as shown in FIG. 1 to
the valve-open position, as shown in FIG. 2. The control member
(60) accelerates the shuttle (53) into the full open valve position
when the piston (14) is at the top end of the piston stroke, as
will hereafter be further explained in greater detail.
With the outlet fitting (27) attached to the suction side of a
conventional pump and assuming the valve (32) is closed, air is
drawn from the chamber (30) above the piston (14), forcing the piston
(14) to rise from the bottom position with the valve assembly (15)
in its valve-closed position, as shown in FIG. 3 and 4 respectively.
The piston (14) continues to rise, as illustrated in FIG. 1 until
the control member (60) in the valve assembly (15) engages the spring
(36) and forces the valve shuttle (53) to move into the valve-open
position with the valve head (52) lifted off the valve seats (50)
and (51), as shown in FIGS. 5 and 7 respectively. When the spring
(36) engages the control member (60), as shown in FIG. 6 the spring
(36) is compressed and the control member (60) is urged to move
from the annulus (61) to the annulus (62). At this juncture, pressure
has built up in the chamber (30) to cause the compression spring
(36) to forcibly move the shuttle (53) to the valve-open position.
The increased diameter of annulus (62) provides an open path for
fluid flow between the chamber (30) and the lower chamber (70) through
the valve openings (48) and (49). The compression spring (36) urges
the control member (60) downward until it engages the sleeve (47).
Thus, the action of the compressed spring (36) initiates and accelerates
the downward movement of the piston (14), which continues to drop
by gravity, as illustrated in FIG. 2 until the valve head (52)
engages the spring (40) at the bottom end of the flowtube (12).
During the descent of the piston (14), the valve head (52) is maintained
in the valve-open position by the compression spring (56). Upon
contacting the spring (40), the valve head (52) is forced back into
the valved-closed position against the force of the compression
spring (56) and the cycle repeats itself, causing the piston to
ascend, assuming the valve (32) has not been opened.
In the embodiment of FIG. 8 an additional fitting (72) is used
to provide positive fluid flow into the flow meter (10) through
passageway (74). The air flows through the passageway (74), past
the air filter (21), into the flowtube (12), pushing the piston
(14) upward until the piston (14) reaches the top of the piston
stroke and the valve assembly is activated to open the valve, thereby
causing the piston (14) to drop to the lower position, as explained
heretofore. A switch or normally closed poppet valve (76) may also
be used to provide a bypass for the flow meter (10). |