Abstrict A 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 opposite ends of the flowtube. The
valve assembly contains a latching mechanism, preferably a magnetic
latch, to provide a positive seal across the piston. The magnetic
latch is opened when the piston strikes a spring-loaded stop at
the top end of the piston stroke, and is closed at the bottom end
of the piston stroke.
Claims What is claimed is:
1. A piston flow meter for measuring fluid flow through the flow
meter comprising:
(a) a hollow flowtube having a top end 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) stop means located at said top end and said bottom end of said
flowtube respectively;
(d) means for connecting said flowtube to an external pump for
directing a fluid through said flow meter at a flow rate to be measured
by said flowmeter;
(e) a valve assembly integrated in said piston for movement in
concert therewith, with said valve assembly having a valve open
and valve closed position, said valve assembly comprising: a valve
body, at least one valve passageway for providing fluid access through
said piston in the valve open position, a valve head, a movable
valve stem slidable within said valve body and connected to said
valve head and latching means for holding said valve in one of said
valve positions until said valve stem strikes said stop means, said
latching means having a permanent magnet member and a steel member
with one member connected to said valve stem and the other member
connected to said valve body such that upon reaching said top end
said permanent magnet is forcibly released from said steel member
allowing said piston to reverse direction and to descend by gravity
to said bottom end and upon reaching said bottom end said piston
is caused to reverse direction and ascend back to said top end with
said permanent magnet reengaged to said steel member; and
(f) photoelectric sensor means arranged at different positions
along said flowtube, relative to said bottom position and elevated
position, for detecting the presence of said piston at each position.
2. A piston flow meter, as defined in claim 1 wherein said fluid
is a gaseous medium such as atmospheric air.
3. A piston flow meter, as defined in claim 2 wherein valve assembly
further comprises a compression spring mounted between said valve
head and said valve body for urging said valve assembly into said
valve-open position.
4. A piston flow meter, as defined in claim 3 wherein said magnet
is connected to said valve stem, and wherein said steel member is
connected to said valve body such that when said valve assembly
is in the valve-closed position, said steel member is magnetically
latched to said magnet.
5. A piston flow meter, as defined in claim 4 wherein said valve
assembly further comprises a manually adjustable member extending
from said valve body, to which said steel member is affixed for
adjusting the relative magnetic latching force between said magnet
and said steel member.
6. A piston flow meter, as defined in claim 5 wherein said valve
stem has an oblong opening in which said permanent magnet is mounted.
7. A piston flow meter, as defined in claim 3 wherein said stop
means comprises a compression spring.
8. A piston flow meter, as defined in claim 7 wherein said stop
means further comprises a flexible ball contained in said compression
spring to contact said valve head at said bottom end of the flow
meter and to contact said valve stem at said top end of said flowmeter.
9. A piston flow meter as defined in claim 7 wherein said means
for connecting said flowtube to an external pump for directing a
fluid through said flow meter comprises a fitting and a popper valve
with said popper valve having an open and closed position for providing
manual and continuous operation of said flowmeter.
10. A piston flow meter as defined in claim 9 wherein when said
poppet valve is in the closed position said flowmeter operates continuously.
Description BACKGROUND OF 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, in which an accurate
knowledge of the sampled air quality determines the exposure level
to various contaminants. The most widely accepted, primary standard
method of flow measurement 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:
a) Initial breakaway friction;
b) Acceleration and deceleration of the piston after breakaway
(until equilibrium is reached);
c) Running friction; and
d) Fixed pressure loading determined by the mass of the piston.
The resolution of the above condition presents a load to the air
flow system being measured. The arrangement of the present invention
incorporates a valve and a magnetic latch within the piston assembly
which minimizes the initial breakaway friction and acceleration
forces of the piston on reversing its direction at the bottom end
of the piston stroke, and is accelerated downward at the top end
of the piston stroke. 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 down-stroke of the piston.
SUMMARY OF THE INVENTION
An improved positve displacement 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 on reaching the end of
the piston stroke corresponding to the elevated position, and is
closed on 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 air flow in the flowtube. Optical detectors are
arranged at predetermined positions along the flowtube for detecting
the rate of movement of the piston between the predetermined positions.
The positive displacement 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) stop means located at said top end and said bottom end of said
flow meter, respectively;
(d) means for connecting said flowtube to an external pump for
directing a fluid through said flow meter at a flow rate to be measured
by said flow meter;
(e) a valve assembly integrated in said piston for movement in
concert therewith, with said valve assembly having a valve-open
and valve-closed position, said valve assembly comprising: a valve
body, at least one valve passageway for providing fluid access through
said piston in the valve-open position, a valve head, a movable
valve stem, slidable within said valve body and connected to said
valve head, and latching means for holding said valve assembly in
said valve-closed position until said valve stem strikes said stop
means, such that upon reaching said top end, said piston is caused
to reverse direction and to descend by gravity to said bottom end,
and upon reaching said bottom end, is caused to reverse direction
and ascend back to said top end; and
(f) photoelectric sensor means arranged at different positions
along said flowtube, relative to said bottom position and elevated
position, for detecting the presence of said piston at each such
position.
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 the valve-open position during its descent from the top
to the bottom end of the flowtube;
FIG. 2 is an enlarged, fragmentary view of a section of the flow
meter of the present invention, for illustrating the piston, valve
assembly, and magnetic latch of FIG. 1 with the valve shown in
the valve-open position during the descent of the piston to the
bottom end of the flowtube; and
FIG. 3 is another enlarged, fragmentary view of a section of the
flow meter of the present invention, for illustrating the piston,
valve assembly, and magnetic latch of FIG. 1 with the valve shown
in the valve-closed position during the ascent of the piston from
the bottom end to the top end of the flowtube.
DETAILED DESCRIPTION OF THE INVENTION
The flow meter of the present invention is identified by the reference
numeral (10), with its corresponding parts identified in FIGS. 1-3
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) which, in concert with the piston (14), reciprocates
from a position adjacent the bottom end (16) of the flowtube (12)
to an elevated position adjacent to the upper end (24) of the flowtube
(12) and back.
The flowtube (12) is supported in a substantially vertical position,
with its bottom end (16) mounted on a platform (18), supported on
a base (17) and sealed by an O-ring (19). An internal chamber (20),
formed between the base (17) and the platform (18), communicates
with an inlet fitting (72), having a passageway (74) for providing
access to an external pump (not shown), or to the atmosphere, based
on the position of a manually operated poppet valve (76) interconnected
between the inlet fitting (72) and the chamber (20). The platform
(18) has a hollow member (38) extending into the flowtube (12) to
provide a passageway for ingress and egress of air from the flow
meter (12) at the bottom end thereof. An air filter (21) is located
in the chamber (20) to filter incoming air from the passageway (74)
to the flowtube (12), and is secured by an O-ring (22) against leakage.
The top end (24) of the flowtube (12) has a cover plate (23) press-fitted
in the flowtube (12), and is sealed from the atmosphere 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 external pump (not shown) is intended to be connected
to fitting (27) or to the fitting (72). For simplicity, the present
invention will be described based on connecting fitting (27), to
the external vacuum pump, with fitting (72) open to the atmosphere.
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).
The poppet 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 opening the
valve assembly (15), as will be explained in detail hereafter. The
hollow member (38), extending from the platform (18), also supports
a compression spring (40) for stopping the piston (14) at the lower
end of the piston stroke. The compression spring (36) contains a
flexible ball (70) fitted within the spring (36) to assure positive
contact with the valve stem (53) of the valve assembly (15). A similar
flexible ball (72) is located in the compression spring (40).
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 a conventional 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.
2 and FIG. 3 includes a valve body (46) having one or more valve
openings (48) and (49), or a single annular opening, an elastomeric
valve head (52), a valve stem (53) extending from the valve head
(52) through the valve body (46), and a compression spring (56)
seated in the valve body (46) around the valve stem (53). A stop
washer (47) is placed between the valve head (52) and the valve
stem (53) to hold the compression spring (56)in place. The valve
head (52) may also have a valve stop (60) to provide positive contact
with the flexible ball (72) at the bottom end of the piston stroke,
The valve body (46) is formed as an integral part of the piston
(14), and has an externally threaded boss (61) extending from the
valve body (46) which is connected to an internally threaded tubular
section (62). The attachment of the tube (62) to the boss (61) is
manually adjustable. A steel pin (64)is secured to the inside diameter
of the body of the tube (62) so that it projects outwardly, transverse
to the longitudinal axis of the tube (62). The valve stem (53) has
an oblong hole (65), as shown in FIG. 3 with a magnet (66) secured
to the valve stem (53) at one of the longitudinal ends of the hole
(65). The magnet (66), in combination with the steel pin (64), functions
as the magnetic latch (50). The position of the steel pin (64) relative
to the magnet (66) is adjustable by adjusting the threaded connection
of the tube (62) on the boss (61).
With the outlet fitting (27) attached to the suction side of a
conventional vacuum pump and assuming the valve (32) is in the closed
position, air will be drawn from the chamber (30) above the piston
(14), forcing the piston (14) to rise, as shown in FIG. 3 from
a position at the bottom end of the flow meter, with the valve assembly
(15)in its valve-closed position, in which the steel pin (64) is
magnetically held against the magnet (66). The piston (14) upon
reaching the upper end of the flow meter will engage the flexible
ball (70) in the compression spring (36), as illustrated in FIG.
3 and continue to rise forcing the spring (36) to compress until
the spring compression force is of a sufficient magnitude to overcome
the magnetic latch (50) and to force the valve assembly (15) into
the valve-open position by pushing the valve stem (53) downward
to lift the valve head (52) off the valve openings (48) and (49).
The compression spring (36) not only causes the valve to open by
overcoming the magnetic latching force, but also initiates and accelerates
the downward movement of the piston (14), which continues to drop
by gravity, as illustrated in FIG. 2 until the stop (60) on the
valve head (52) engages the flexible ball (72) extending from the
compression 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 ball (72) the valve head (52)is forced back into
the magnetically latched valved-closed position against the force
of the compression spring (56) and the cycle repeats itself, causing
the piston to ascend. The magnetic latching force is adjustable
by adjustment of the tube (62). The compression spring (40) at the
bottom end of the flowtube assists the piston to reverse direction,
giving it a lift off, which minimizes any existing breakaway friction.
Any fluid, preferably gaseous, may be introduced into the flowmeter
through either fitting (27) or (72) in accordance with the detailed
description of the invention using an external pump (not shown).
The flow meter of the present invention will accurately measure
the flow rate of the fluid pumped through the flowmeter over a wide
range of flow rates. Although a magnetic latch (50) is preferred
a non-magnetic latching mechanism may be used which may be mechanical
or electronic such as e.g. a solenoid or an electronic switch. This
is however less desirable compared to a magnetic latch which permits
the system to operate without wires or external interference i.e.
as a closed system. A magnetic latch is also less expensive and
less complicated compared to an electronic device. |