Abstrict A piston flow meter for measuring the flow of a gaseous fluid,
such as ambient air, through the flow meter by timing the displacement
of a piston in a hollow precision bore flowtube over a fixed distance.
The piston is reciprocated from opposite ends of the flowtube by
a poppet valve which controls the direction of movement of the piston
in the flowtube. Ambient air is drawn into the flow tube from the
atmosphere by an external pump connected to an inlet at one end
of the flowtube. The flow meter includes a damping assembly for
damping pulsations in gas flow through said flowtube with the damping
assembly preferably located adjacent the pump inlet to the external
pump. The damping assembly comprises damping means which includes
a porous member and at least one elastic diaphragm contiguous thereto
with the porous member having a multiplicity of voids and a plurality
of open channels extending through the porous member to the elastic
diaphragm.
Claims What is claimed is:
1. A positive displacement piston flow meter comprising:
(a) a hollow flowtube having two opposite open ends;
(b) a piston disposed in said flowtube for movement between a first
position relative to one of said open ends and a second position
relative to the opposite open end thereof;
(c) means for connecting said flowtube to an external pump for
directing a gaseous fluid through said flow meter at a flow rate
to be measured by said flow meter;
(d) valve means for controllably reversing the direction of movement
of said piston in said flowtube between said first and second position
respectively; and
(e) damping means for damping pulsations in gas flow through said
flowtube, said damping means comprising at least one nonporous elastic
diaphragm responsive to pressure pulsations in said flowtube and
a porous member adjacent thereto with the porous member having a
multiplicity of voids and a plurality of open channels extending
to the elastic diaphragm and distributed throughout the porous member.
2. A piston flow meter as defined in claim 1 wherein said hollow
flowtube is vertically oriented.
3. A piston flow meter as defined in claim 2 further comprising
photoelectric sensor means arranged at different positions along
said flowtube for detecting the presence of said piston at each
such position.
4. A piston flow meter, as defined in claim 2 wherein said gaseous
fluid is atmospheric air.
5. A piston flow meter, as defined in claim 4 wherein said plurality
of open channels are arranged in a substantially parallel relationship
to one another.
6. A piston flow meter, as defined in claim 5 further comprising
another elastic diaphragm with said plurality of open channels extending
therebetween.
7. A piston flow meter, as defined in claim 4 wherein valve means
comprises a valve having a valve open position and a valve closed
position, magnetic latch means for holding said valve in one of
said positions and a flexible string connected to said piston and
to said valve stem for automatically operating said magnetic latch
means as the piston is moved from one end of the flow tube to the
other.
8. A piston flow meter, as defined in claim 7 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.
9. A piston flow meter, as defined in claim 8 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.
10. A piston flow meter, as defined in claim 7 wherein said flexible
string has one end affixed to a member mounted in a compression
spring extending from said valve stem.
11. A piston flow meter, as defined in claim 10 wherein said flexible
string has its opposite end affixed to a member attached to said
piston.
12. A piston flow meter as defined in claim 1 wherein said means
for connecting said flowtube to an external pump comprises an fitting
with said damping means located in close proximity to said inlet
fitting.
13. A piston flow meter as defined in claim 12 wherein at least
one of said parallel channels is in direct unrestricted gaseous
communication with said inlet fitting to the external pump.
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 having a damping assembly for smoothing out air
flow pulses through the flow meter.
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. Accordingly, a soap film flow meter comes closest
to meeting the unique requirements of the ideal calibrator. However
from the standpoint of convenience the use of a positive displacement
flow meter in which a piston is reciprocated within a flowtube may
have some practical advantages over the bubble flowmeter.
Nevertheless, the measurement of air flow using a positive displacement
reciprocating piston flow meter is susceptible to errors due to
the following:
a) Initial piston breakaway friction;
b) Acceleration and deceleration of the piston after breakaway
(until equilibrium is reached);
c) Running friction within the flowtube; and
d) Fixed pressure loading determined by the mass of the piston.
Each of the above conditions present a load to the air flow system
being measured. Moreover the suction pump which is used to draw
air through the flowtube may cause air pulsations which in a positive
displacement flow meter presents a dynamic load to the system. Various
techniques for damping the air pulsations have been suggested heretofore
with little success.
SUMMARY OF THE INVENTION
In accordance with the present invention it has been discovered
that air pulsations through the flowtube may be minimized by incorporating
a damping assembly in the flowmeter in communication with the air
passageway to the external pump and responsive to pressure fluctuations
in the flowtube. The preferred damping assembly of the present invention
comprises a damping member composed of a porous element having a
multiplicity of voids and a plurality of substantially parallel
channels extending through the porous element with diaphragm means
adjacent thereto.
The positive displacement piston flow meter of the present invention
broadly comprises:
(a) a hollow flowtube having two opposite open ends;
(b) a piston disposed in said flowtube for movement between a first
position relative to one of said open ends and a second position
relative to the opposite open end thereof;
(c) means for connecting said flowtube to an external pump for
directing a gaseous fluid through said flow meter at a flow rate
to be measured by said flow meter;
(d) valve means for controllably reversing the direction of movement
of said piston in said flowtube between said first and second position
respectively; and
(e) damping means for damping pulsations in gas flow through said
flowtube, said damping means comprising at least one elastic diaphragm
responsive to pressure pulsations in said flowtube and a porous
member contiguous thereto with the porous member having a multiplicity
of voids distributed throughout the porous member and a plurality
of open channels.
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 a preferred embodiment
of the piston flow meter of the present invention with the piston
shown at the top of the piston stroke;
FIG. 2 is an exploded fragmentary view of a section of FIG. 1 illustrating
the assembly of the preferred poppet valve for controlling the movement
of the piston in the flowmeter of the present invention;
FIG. 3 is a bottom view of the damping element in the damping assembly
of FIG. 1; and
FIG. 4 is another view in vertical section of the piston flow meter
of FIG. 1 with the piston shown rising from the bottom of the piston
stroke.
DETAILED DESCRIPTION OF THE INVENTION
The flow meter of the present invention is identified in FIG. 1
by the reference numeral (10), with its corresponding parts identified
in each of the other figures by the same reference numbers. The
flow meter (10) as shown in FIGS. 1 2 and 4 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 reciprocates
from a position adjacent the bottom end (16) of the flowtube (12)
to an elevated position adjacent to the top end (24) of the flowtube
(12) and back. The piston (14) separates the flowtube (12) into
an upper piston chamber (13) located between the piston (14) and
the top end (24) of the flowtube (12) and a lower piston chamber
(15) located between the piston (14) and the bottom end (16) of
the flowtube (12).
The flowtube (12)is preferably supported in a substantially vertical
position between a lower housing (17) and an upper housing (18)
although it should be understood that the flowtube (12) may equally
be supported for operation in a horizontal position. In the vertical
orientation the bottom end (16) of the flowtube (12) is mounted
in the lower housing (17) and sealed from the atmosphere by an O-ring
(19). The housing (17) has an internal chamber (20) which provides
access to the atmosphere through an access passageway (21). The
internal chamber (20) is accessable to the lower piston chamber
(15) in the flowtube (12) through an air filter (22) and through
openings (23) in the housing (17). The air filter (22) is supported
by an o-ring (24a) held in place by a support ring (27) affixed
to the housing (17). The housing (17) also includes a damping assembly
(25) comprising a damper element (26) of a foam like-material containing
a multiplicity of voids (not shown) and a plurality of open channels
30 extending between elastic diaphragm members (32) on opposite
sides of the element (26). A bottom plate 34 holds the damping assembly
25 in place in the housing 17. The composition of the damper element
(26) and its function will be discussed in greater detail later
in the specification in connection with FIG. 3.
The top end (24) of the flowtube (12) is seated in housing (18)
and sealed from leakage to the atmosphere by an o-ring (33). Housing
(18) contains a poppet valve (28) for controlling the ascent and
descent of the piston (14) in the flowtube (12). A preferred poppet
valve arrangement is taught in U.S. Pat. No. 5295396 the disclosure
of which is herein incorporated by reference. In general the poppet
valve (28), as more specifically shown in FIG. 2 includes a valve
body (36) including one or more valve openings (38) and (39) or
a singular annular opening, an elastomeric valve head (40), a movable
valve stem (41) which extends from the valve head (40) through the
valve body (36), and a compression spring (42) seated in the valve
body (36) around the valve stem (41).
The movable valve stem (41)is an elongated tubular member with
an oval slot (not shown) in which a permanent magnet (45) is mounted.
A mounting bracket (47), of cylindrical geometry extends from the
valve stem (41) into the upper piston chamber (13). A flexible string
(48) flexibly connects the mounting bracket (47) to the movable
piston (14). The flexible string (48)is connected at one end to
a spherical member (49) placed within a compression spring (52)
mounted over the free end of the bracket (47). The compression spring
(52) acts as a stop for the piston (14) during its ascent. The other
end of the string (48) is connected to a spherical member (50) which
is press fitted into an opening (53)in the piston (14). The flexible
string (48) functions to automatically close the poppet valve (28)
by pulling the movable valve stem (41) during the descent of the
piston (14) when the piston (14) reaches a position at or near the
bottom end (16) of the flowtube (12).
A cylindrical collar (55) extends over a depending section (56)
of the valve body (36) surrounding the permanent magnet (35). The
collar (55) has a steel pin (57) which extends through the oval
slot (not shown) of the valve stem (41) to form a magnetic latch
for holding the valve stem (41) in the valve closed position as
shown in FIG. 4 with the flexible valve head (40) covering the valve
opening(s) (3839) in the valve body (36) against the force of the
compression spring (42) until the piston (14) rises back to a position
to open the valve (28) and to unlatch the magnet (35) from the steel
pin (57) as shown in FIGS. 1 and 2. The collar (55) is adjustable
to vary the magnetic latching force.
Two hollow tubes stand pipes (60) and (62) are respectively connected
between the upper housing (18) and the lower housing (17). The hollow
tube (60) communicates with a passageway (64) in the upper housing
(18) and with the internal chamber (20) of the lower housing (17)
through the air filter (22). The passageway (64) is in direct communication
with an opening (65) above the valve head (40). A flexible diaphragm
(66) is supported in the upper housing (18) in communication with
the opening (65) above the valve head (40). When the valve (28)
is open, as shown in FIGS. 1 and 2 the opening (65) is accessable
to a passageway (68) in housing (18) through the valve openings
(38) and (39). The passageway (68) leads into the upper piston chamber
(13) of the flowtube (12) and is in communication with hollow tube
(62) assuming the manually operable vent valve (69) is closed which
is normally the case. The manually operable vent valve (69) permits
single shot manual operation. The tube (62) communicates with an
access passageway (70) to a pump inlet fitting (72) to which an
external pump (not shown) is connected. The external pump (not shown),
preferably a suction pump, draws gas (ambient air) from the inlet
fitting (72) of the flowtube (10). The damping assembly (25) is
positioned close to the access passageway (70) with one of the open
channels (30) in direct communication with the inlet to the external
pump.
As more clearly shown in FIG. 3 the damper element (26) in the
damping assembly (25) is an open cell porous foam member preferably
containing about 80 PPI (pores per inch). A multiplicity of open
channels (30) is formed in the foam element (26). The number of
open channels (30) and their size (diameter) may vary although a
diameter of about 3/8 inch has been found to be satisfactory. The
open channels (30) should be uniformly distributed and should preferably
lie parallel to one another in a preferred direction substantially
along the longitudinal axis of the flowmeter (10) between the two
nonporous elastic diaphragms (32) (32) although the orientation
of the channels 30 is not at all critical to the invention. The
damping assembly (25) acts as an accumulator to smooth out pulsations
in air flow through the flowmeter (10). The upper nonporous elastic
diaphragm (32) separates the chamber (20) in the lower housing (17)
from the access passageway (70) to the external pump. The damping
assembly (25) is sensitive to pressure variations in the flowtube
(10) with the parallel channels (30) substantially increasing the
sensitivity of the damping element 26 to changes in air flow to
and from the external pump so as to minimize flow variations.
In operation, referring to FIG. 4 with the valve (28) shown in
the closed position and the pump inlet fitting (72) assumed to be
attached to the suction side of a conventional vacuum pump and the
manual vent valve (69) assumed to be in the manually closed position,
air is drawn from the atmosphere through the air filter (22) and
openings (23) into the lower piston chamber (15) of the flowtube
(12) forcing the piston (14) to rise from a position at the bottom
end of the flow meter (10) toward the upper end thereof. Upon reaching
the upper end of the flow meter (12) the piston (14) will force
the valve (28) into the open position as shown in FIGS. 1 and 2
by pushing the valve stem (41) upward to lift the valve head (40)
off the valve openings (38) and (39) overcoming the magnetic latching
force. Once the valve (28) is open air is drawn through the tube
(60) to the passageway (68) and to the tube (62) thereby bypassing
the valve (28) and the piston (14) begins to drop by gravity toward
the bottom end of the flow tube (10). During the descent of the
piston (14), the valve head (40)is maintained in the valve-open
position by the compression spring (42). At the end of the piston
stroke the flexible string (48) automatically closes the valve (28)
by pulling the movable valve stem (41) down against the force of
the compression spring (42) at a time when the piston (14) reaches
a position at or near the bottom end (16) of the flowtube (12) and
relatches the magnetic magnet (35) to the steel pin (57) so that
the cycle may repeat itself, causing the piston (14) to ascend.
Single stroke operation may be achieved by opening and closing the
manual vent valve (69).
The flow meter (10) of the present invention will accurately measure
the flow rate of a fluid pumped through the flow tube (12) over
a wide range of flow rates. To measure flow through the flow tube
(12) two sets of conventional optical LED photoelectric sensor elements
(80) and (82) are positioned along the flowtube (12) spaced a fixed
distance apart and operate to measure the displaced transit time
of the leading and/or trailing edge of the piston (14) as it moves
along the flowtube (12) between the two sets of sensor locations.
The operation and and method of calculating transit time and flow
rate is conventional and will not be discussed in this application. |