Abstrict An improved gas flow meter which measures the time of travel of
a soap film through a fixed volume to compute gas flow rate. A portable
meter includes infrared sensors which detect the passing of a soap
film, a timing circuit to determine the time of travel between the
beginning and end of the fixed volume and a method of blocking the
infrared portion of ambient light to allow for use outdoors. The
device has a unique method of generating the soap bubble and for
breaking the soap bubble at the end of the fixed volume. The unique
flow unit, including flow cell and fixed sensors, is calibrated
to determine the fixed volume which is stored in a nonvolatile memory
location as a part of the flow unit.
Claims What is claimed is:
1. A flow unit for use in a gas flow meter comprising a flow tube
of fixed inner and outer diameter having a lower end and an upper
end at which is located an upper chamber, and a sensor unit permanently
attached to said flow tube, including at least two fixed sensor
locations positioned between said ends and a nonvolatile memory
location for storing the value of the fixed volume of the flow tube
between the two sensor locations;
optical sensors at said two sensor locations for detecting the
presence of gas flow in said tube; and
wherein the top of said upper chamber includes a cap for filtering
out infrared components of ambient light.
2. The flow unit of claim 1 wherein said top of said upper chamber
additionally comprises a protrusion extending from said top into
said flow tube and wherein the diameter of said protrusion is less
than the inner diameter of said flow tube.
3. The flow unit of claim 1 additionally comprising a gas flow
outlet in said upper chamber.
4. A flow unit for use in a gas flow meter comprising:
a flow tube of fixed inner and outer diameter having upper and
lower ends;
a sensor unit permanently attached to said flow tube and comprising
at least two fixed sensor locations positioned between said ends
and a nonvolatile memory location for storing the value of the fixed
volume of the flow tube between the two sensor locations;
a lower chamber associated with said lower end of said flow tube,
said lower chamber having a reservoir of soap solution; and
means for applying a film of said soap solution comprising a cup
with an annulus having a diameter which is smaller than the inner
diameter of said flow tube.
5. The flow unit of claim 4 further comprising a gas flow inlet
in said lower chamber.
6. A gas flow meter comprising:
a flow unit comprising a flow tube with a fixed inner diameter
and having upper and lower ends, said tube having at least first
and last fixed sensor locations positioned between said upper and
lower ends, and a sensor unit permanently attached to said flow
tube including at least one nonvolatile memory location for storing
the fixed volume of said flow tube between said sensor locations;
at least first and last sensors provided at said fixed sensor locations;
bubble generator comprising a cup with an annulus having a diameter
which is less than the inner diameter of said flow tube and means
for applying a film of soap to said annulus; and
a system associated with said sensor locations for measuring the
time for a bubble to pass through said fixed volume and adapted
to produce an output indicative of such measured time.
7. The gas flow meter of claim 6 wherein said flow unit additionally
comprises means for breaking a bubble after said bubble has passed
said last sensor said means being located at the upper end of said
flow tube.
8. The gas flow meter of claim 6 wherein said sensors are infrared
optical sensors.
9. The gas flow meter of claim 6 further comprising a base unit
adapted to receive said flow unit.
10. The gas flow meter of claim 6 further comprising means responsive
to the output produced by said system for determining the flow rate
of gas through said flow tube.
11. The gas flow meter of claim 10 wherein said means for determining
the flow rate of gas through said flow tube includes means for retrieivng
said fixed volume from said nonvolatile memory and means for calculating
the flow rate of gas through said flow tube utilizing said fixed
volume and said time value.
12. A gas flow meter comprising:
a flow unit comprising a flow tube withya fixed inner diameter
and having upper and lower ends, said tube having at least first
and last fixed sensor locations positioned between said upper and
lower ends, and a sensor unit permanently attached to said flow
tube including at least one nonvolatile memory location for storing
the fixed volume of said flow tube between said sensor locations;
at least first and last infrared optical sensors provided at said
fixed sensor locations;
a bubble generator for providing a bubble to said flow unit in
the presence of gas flow through the flow unit;
a system associated with said sensor locations for measuring the
time for a bubble to pass through said fixed volume and adapted
to produce an output indicative of such measured time; and
filter means disposed on said upper end of said flow tube for filtering
out the infrared components of ambient light.
13. A method for measuring the flow rate of gas in a gas flow meter
having a flow unit comprising a flow tube having a fixed inner diameter,
a fixed volume and upper and lower ends and a sensor unit permanently
attached to said flow tube including at least first and last sensors
positioned between said ends and a nonvolatile memory for storing
the value of said fixed volume of the flow tube between said first
and last sensors, comprising the steps of:
storing the fixed volume of said flow tube in said at least one
non-volatile memory location;
introducing a bubble into a first end of said flow tube by providing
a soap film having a diameter which is less than the inner diameter
of said flow tube and transferring said soap film to the flow tube
in the presence of gas flow;
measuring the time for the bubble to traverse said fixed volume;
and
determining the flow rate from said time measurment and said stored
value of the fixed volume of the flow tube.
14. The method of claim 13 wherein said gas flow meter further
compriss means for measuring the time for a bubble to traverse said
fixed volume between said first and last sensors and wherein said
step of measuring the time includes the steps of:
first sensing said bubble at said first sensor;
initiating said means for measuring the time upon said first sensing;
last sensing said bubble at said last sensor; and
stopping said means for measuring the time upon said last sensing.
15. The method of claim 13 wherein said determining step includes;
retrieving said fixed volume value from siad nonvolatile memory;
and
combining said time measure with said fixed volume value to calculate
the gas flow rate.
Description FIELD OF THE INVENTION
This invention relates in general to gas flow meters and, in particular,
to systems which operate on the principle of measuring the time
displacement of a soap film between two points in a flow tube having
a fixed volume, with the time displacement being converted into
a measurement of the rate of flow of the gas.
BACKGROUND OF THE INVENTION
In the past many methods have been used to measure the flow rate
of a gas. One of these methods, which has been made portable, consists
of a flow tube, for which the volume is known very accurately between
two fixed points which are provided with sensors, a frictionless
piston made up of a soap film, and a base unit to which the flow
tube is attached, including means for recording the time of travel
of the frictionless piston between the two points. Generally, a
reservoir of soap solution is provided near the base of the flow
tube. A film of the soap is applied to a "bubble maker",
a generally circular ring which is suspended above the reservoir.
Gas filling the flow tube causes the soap film to move off of the
ring and to proceed through the flow tube, passing the sensors one
at a time. The time of travel of the film, or "bubble",
between the sensors defining the fixed volume or critical volume
area is proportional to the rate of flow of the gas through the
flow tube. Initial "wetting" of the sidewalls of the flow
tube has been taught as a method for reducing the friction acting
against the film as it is carried up the tube by the gas flow. To
that end, the prior art has taught that it is preferable to allow
several film bubbles to traverse the flow tube and burst prior to
sensing the passage of the film and measuring the flow rate.
Numerous devices which have been developed in the past, one of
which is illustrated in U.S. Pat. No. 4860590 issued to Buck,
use optical sensors to detect when the soap film crosses the fixed
points. The systems may further include a microprocessor based timing
circuit which receives the detection signals from the sensors and
records the time at which each sensor was passed, thereby yielding
the time that the bubble spent traversing the critical volume area
of the tube. The microprocessor in the base unit can additionally
calculate the rate of flow of the gas by dividing the fixed volume
of the tube by the measured time required for the soap film to pass
through that volume. Thus, the flow rate of the gas is computed
based on the fixed volume of the flow tube and the time required
for a bubble to traverse that volume. Devices of this type typically
claim accuracies of +/-0.5%.
Sources of error are inherent in the systems which have been used
in the past. One source of error, particularly associated with mass-produced
meters, is the inability to readily establish the volume of the
flow tube. There is great difficulty in maintaining strict tolerances,
and the cost of manufacturing flow tubes with precise dimensions
having specifically placed sensors can be prohibitive. Absent strict
manufacturing standards however, one cannot be guaranteed that a
given flow tube has the exact intended volume between the sensors
once it is assembled with the base unit. In most of the prior art
metering systems, the inexact volume of the tube is compensated
for by moving the sensors along the height of the tube until the
volume readings indicate the desired preset volume value. In the
above-referenced Buck patent, there is taught a means by which the
volume of an individual flow tube can be approximated and stored
by the microprocessor and switches associated with the base unit
once the meter has been assembled. The Buck system provides a gas
having a known flow rate through the assembled meter and calculates
the flow rate for that gas as it passes through the flow tube' s
fixed volume, using the stored value of the "ideal" volume
of the tube in the calculation (i.e., the volume which the flow
tube was manufactured to contain). The calculated flow rate is then
compared to the known flow rate and any deviation is assumed to
be due to a difference between the ideal and the actual volume magnitude
of the flow tube. The difference in the magnitudes is then compensated
for by adjusting the stored value of the volume to reflect the actual
volume of the flow tube. The stored value is adjusted by DIP switches
(representing a matrix of volume values) which are coupled to the
microprocessor. Thereafter, the microprocessor will calculate the
flow rate using the actual measured volume of the flow tube which
is the combined value of the ideal volume stored in the microprocessor
of the base unit and adjustments as stored the DIP switches coupled
thereto. This latter Buck system does not require that the sensors
be moved during the calibration, which is preferable given the sensitivity
of the sensor components and the alignment challenges attending
the placement of same. However, in the Buck system, any time that
a different flow tube is provided to the base unit, re-calibration
and consequent re-adjustment of the DIP switches is required. What
is desirable is a means to precisely determine the volume of the
flow tube and digitally record this value in nonvolatile memory
associated with the tube itself rather, that the base unit, thus
allowing for exacting flow rate calculations without maintaining
exact machining dimensions and without the necessity of repositioning
system components, thereby reducing manufacturing and assembly concerns
and costs while still achieving a high accuracy with the flow device.
Another source of error in gas flow meters is the generation of
bubbles, soap films introduced into the flow tube in the absense
of gas flow. This can result when a film of the soap has been applied
to the bubble maker and the ring comprising the bubble maker has
substantially the same diameter as the flow tube. Mere contact with
the sidewalls of the flow tube may result in transfer of the soap
film to the flow tube, which can effect sensing of "flow"
in the absense of any gas flowing in the tube. Furthermore, the
presence of the film in the tube will necessarily affect the next
intended measurement.
A further design inefficiency of gas flow meters involves the length
of the flow tube, beyond the critical volume area between the sensors.
Prior flow meter designs have had difficulty in completely bursting
the soap film after it has traversed the critical volume area of
the flow tube. Typically, the soap film is allowed to proceed to
an upper surface, beyond the critical volume area, at which the
film essentially collapses or bursts with the soap solution then
running back along a given path to the reservoir below. A difficulty
is encountered when the films, or the so-called "bubbles",
don't collapse or burst completely, resulting in a multitude of
small malformed bubbles at the upper end of the tube. Known designs
encourage the formation of small bubbles since the bursting surface
at the upper end of the tube generally is joined to the upper sidewalls
of the tube itself. The build up of smaller bubbles can result in
excessive flow of bubbles and/or liquid along the sidewalls of the
flow tube and consequent interference with the sensors. The incomplete
bursting problem therefore necessitates leaving considerable time
between measurements so that the small bubbles have sufficient time
to eventually dissipate before their presence can interfere with
the sensors and the subsequent measurements. Increasing the time
between successive flow measurements, to allow the small bubbles
to burst themselves, can be time consuming and therefore unacceptable
to many users of the flowmeter. An alternative solution, which has
been adopted by many flow meter manufacturers, is to increase the
length of the flow tube from the critical volume area, at the later
encountered sensor, to the upper end of the tube at which the film
is to be burst. Due to the portability of the device, it is evident
that the ability to fully burst the bubble at a controlled point
would desirably reduce the size of the flow tube and the time between
successive measurements.
Another problem is particularly associated with the prior art flow
meters which utilize optical sensors. Many of the prior art portable
flow meters which incorporate optical sensors have difficulty operating
in varying ambient light conditions. A large amount of background
light, such as the condition which exists when operating out of
doors or near an outside window, causes the optical sensors to erroneously
fail to detect a bubble passing through the tube. Although prior
art references, such as the Buck patent, teach that the use of infrared
sensors can eliminate the problem, infrared sensors are nonetheless
sensitive to the infrared components of ambient light which can
adversely affect the operation of the sensors. It is therefore desirable
to have a method of filtering the infrared portion of the ambient
light, to an acceptable level for the infrared emitter detector
pairs which are incorporated into the flow meter device.
OBJECTIVES OF THE INVENTION
It is therefore an objective of the present invention to provide
an improved portable primary gas flow meter which is inexpensive
to produce, operates reliably and is accurate.
It is a further objective of the present invention to eliminate
the necessity of unused volume in a gas flow meter between the upper
sensor and the bubble breaker at the upper end of the flow tube.
It is-yet another objective to store the exact volume of the flow
tube, between fixed sensor locations, as a digital number in nonvolatile
memory associated with the flow tube itself.
Still another objective of the invention is to block ambient infrared
light to allow a portable flow meter having optical sensors to be
used in conditions of ambient light which would otherwise interfere
with the sensors.
Yet another objective of the present invention is to provide a
gas flow meter which does not permit transfer of soap film to the
flow tube in the absense of gas flow.
SUMMARY OF THE INVENTION
These and other objectives of the invention are realized in a primary
gas flow meter including: a compact flow tube having a nonvolatile
memory associated therewith for storing the exact volume between
the fixed sensors of the flow tube and a reliable means for completely
breaking the soap film said means consisting of an abutment at the
upper end of the flow tube which has no contact with the inside
of the flow tube; a bubble maker consisting of a cup with an annulus,
having a diameter smaller than that of the flow tube, which reliably
produces a single soap film when depressed into a soap solution
and transfers the film to the flow tube only when a gas flow condition
exists; optical sensors for sensing the passage of the film or bubble;
and, an infrared filter means to shield the optical sensors from
infrared components of ambient light. The soap film or bubble will
travel into the flow tube only in the presence of gas flow and will
maintain contact with the flow tube being propelled by the flowing
gas along the tube at a rate of travel which is proportional to
the flow rate of the gas. A pair of infrared optical detectors and
emitters will detect the passage of the bubble through the flow
tube and control the timing circuit. A means for filtering ambient
light, to prevent the infrared portion of the light from entering
the detectors and causing a fuilure to detect the soap film, consists
of a filter incorporated into the top chamber of the flow tube (which
may additionally include bubble bursting apparatus) and/or incorporated
into the housing cover of the flowmeter assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will hereinafter be described in further detail with
specific reference to the ataches figures wherein:
FIG. 1 is a front elevation view of the inventive gas flow meter
illustrating the microcomputer housing, flow cell, sensor block
and light filtering housing cover.
FIG. 2 is a simplified view of the flow unit illustrating the sensor
block the flow tube, a first chamber having a soap solution reservoir,
and upper second chamber and an infrared light blocking cap for
the flow units'upper chamber.
FIG. 3 illustrates the sensor block.
FIGS. 4A and 4B illustrate the bubble breaker and buble forming
cup.
DETAILED DESCRIPTION OF THE INVENTION
With reference to FIG. 1 the basic components of one form of the
present invention are illustrated. Gas flow meter 10 comprises a
flow cell 11 a soap film detecting sensor block 15 a microprocessor
based dsplay 14 and an infrared light blocking cover 12. The features
of the flow cell, 11 are further detailed in FIG. 2 wherein flow
cell 11 comprises a flow tube 17 having a first end 156 which protrudes
into reservoir chamber 22 and a second end 18 which terminates
in upper chamber 21. The upper chamber 21 has an infrared light
blocking cap 39 in accordance with the teachings of the present
invention. A gas flow inlet 20 is disposed within reservoir chamber
22 while gas flow outlet 19 is disposed within the upper chamber
21. In operation, as in the prior art, the gas is provided to the
reservoir chamber 22 via gas flow inlet 20. A soap film, which has
been applied to the first end, 16 of the flow tube is made to travel
up the flow tube due to the flow of the gas. The film's passage
will be detected by sensors, further illustrated in FIG. 3 and
will terminate at the second end, 18 of the flow tube. When the
film has collapsed, or burst, in the upper chamber 21 the gas is
exhausted out of gas flow outlet 19.
Turning to FIG. 3 sensor block 15 comprises of flow cell clamp
29 held in place by screw 30. A circuit board 36 has the drivers
for the infrared emitters 32 and 33 and infrared detectors 34 and
35. Circuit board 36 also maintains the digital value of volume
which is associated with the particular flow tube 17 and sensor
block 15 that it has been calibrated on. When assembled, the flow
cell and sensor assembly, the so-called "flow unit", is
calibrated not by estimating or adjusting as in the past, but by
measuring the actual volume of the flow tube between the fixedly
mounted sensors and storing that volume value in a nonvolatile memory
location associated with the flow cell and sensor assembly. In accordance
with the present invention, a flow cell and sensor block comprise
a single interchangeable flow unit for use with any base unit, together
forming a gas flow meter. A variety of flow cell and sensor block
pairs, or flow units, with sensitivity , etc. corresponding to a
needed range of flow rates, can be used with any microprocessor
based display 14 base unit. As discussed above, the optical sensors
readily detect the passage of the soap film traversing the flow
tube. It is to be noted that many of the inventive features of the
present invention are not limited to a gas flow meter which uses
optical sensors. Other sensors which are known to be equivalents
in the art may readily be substituted for the optical sensors without
departing from the teachings and claims of the present invention.
Referring to FIGS. 4A and 4B, the details of the inventive bubble
maker and bubble breaker components will now be described. Flow
cell 11 depressing button 23 compresses spring 24 causing shaft
25 to lower cup 26 into soap solution 36. Releasing button 23 allows
cup 26 to raise to its resting position on the bottom of tube 17.
Upon releasing the button, the annulus 38 of cup 26 will have applied
to it a film of soap. It is also possible to construct a system
wherein the soap solution 36 is brought up to the cup 26 to similarly
apply the solution to the annulus 38 of the cup. Clearly the method
of application of the film to the cup is not critical to the present
invention. What is critical is that the diameter of the annulus
38 be less than the interior diameter of the flow tube 17. A gas
flow condition causes a soap film to be transferred from annulus
38 to the first end of flow tube 17. In the absence of gas flow
the soap film remains on annulus 38 since the film will not be
in physical contact with the sidewalls of the flow tube. As noted
above, the specific relationship between the diameter of the annulus
and the interior diameter of the flow tube insures that no film
will be transferred to the flow tube in the absense of gas flow
in the system. Consequently, the system will not falsely detect
gas flow. In accordance with principles known in the art, one cannot
restrict the gas flow, by placing too narrow a passageway in the
gas flow meter assembly, and expect accurate measurements. Similarly,
the lower limit for the diameter of the annulus is the point at
which the flow of gas through the assembly would be compromised.
Bubble breaker 28 is located in upper chamber 21 and protrudes
into the second, upper end 18 of tube 17. The bubble breaker 28
completely bursts the soap film upon contact. Since there is no
physical contact between the edges of the bubble breaker and the
flow tube sidewalls, the formation of smaller bubbles and the problems
attendant thereto are avoided. Moreover, the initial wetting of
the sidewalls is facilitated by the complete bubble breaking at
breaker 28. By virtue of the use of the present bubble breaker,
there is no need to extend the physical length of the tube nor to
wait an extended period between measurements due to the formation
of small bubbles.
In operation, a film of the soap solution is applied to the annulus
38 of cup 26 either by the operation of plunging the cup into a
reservoir of the solution as described above, or by moving the reservoir
to the cup. The film spans the diameter of the annulus 38 but does
not span the diameter of end 16 of the flow tube. Upon the flow
of gas through inlet 20 the film is transferred from the annulus
38 to the flow tube 17 and travels up the sidewalls of the tube
at a rate proportional to the gas flow rate. Under initial conditions,
the film may encounter too much friction to complete its traversal
of the flow tube, since the soap film will tend to wet the sidewalls
and thin the bubble itself until it bursts. As noted above, several
bubbles should ideally be sent up the flow tube until all of the
sidewalls have been wetted, by both the film wetting the sidewalls
due to the friction along the walls and by the downflow of solution
caused when bubbled successfully traverse the length of the flow
tube and are completely burst at bubble breaker 28. The process
of forming soap films is continued until the sidewalls of tube 17
are fully wetted. This is evident when the soap films completely
traverse the length of tube 17 and burst upon making contact with
bubble breaker 28. Once tube 17 is fully wetted, the measurement
is begun by producing a single soap film on annulus 38 and transferring
it to end 16 of tube 17. The soap film is propelled by the gas through
tube 17 crossing infrared sensor pair 32 34 causing circuit 36
to send a signal to microprocessor 14 which starts the timing circuit.
The time of travel is recorded, for use in the flow rate computation,
when the soap film crosses infrared sensor pair 33 35 causing circuit
36 to send a signal to microprocessor 14 which stops the timing
circuit. The soap film is completely burst when it contacts bubble
breaker 28 at or near end 18 of tube 17. The gas flow rate is computed
by knowing the volume encompassed by tube 17 between sensor pair
32 34 and sensor pair 33 35 which value is provided to the microprocessor
14 from its nonvolatile memory location on board 36 and dividing
this value by the recorded time of travel of the soap film between
the sensor pairs.
The accuracy of the gas flow rate measurement is a function of
the accuracy and precision of the timing circuit mathematically
combined with the accuracy and precision of the volume of tube 17
as stored in the nonvolatile memory 36. The ability to store, in
nonvolatile memory 36 the exact volume of tube 17 is necessary
due to the production tolerances on the diameter of tube 17 and
the production tolerances in the placement of the sensor pairs.
When using optical sensors, as discussed above, it is desirable
to filter out the infrared components of the ambient light. Infrared
blocking top 39 as illustrated in FIG. 2 can be provided as part
of the interchangeable flow unit, comprised of the calibrated flow
cell and sensor block. Furthermore, an infrared light blocking cover
12 can additionally or alternatively be provided to the gas flow
meter assembly 10 comprised of the flow unit and the base or microprocessor
unit 14.
The present invention has been described with reference to several
exemplary embodiments. It is to be recognized by one having skill
in the art that modification may be made to the described embodiments
without departing from the spirit and scope of the invention as
defined by the appended claims. |