Abstrict An improved fluid flow meter for measuring a liquid flow rate is
disclosed. It comprises a container having an interior cavity, an
inlet opening, a vent opening, a first passageway and a second passageway.
The first passageway has an upper opening which is sealed by a pressure
transducer and has a lower opening which opens into the interior
cavity below the inlet and vent openings. A pressure signal produced
in the first passageway by liquid in the cavity is converted into
a flow rate by a processor which is connected to the pressure transducer.
The second passageway has a lower opening in communication with
the interior cavity above the lower opening of the first passageway
and an upper opening below the upper opening of the first passageway
which is connected to a downwardly depending drain tube to drain
the level of liquid in the cavity down by siphon action. In one
embodiment the vent opening and the second passageway are in communication
with the upper portion of a second cavity which is provided below
the first cavity and has a lower outlet opening.
Claims I claim:
1. A disposable container for use in a meter pump apparatus in
which a pressure transducer produces a signal which is proportional
to an air pressure within the container which varies in accordance
with the volume of liquid within the container, comprising:
an interior chamber defining a first cavity and having an upper
portion with an inlet opening for admitting the liquid into the
first cavity;
an integral first conduit defining a passageway and having a lower
opening which opens into the first cavity below the inlet opening
and an upper opening which is suitable to operatively mount the
pressure transducer so that the upper opening is in communication
with the first cavity below the inlet opening and the air pressure
in the first passageway varies in accordance with the volume of
liquid in the first cavity when the level of the liquid is above
the lower opening of the first passageway and the upper opening
of the first passageway is sealed by the pressure transducer;
an integral second conduit defining a second passageway and having
a lower outlet opening in communication with the first cavity above
the lower opening of the first passageway and an upper outlet opening
below the upper opening of the first passageway;
an integral second chamber defining a second cavity below the upper
outlet opening of the second conduit and having a lower opening
which is suitable for connection to a feed tube;
an integral third conduit defining a third passageway between the
upper outlet opening of the upper portion of the second passageway
and the upper portion of the second cavity;
an integral fourth conduit defining a fourth passageway between
the upper portion of the first cavity and the upper portion of the
second cavity for providing communication between the first cavity
and the second cavity so that when the second cavity is primed with
a quantity of liquid, a partial vacuum is developed in the container
and the admission of liquid into the first cavity pumps a similar
quantity of liquid from the second cavity and liquid is transferred
from the first cavity to the second cavity by siphon action when
the level of liquid within the first cavity reaches the upper outlet
opening of the second passageway; and
means for controlling the flow of liquid into the first cavity
to meter the volume of liquid pumped from the second cavity.
2. A disposable container as in claim 1 further comprising:
means for communicating the air pressure in the first cavity to
the pressure transducer.
3. A disposable container for use in a meter pump apparatus in
which a pressure transducer produces a signal which is proportional
to an air pressure within the container which varies in accordance
with the volume of liquid within the container, comprising:
an interior chamber defining a first cavity and having an upper
portion with an inlet opening for admitting the liquid into the
first cavity;
an integral first conduit defining a passageway and having a lower
opening which opens into the first cavity below the inlet opening
and an upper opening which is suitable to operatively mount the
pressure transducer so that the upper opening is in communication
with the first cavity below the inlet opening and the air pressure
in the first passageway varies in accordance with the volume of
liquid in the first cavity when the level of the liquid is above
the lower opening of the first passageway and the upper opening
of the first passageway is sealed by the pressure transducer;
an integral second conduit defining a second passageway and having
a lower outlet opening in communication with the first cavity above
the lower opening of the first passageway and an upper outlet opening
below the upper opening of the first passageway;
an integral second chamber defining a second cavity below the upper
outlet opening of the second conduit and having a lower opening
which is suitable for connection to a feed tube;
an integral third conduit defining a third passageway between the
upper outlet opening of the upper portion of the second passageway
and the upper portion of the second cavity;
an integral fourth conduit defining a fourth passageway between
the upper portion of the first cavity and the upper portion of the
second cavity for providing communication between the first cavity
and the second cavity so that when the second cavity is primed with
a quantity of liquid, a partial vacuum is developed in the container
and the admission of liquid into the first cavity causes the expulsion
of a similar quantity of liquid from the second cavity and liquid
is transferred from the first cavity to the second cavity by siphon
action when the level of liquid within the first cavity reaches
the upper outlet opening of the second passageway; and
means for directly communicating the air pressure in the upper
portion of the first cavity to the pressure transducer.
Description FIELD OF THE INVENTION
This invention relates to fluid flow meters and more particularly
to a fluid flow meter for medical applications.
BACKGROUND OF THE INVENTION
Diagnosis of kidney and urinary tract infections is often facilitated
by a knowledge of the patient's urinary flow rate. Urinary flow
rate data is particularly important in critical care situations,
and in particular when the patient has just undergone surgery since
the patient may not be conscious and able to verbalize any symptoms.
In critical care situations, urine is usually continuously drained
from the body via a Foley type catheter and the urine output is
usually determined by visually observing the amount of urine drained
from the patient via the catheter into a drainage receptacle.
Examples of prior art drainage receptacles for enabling the quantity
of urine drained via a Foley type catheter to be determined are
found in U.S. Pat. Nos. 4301813 4095589 4085616 and 4000649.
Each of the prior art drainage receptacles described in the above
listed patents includes a transparent urine receiving chamber having
a graduated scale thereon for indicating the quantity of urine entering
the chamber. The urine flow rate is determined in such devices by
observing the quantity of urine entering the chamber of the drainage
receptacle over a period of time.
While the prior art devices are satisfactory for this purpose,
there are several disadvantages associated with measuring urine
in this manner. The most obvious disadvantage is that the urine
flow is detected visually. This requires either a nurse, a doctor
or a technician to observe the patient for a set period of time.
In practice, a nurse generally records the volume of urine at 15
minute intervals. However, inaccuracies may arise due to failure
to maintain an exact interval between observations. In addition,
an exact measurement may not always be obtained due to the imprecisions
in reading the drainage receptacle scale. More importantly, the
use of prior art urine flow measuring techniques do not provide
an instantaneous measure of excessive urine output unless the nurse,
doctor or technician is present to observe the excessive urine output.
BRIEF SUMMARY OF THE INVENTION
Briefly, in accordance with the preferred embodiment of the invention,
a fluid flow meter for measuring fluid flow and in particular for
measuring urine flow includes a container comprising an interior
chamber which defines an interior cavity. The interior chamber has
an upper portion with an inlet opening for admitting a liquid, such
as urine, into the cavity. The upper portion is also provided with
a vent opening for admitting air at atmospheric pressure into the
cavity. A first conduit defines a passageway within the container
which has a lower opening which opens into the interior cavity below
the inlet and vent openings and which has an upper opening which
is suitable to mount a pressure transducer to measure the air pressure
in the first passageway. The air pressure in the first passageway
varies according to the volume of liquid in the interior cavity
so that the pressure transducer output signal can be used to determine
the flow rate of liquid into the container.
To prevent the container from overflowing, a second passageway
can be provided having a lower outlet opening above the lower opening
of the first passageway and having an upper outlet opening below
the upper opening of the first passageway. The upper outlet opening
of the second passageway is suitable to be connected to a urine
drainage bag below the container. With this structure, when the
level of urine reaches the level of the upper outlet opening of
the second passageway, urine will be drained from the interior cavity
by siphon action into the urine drainage bag.
A container of the invention thus described can be inexpensively
manufactured from polystyrene plastic or the like so as to make
it economical to dispose of after use, thereby avoiding the need
for cleaning.
The fluid flow meter described above can also be advantageously
modified to yield a meter pump apparatus for intravenous fluid which
not only measures the rate of intravenous fluid flow but which also
precisely delivers the fluid flowing therethrough while assuring
total sterility. To obtain such a meter pump apparatus, a second
chamber defining a second cavity is added to the fluid flow meter
previously described so that the second cavity is located in part
below the first cavity. The second cavity has an outlet at its lower
end, the outlet being coupled by a feed tube or the like to the
patient so that intravenous fluid may be carried from the second
cavity into the patient. The upper portion of the second cavity
is in communication with the first cavity via two passageways. One
of those passageways is an equalizer passageway which is in communication
with the vent opening and seals the vent opening from the atmosphere.
The other passageway is a siphon passageway which communicates with
the upper outlet opening of the second passageway to help provide
the siphon action which lowers the level of liquid within the first
cavity. When a partial vacuum is developed in the container by priming
it and a liquid is introduced into the first cavity through the
inlet, the air in the first cavity is forced therefrom into the
second cavity via the equalizer passageway to effectively pump the
liquid in the second cavity into the patient. The continued introduction
of liquid into the first cavity causes the first cavity to fill
to the level of the upper outlet opening of the second passageway
so that the liquid is transferred from the first cavity to the second
cavity by siphon action via the siphon passageway to replenish the
volume of liquid in the second cavity. By regulating the flow rate
of liquid entering the first cavity through the use of a flow control
valve, the volume of fluid entering the patient can be precisely
metered. Since the entire system is closed, sterility is assured.
It is an object of the invention to provide a fluid flow meter
for measuring a rate of fluid flow;
It is yet another object of the invention to provide a fluid flow
meter for measuring a rate of fluid flow electrically;
It is yet another object of the invention to provide a fluid flow
meter for electrically measuring a rate of fluid flow which is inexpensive
to manufacture so as to make disposal after use economical.
It is yet another object of the present invention to provide an
improved meter pump apparatus for not only measuring fluid flow
therethrough but for pumping fluid therethrough.
It is yet another object of the present invention to provide a
meter pump for not only measuring the rate of fluid flow but for
precisely metering fluid flow therethrough;
Other objects and advantages will become apparent from the following
detailed description and from the drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a fluid flow meter apparatus of the
present invention;
FIG. 2 is a cross-sectional view of the container of the fluid
flow meter apparatus of FIG. 1 taken along the plane of the line
2--2 thereof;
FIG. 3 is a flow chart of a program for the processor of the fluid
flow meter of FIG. 1;
FIG. 4 is a block diagram of a meter pump apparatus for measuring
the rate of fluid flow therethrough and for precisely metering the
fluid flowing therethrough; and
FIG. 5 is a cross-sectional view of the container of the meter
pump apparatus of FIG. 4 taken along the plane of the line 4--4
thereof.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a fluid flow meter 10 of the present invention
which includes a container 11. Referring also to FIG. 2 the container
11 is made up of two rigid wall sections 11a and 11b which are joined
at their edges. The wall sections 11a and 11b are preferably made
of a transparent resinous material such as polystyrene. The interior
surfaces of the wall sections 11a and 11b are coated with a hydrophilic
coating 12. In practice, the hydrophilic coating 12 comprises a
coating sold under the trade designation "HYDROMER" which
is a polyurethane polyvinyl pyrollidone interpolymer manufactured
by Biosearch, Whitehouse, New Jersey. Note that in some applications
it may be desirable to add a hydrophilic additive directly to the
resinous material of the wall sections so as to avoid the need to
coat the surfaces with a hydrophilic coating.
The wall sections 11a and 11b provide two interior baffles 13 and
14. The baffles 13 and 14 along with the exterior walls of the container
11 make up an interior chamber which defines a cavity 15. The chamber
has an upper portion with an inlet opening 16. The inlet opening
16 is suitable to be connected to a tube 17 that carries the liquid,
the flow rate of which is to be measured, into the cavity 15. The
tube 17 may be connected to a catheter (not shown) such as a Foley-type
catheter, which is in-dwelling in a patient so that the liquid,
such as urine, can flow from the patient through the catheter and
the connecting tube 17 and into the container 11. The upper portion
of the interior chamber is also provided with a vent opening 18
preferably above the inlet 16. In the container 11 the vent opening
18 admits air at atmospheric pressure into the interior cavity 15.
The baffle 13 and the right hand portions of the exterior walls
of the container 11 make up a first conduit which defines a first
passageway 19. The passageway 19 extends for substantially the entire
height of the cavity 15 and opens into the cavity 15 well below
the inlet and vent openings at a lower opening 20 which is relatively
closely adjacent to the bottom of the container 11. The passageway
19 also has an upper opening 21 which opens to the exterior of the
container 11. The upper opening 21 is thereby in communication with
the cavity 15 below the inlet opening 16 and the vent opening 18.
The upper opening 21 is suitable to easily mount a pressure transducer
22 in operative relationship so that the upper opening 21 is sealed
by the transducer 22 and the transducer 22 can measure the air pressure
within the passageway 19. The transducer 22 provides an electrical
signal in accordance with the air pressure in the passageway 19.
The transducer should be capable of following the air pressure as
closely as possible to provide a near instantaneous indication thereof.
Note that this arrangement shields the pressure transducer 22 from
urine contamination so that it need not be cleaned and is reuseable.
Either gauge or absolute pressure could be sensed by the pressure
transducer 22. However, as will become clear from the following
description, it is preferable to sense gauge pressure since atmospheric
pressure acts on the liquid within the cavity 15. If absolute pressure
were measured, the atmospheric pressure would be taken account of
in the processing required to convert the pressure signal into a
volume. A transducer 22 which was found suitable in practice is
the transducer sold under the trade designation Microswitch No.
142PC01G. This transducer has a measuring port 22a and a gauge port
22b. In practice, the measuring port 22a is mounted to the opening
21 and the gauge port 22b is left open to the atmosphere to measure
gauge pressure.
As liquid is admitted into the cavity 15 through the inlet opening
16 the level of liquid within the cavity rises. When the level
of liquid in the cavity is below the lower opening 20 the pressure
in the passageway 19 will be equal to atmospheric pressure due to
the atmospheric vent 18. However, when the level of liquid rises
above the opening 20 the passageway 19 becomes sealed from atmospheric
pressure by the liquid. As the level of liquid in the cavity 15
rises above the opening 20 some of the liquid will enter the passageway
19 thereby compressing the air in the passageway and the air pressure
in the passageway 19 will rise. The air pressure within the passageway
19 will depend upon the height of the liquid within the cavity 15
above the opening 20 which, in turn, is dependent upon the volume
of the liquid within the container 11. Therefore, any given volume
of liquid within the container 11 will produce a corresponding air
pressure within the passageway 19.
While the air pressure within the passageway 19 can be used to
determine the volume of liquid within the container 11 it can also
be applied to determine the rate of change of the volume of liquid
within the container, which equals the flow rate into the container.
Designating the volume of liquid within the container as V, the
flow rate into the container as F and time as t, the flow rate is
related to the change in volume, or .DELTA.V, by the following relationship:
##EQU1## so that
Now, designating the area occupied by the free surface (the surface
exposed to atmospheric pressure) of the liquid within the container
11 by A, and assuming A to be constant for the height of the container
above the opening 20 to simplify the analysis, and designating the
height of the liquid level above the opening 20 by h, the change
in volume .DELTA.V is related to the change in height .DELTA.h as
follows:
ti .DELTA.V=A.DELTA.h,
therefore A.DELTA.h=F.DELTA.t so that ##EQU2##
.DELTA.V is not exactly equal to A.DELTA.h because as the level
of the liquid surface rises, some of the liquid will enter the passageway
19 to further compress the air therein. This introduces an approximation
into the analysis but can be made negligible if the cross sectional
area of the passageway 19 is small compared to the cross sectional
area of the free surface of the liquid. Also, the error attributable
to this approximation can be eliminated by the processing techniques
explained below.
The rate of change of the height of the liquid level is represented
by .DELTA.h/.DELTA.t. Therefore if A is substantially constant,
it is shown that the rate of change of the height of the liquid
level within the container 11 is directly proportional to the flow
rate, F. Since the air pressure in the passageway 19 varies in accordance
with the height of liquid within the cavity 15 the rate of change
of the air pressure within the passageway 19 can be used as an indication
of the flow rate into the cavity 15.
As previously mentioned, the output signal of the transducer 22
represents the instantaneous air pressure within the passageway
19. This signal could be input to a strip chart recorder or other
output means to provide a graphical depiction of the pressure within
the passageway 19 over a period of time. Since the graph would depict
the pressure over time, one could get an indication of the rate
of change of the pressure and therefore of the rate of change of
the volume in the container 11 at a given instant by observing and/or
measuring the slope of the graph at that instant.
Also, as previously mentioned, the output signal of the transducer
22 which is representative of the air pressure within the passageway
19 can be used to determine the height of liquid within the container
11 and can therefore also be used to determine the flow rate into
the container. The exact relation between the air pressure in the
passageway 19 and the height of the free surface of the liquid above
the opening 20 is complicated by the fact that air is a compressible
gas so that a change in the height of the free surface of the liquid
causes some of the liquid to enter the passageway 19. While this
complicates the relationship, the relationship is derivable and
once derived, could be used to convert the air pressure in the passageway
19 into the height of the free surface or into the volume of the
liquid within the container. The derived height or volume function
could then be differentiated with respect to time to determine the
flow rate.
However, it is not necessary to derive the relationship between
the air pressure in the passageway 19 and the height or volume of
the liquid within the container 11. It is not necessary because
any volume of liquid of a given density within the container 11
which has a free surface above the opening 20 produces an air pressure
within the passageway 19 which is repeatable. That is, for a known
volume of liquid within the container 11 the air pressure within
the passageway 19 can be experimentally measured. This can be repeated
for many different known volumes of liquid and the resulting air
pressures can be measured and recorded. Then later, when the volume
within the container of the same liquid or of a liquid with substantially
the same density is desired to be determined, the air pressure in
the passageway 19 can be measured and compared with the earlier
experimentally obtained results to obtain the volume of the liquid.
This can be done for a series of unknown volumes and the resulting
volume-time function can then be differentiated, such as by measuring
the slope of the function at a given time, to yield the flow rate.
While this method of manually determining the flow rate is possible,
it is tedious and time consuming. Therefore, a processor 23 is provided
to convert the pressure transducer output signal into the flow rate.
The pressure transducer 22 is connected to an analog to digital
converter 24 which converts the pressure transducer output signal
into a digital signal which is suitable to be input to a central
processing unit 25. The central processing unit 25 is programmed
to convert the digitized pressure signal into a flow rate and to
output the flow rate.
Referring to FIG. 3 the preferred processing technique employs
a central processing unit 25 which has an internal clock which can
generate a read signal at set intervals such as the microcomputer
sold under the trade designation Intel 8051. The processing unit
is programmed to await the signal from the internal clock to begin
the portion of the program wherein the pressure signal is read and
processed into the flow rate. The first step in this portion of
the program is to generate an enable signal to be carried by an
enable line 26 to the analog to digital converter 24. The analog
to digital converter 24 would then convert the pressure signal from
the transducer 22 into a digital signal which would be carried by
a data bus 27 to the processing unit 25 where it is read.
The CPU 25 converts the digitized pressure signal into a volume
using a look-up table. In the look-up table, the experimentally
derived pressure-volume data is stored in associated memory addresses.
The processor compares the input pressure value to the stored pressure
values and stores the memory address of the experimental pressure
value which is closest to the input pressure value. The processor
then reads the volume from the memory address corresponding to the
memory address of the closest experimental pressure value.
Returning to the mainstream of the flowchart in FIG. 3 the corresponding
volume is stored in a third register. Each value stored in an address
in the third register is separated in time from the immediately
adjacent values by the time period between read signals from the
internal clock. Therefore, the flow rate can be computed by taking
the difference between two successive values and dividing it by
the time period between the two values. For reasons to become apparent
below, if the change in volume is negative, that is, if the volume
in the container 11 decreases, it is desirable to disregard the
change. In that case, the processor outputs the last flow rate and
returns to await the next read signal.
If the change is zero or positive, the change in volume .DELTA.V
is divided by the time interval between read signals .DELTA.t to
yield the flow rate. The flow rate is then stored and displayed,
and the processor returns to await the next read signal from the
internal clock.
While any of a number of well known displays could be used, the
preferred display is a strip chart recorder 30 to provide a histogram
of the flow rate. The recorder 30 is interfaced to the CPU 25 by
a digital to analog converter 31. When the CPU 25 reaches the display
instruction in the program, it generates an enable signal which
is carried by line 32 to the digital to analog converter 31 and
enables the converter 31 to read the digital flow rate from the
data bus 27. The converter 31 then converts the digital flow rate
signal into an analog signal which is suitable for input to the
recorder 30 via line 35. Also, the recorder 30 could be provided
with an alarm which would light or sound whenever the flow rate
was not within a preselected range.
The foregoing description adequately describes a container of the
present invention. However, as liquid enters the container 11 the
container 11 would eventually fill up and the liquid would exit
the container via the vent opening 18 without other means provided.
Therefore, in the preferred embodiment, the baffle 14 and the left
hand portion of the exterior walls of the container 11 make up a
second conduit which defines a second passageway 36. The second
passageway 36 has a lower outlet opening 37 which opens into the
cavity 15 above the lower opening 20 of the first passageway 19.
The second passageway 36 also has an upper outlet opening 38 which
is below the upper opening 21 of the first passageway 19 and opens
to the exterior of the container. The upper outlet opening 38 is
suitable to be connected to a drainage tube 40 which extends below
the upper outlet opening 38 to a lower end (not shown). The lower
end of the drainage tube 40 is below the lower outlet opening 37
and is connected to a drainage bag (not shown).
The second passageway 36 is provided so that the container 11 will
automatically be partially drained. As liquid enters the container
11 and the level of liquid within the cavity 15 rises, the level
of liquid within the passageway 36 will rise with it. Eventually,
the liquid level within the passageway 36 will reach the level of
the outlet opening 38. When this occurs, the liquid will exit the
container 11 via the drainage tube 40 and create a suction which
draws the level of the liquid within the cavity 15 down to the level
of the lower outlet opening 37 by siphon action.
It should be noted that the area of the container 11 above the
lower opening 37 and below the upper opening 38 is constant. This
may be desirable in those applications where a derived equation
is used to convert the pressure transducer 22 output signal into
the flow rate to simplify the calculations. However, with the preferred
processing technique of FIG. 3 this is irrelevant.
As the container 11 is drained by siphon action via the passageway
36 the liquid level within the cavity 15 will fall thereby causing
the air pressure within the passageway 19 to fall. Any output attributable
to the falling pressure would be meaningless and would be simply
disregarded by an observer. However, in the preferred technique
of FIG. 3 the processor disregards any negative changes in volume.
It simply outputs the last flow rate and returns to await another
read signal from the clock, as described above.
The container 11 can be inexpensively manufactured from injection
molded polystyrene, thus, the cost of the container can be made
low so that it is economical to dispose of the container after use.
By making it economical to dispose of the container, the need for
cleaning the container is obviated.
Although the flow measuring apparatus 10 is intended for flow measurement,
the apparatus 10 can be modified as illustrated in FIGS. 4 and 5
to provide a meter pump 42 which not only measures the flow rate
of an intravenous liquid but also precisely pumps the liquid into
the patient without complex and expensive equipment. The entire
description thus far pertains to the meter pump 42. Therefore, like
elements have been designated with corresponding numerals. However,
the meter pump 42 comprises structure in addition to the flow measuring
apparatus 10 so that further description is necessary.
The container 11' of the meter pump 42 is also made in two pieces
11a and 11b with all inside surfaces covered with a hydrophilic
coating 12'. The container 11' further comprises wall sections which
define a second cavity 43 which is located below the lower opening
37'. The lower end of the wall sections defining the cavity 43 are
suitable to be connected to a feed tube 44 which is to carry the
liquid from the container 11' to the desired destination, such as
a patient, below the cavity 43.
The upper end of the cavity 43 is in communication with the cavity
15' via a siphon passageway 45 and an equalizer passageway 46. The
siphon passageway 45 is in communication with the passageway 36'
and has an end opening 47 which is below the opening 37' of the
passageway 36'. The conduits forming the equalizer passageway 46
are integral with those forming the vent opening 18' so that the
passageway 46 is in communication with the vent opening 18'.
Note that it is not necessary that any of the conduits forming
the various cavities and passageways of the containers 11 and 11'
be integral with one another. However, it is necessary that the
container 11 be open to the atmosphere, as provided by the vent
opening 18 and that the container 11' be closed to the atmosphere,
as will become apparent.
The meter pump 42 must be primed, preferably before the tube 44
is connected to a patient. Priming can be accomplished by the use
of a flow control valve 50. The tube 17' is connected to a liquid
source, such as an intravenous fluid bag, and the valve 50 is adjusted
to allow a free flow of liquid into the cavity 15'. Any time after
the level of the liquid in the cavity 15' surpasses the opening
38' and the liquid begins to exit the cavity 15' by siphon action
via passageways 36' and 45 the valve is adjusted to provide a restriction
to the flow through the inlet 16'. The liquid exiting the cavity
15' will collect in the cavity 43 and in the tube 44. Some of the
liquid first exiting the cavity 15' will also be expelled out the
patient end (not shown) of the tube 44.
However, not all of the liquid exiting the cavity 15' will be immediately
expelled. As some of the liquid is expelled, a partial vacuum will
develop in the cavity 43 which is communicated to the cavity 15'
by the passageway 46. Eventually the vacuum will be sufficient to
support the column of liquid in the tube 44 and the cavity 43. When
this equilibrium vacuum is reached, the flow rate of the liquid
expelled from the tube 44 will be exactly equal to the flow rate
of the liquid entering the cavity 15' via the tube 17'. Since the
flow rate of the liquid entering the cavity 15' can be adjusted
by the control valve 50 the flow rate of the liquid being expelled
from the tube 44 is adjustable. Once an uninterrupted flow of liquid
(there may at first be air bubbles in the tube 44) is being expelled
from the tube 44 the tube can be connected to the patient or other
receptacle.
The meter pump 42 is referred to as a "pump" for the
following reasons. Once the partial equilibrium vacuum is developed
in the cavities 15' and 43 the flow rate of liquid into the cavity
15' will "pump" a like flow rate out of the cavity 43
to maintain the total volume of air and liquid in the container
11' constant. For example, immediately after the liquid has been
siphoned from the cavity 15' into the cavity 43 the volume of liquid
in the cavity 15' is at a steady state minimum value and the volume
of liquid in the cavity 43 is at a steady state maximum value. If
one drop of liquid is then admitted to the cavity 15' from the tube
17', it will displace a like volume of air in the cavity 15' which
will momentarily cause the air pressure in the cavity 15' to rise.
This pressure increase is communicated to the cavity 43 by the equalizer
passageway 46 which causes a drop of liquid to be expelled from
the cavity 43 out through the tube 44. Note that the drop admitted
to the cavity 15' increases the volume, and therefore the height,
of the liquid in the cavity 15 and that the drop expelled from
the cavity 43 diminishes the volume in the cavity 43. However, when
the height of liquid within the cavity 15' reaches the opening 38',
the volume of liquid in the cavity 43 is replenished by siphon action,
and the pumping cycle starts over again.
While this pumping is taking place, the meter portion of the meter
pump 42 is operating as described in connection with the flow measuring
apparatus 10. However, the meter pump 42 differs from the apparatus
10 in that it is not open to the atmosphere. Instead, the meter
pump 42 is a closed system in which a partial vacuum acts on the
liquid in the cavity 15'. Since the most relevant pressure value
is the pressure attributable to the height of liquid within the
cavity 15', which is equal to the difference between the air pressure
in the passageway 19' and that in the cavity 15', it is desirable
to directly measure that difference. This can be accomplished by
providing communication between the gauge port 22b' of the pressure
transducer 22' and the passageway 46 with a tube 51. Alternatively,
as with the apparatus 10 the air pressure in the cavity 15' could
be measured and input to the processor where it would be accounted
for.
Note that the flow control valve 50 could be controlled by the
processor 23' in a feed back type of control. This may be desirable
in applications where a very accurate and constant flow rate is
required. For this type of control, the control valve 50 would have
an input which was connected to a digital to analog converter of
the processor 23'. The desired flow rate would be programmed into
the processor 23' and it would compare the measured flow rate to
the desired flow rate to determine the correction signal to be input
to the flow control valve 50 via the digital to analog converter.
It is important to understand that the container 11' is completely
sealed from the atmosphere and therefore from contaminants. Thus,
the sterility of the liquid flowing through the container 11' is
assured, thereby reducing the possibility of infection.
Many modifications and variations of the invention will be apparent
to those skilled in the art. It is therefore intended that the scope
of the invention is not to be determined by the description of the
preferred embodiments, but by the claims which follow.
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