A flow meter system that calculates mass flow rate based only on
a single pressure signal. A flow controller is arranged in parallel
with a restriction such that a constant pressure differential is
maintained across the restriction. The pressure, and temperature
if not controlled, of the fluid flowing through the restriction
is measured on either side of the restriction. The pressure is compared
to a plot of pressure versus mass flow rate calculated for the specific
restriction and fluid being measured. The constant pressure differential
maintained across the restriction yields a linear relationship between
pressure and flow rate. If temperature is not controlled, the plot
of pressure versus mass flow rate will remain linear, but the slope
of the curve will be adjusted based on the temperature of the fluid.
What is claimed is:
1. A method for calibrating a flow meter; said flow meter comprising
an inlet with a diameter and an outlet with a diameter; a flow restrictor
having a restriction chamber and a pressure balancing system interposed
between the inlet and outlet; the restriction chamber having a variable
orifice assembly; a cylindrical restriction wall with a diameter
less than the diameters of the inlet and outlet; and, a pressure
sensor and a temperature sensor upstream from the flow restrictor
providing input to a meter circuit having calibration data; whereby
the meter circuit generates a flow output signal based on the calibration
data and the input from the pressure sensor and temperature sensor;
the steps of said method comprising: connecting the flow meter to
a referenced flow standard; applying a negative gauge pressure to
the outlet; setting a zero point on the meter circuit; applying
pressure upstream of the meter; measuring a flow; entering a gas
specific constant into the calibration data of the meter circuit;
obtaining a maximum flow range by adjusting the variable orifice
assembly for a given inlet pressure; decreasing the flow to ten
percent of maximum flow; measuring the pressure and temperature
and storing results in the meter circuit; increasing the flow by
ten percent increments up to the maximum flow; measuring the pressure
and temperature at each ten percent increment and storing the results
in the meter circuit; calculating a slope of a graph of pressure
versus mass flow rate from the results stored in the meter circuit;
setting the meter circuit by using the slope.
2. The method of claim 1 where the meter circuit further comprises
an arithmetic logic unit and a linearization amplifier and, where
the graph is non-linear, the steps of said method further comprise:
performing a piecewise linearization function on a filtered pressure
signal to obtain a compensated pressure signal for use in setting
the meter circuit.
3. The method of claim 1 where the method further comprises the
following step: measuring a current and storing results in the meter
circuit as a pressure.
The present invention relates to systems and methods for measuring
and controlling mass flow and, more specifically, to such systems
and methods that allow precise measurement of mass flow using a
flow restriction and pressure and temperature sensors.
BACKGROUND OF THE INVENTION
In many disciplines, the mass flow of a fluid must be measured
with a high degree of accuracy. For example, in medical and semi-conductor
manufacturing, gasses and liquids often need to be delivered in
precise quantities to obtain desired results. Meters are used to
measure the mass of the fluid actually delivered.
Conventional pressure-based mass flow meters employ a flow restriction,
a temperature sensor, and pressure sensors for detecting the absolute
pressure upstream of the flow restriction as well as the differential
pressure across the flow restriction. Mass flow is determined from
a table that correlates the pressure and temperature readings with
predetermined mass flow rates. Such systems require at least two
pressure sensors and a temperature sensor to account for fluid density,
fluid velocity, and fluid viscosity under different temperatures
and upstream and downstream pressures.
The need exists for mass flow meters that are simpler and require
less complex calculations to determine true mass flow.
U.S. Pat. No. 5791369 to Nishino et al. discloses a flow rate
controller that, purportedly, requires only one functional pressure
transducer. However, the controller disclosed in the '369 patent
operates only in the sonic flow regime, and this system requires
that the inlet pressure be twice the outlet pressure for the controller
to function properly. The flow controller of the '369 patent thus
operates only with very low flow rates, only with gases, and must
have effective pressure regulation upstream. In addition, the '369
patent discloses the use of a second pressure transducer to determine
when the downstream pressure is more than half of the inlet pressure,
and the controller shuts down when this condition is met.
U.S. Pat. No. 6152162 to Balazy et al. discloses a fluid flow
controller that requires two pressure measurements, one upstream
and one downstream of a flow restrictor. The '162 patent does not
measure mass flow. The '162 patent also employs a filter element
as the flow restriction. Particles in the gas stream can clog the
filter, thereby changing the relationship of pressure drop and flow
characteristics of their flow restriction and possibly deviating
from the initial calibration setting.
U.S. Pat. No. 6138708 to Waldbusser discloses a pressure compensated
mass flow controller. The system described in the '708 patent combines
a thermal mass flow controller with a thermal meter coupled to a
dome-loaded pressure regulator. Another pilot pressure regulator
using an independent gas source loads the dome of the pressure regulator
upstream of the thermal mass flow controller. The pilot regulator
and the mass flow controller are controlled by a microprocessor
so that inlet pressure is controlled in concert with the flow rate
resulting in an inlet pressure independent flow controller.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram depicting an exemplary mass flow meter
of the present invention;
FIG. 2 is an exemplary plot of mass flow through the meter versus
fluid pressure illustrating the operation of the present invention;
FIG. 3 is a somewhat schematic section view depicting an exemplary
mechanical system that may be used to implement a mass flow meter
as depicted in FIG. 1;
FIG. 4 is a block diagram of a meter circuit employed by the mass
flow meter of FIG. 1;
FIG. 5 is a detailed block diagram of an exemplary meter circuit
that may be employed by a mass flow meter employing the principles
of the present invention;
FIG. 6 is a flow diagram representing one exemplary method of calibrating
the mass flow meter of FIG. 1;
FIG. 7 is a plot of mass flow through the meter versus fluid pressure
for several different fluid temperatures illustrating compensation
for different fluid temperatures;
FIG. 8 is an exemplary plot of mass flow through the meter versus
fluid pressure illustrating the basic principles of operation of
the present invention applied to a non-linear mass flow output;
FIG. 9 is a block diagram of an exemplary flow control system employing
a mass flow meter system of the present invention; and
FIG. 10 is a block diagram of an alternate embodiment of a flow
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following discussion is organized in a number of sections.
In the first section, the basic operation and theory of the present
invention will be described in the context of a mass flow meter
system. The second and third sections will describe exemplary mechanical
and electrical systems that may be used to implement the present
invention. The fourth section will describe one method of calibrating
a mass flow meter system constructed in accordance with the principles
of the present invention. The fifth section describes the mass flow
meter described in the first through fourth section used as part
of a mass flow controller. The sixth section describes an alternate
embodiment of a mass flow control system. The final section describes
additional considerations that are typically taken into account
when designing and constructing a particular implementation of the
1. Mass Flow Meter System
Referring initially to FIG. 1 of the drawing, depicted at 20 therein
is an exemplary mass flow meter system constructed in accordance
with, and embodying, the principles of the present invention. The
meter system 20 comprises a mechanical system 22 and an electrical
system 24. The mechanical system 22 comprises a flow restrictor
30 defining a restriction chamber 32 and a pressure balancing system
34. The electrical system 24 comprises a pressure sensor 40 a temperature
sensor 42 and a meter circuit 44.
The mechanical system 22 defines a fluid inlet 50 and a fluid outlet
52. The inlet 50 and outlet 52 are connected to a source or supply
54 of pressurized fluid and a destination 56 of that fluid, respectively.
From the discussion above, it should be apparent that particulars
of the source 54 and destination 56 may vary significantly depending
upon the environment in which the meter system 20 is used. For example,
in a medical environment, the source 54 may be a bottle of pressurized
gas and the destination 56 may be a mixer that mixes the gas with
air and delivered the mixture to a patient using conventional means.
In a manufacturing environment, the source 54 may be a converter
that generates a supply of gas from raw materials and the destination
56 may be a reaction chamber in which the gas is used as part of
an industrial process. In many cases, the supply pressure at the
source 54 and back pressure at the destination 56 may be unknown
The meter system 20 of the present invention is thus intended to
be used as part of a larger system in which pressurized fluid thus
flows from the source 54 to the destination 56 through the mechanical
system 22. The pressure balancing system 34 maintains a constant
differential pressure across the restriction chamber 32.
As depicted in FIG. 1 the exemplary flow restrictor 30 is variable.
In particular, when the meter system 20 is calibrated the flow restrictor
30 defines a predetermined geometry and an effective cross-sectional
area of the restriction chamber 32. In the exemplary system 20
the flow restrictor 30 may be changed to alter the geometry, and
in particular the effective cross-sectional area, of the restriction
chamber 32. In other embodiments of the present invention, the flow
restrictor 30 need not be variable, but instead can be fabricated
with a preset geometry and effective cross-sectional area. This
may or may not include a standard orifice, sonic orifice, laminar
flow element of various geometries, or a variable area restriction.
The use of a preset or variable flow restrictor may affect the process
of calibrating the meter system 20 as will be discussed below.
The pressure balancing system 34 is preferably a flow controller
that utilizes a mechanical regulation system to maintain a constant
differential pressure across the restriction chamber 32 even if
the source and destination pressures are unknown or variable. Such
mechanical flow controllers are disclosed, for example, in U.S.
Pat. No. 6026849 issued Dec. 2 1999 and copending U.S. patent
application Ser. No. 09/805708 filed Mar. 13 2001 and commonly
assigned with the present application. However, the pressure balancing
system 34 may also be an electro-mechanical flow controller as disclosed
in the '708 application. The teachings of the '849 patent and the
'708 application are incorporated herein by reference.
The pressure and temperature sensors 40 and 42 are preferably electro-mechanical
transducers that convert pressure and temperature values into an
electrical signal. These sensors 40 and 42 are operatively connected
to the mechanical system 22 to generate electrical signals indicative
of the pressure and temperature, respectively, of the fluid flowing
through the mechanical system 22.
The meter circuit 44 stores or otherwise has access to calibration
data relating mass flow rate to pressure and temperature for a given
fluid. The calibration data includes a calibration factor calculated
for a given restrictor 30 and a gas constant determined by the characteristics
of the gas flowing through the meter system 20. The gas constant
is based on the specific gas density or viscosity as related to
Based on the calibration data and the pressure and temperature
signals, the meter circuit 44 generates a flow output signal corresponding
to the mass flow of fluid through the mechanical system 22. The
flow output signal may be recorded or displayed or used as part
of a larger circuit for controlling fluid flow from the source 54
to the destination 56.
Referring now to FIG. 2 depicted therein at 60 is a plot of pressure
versus mass flow through the restrictor 30 when the pressure balancing
system 34 is connected across the restrictor 30 as described above.
As seen in the figure, the mass flow output increases linearly with
outlet pressure. This curve 60 is an effect of the ideal gas law,
which relates volume, mass, temperature, and non-linear compressibility
effects together as described by the following equation (1):
Where: P=pressure, m=mass, V=volume, R=Gas Constant (Universal),
T=temperature, and Z=gas compressibility (in the following discussion,
a "." above any of these symbols denotes a mass of volume
Dividing both sides of the ideal gas law equation by time yields
the following rate equation (2):
Solving for the rate equation (2) for mass flow rate yields the
following mass flow rate equation (3): ##EQU1##
Rearranging the terms of the mass flow rate equation yields the
following slope equation (4): ##EQU2##
The slope of equation (4) illustrates the relationship between
the mass flow rate and pressure for a given system and gas. If the
pressure increases, the amount of mass within a certain volume (i.e.,
density) will increase proportionally if temperature remains constant.
Experimental data showed that the temperature only varied by a fraction
of a degree throughout the entire experiment. Since the slope of
the plot remained constant, the end result was the volumetric flow
rate for this device remained constant through the entire pressure
range until the pressure differential pressure (i.e., inlet pressure
minus outlet pressure) approached a critical value.
In contrast, traditional flow meter devices that rely on pressure
measurements must take into account three factors: inlet pressure,
inlet temperature, and pressure differential across an orifice.
The flow rate across an orifice, or similar flow restriction, is
expressed in general terms by the following flow rate equation (5):
Where: p.sub.1 =gas pressure upstream of the restriction p.sub.2
=gas pressure downstream of restriction T.sub.1 =temperature upstream
of restriction D=flow passage diameter d=restriction hydraulic diameter
(effective flow diameter) G=specific gravity or normalized molecular
weight of gas Z=compressibility factor of gas
The term K in this flow rate equation (5) is a factor that is determined
experimentally during the calibration of a given restriction. The
term K is dependent on the geometry of the restriction and expansion
factors of the gas such as Joule-Thompson cooling/heating (i.e.,
the change in temperature caused by a sudden change in pressure).
The flow rate equation (5) is only valid for low flows or restrictions
that do not create large gas velocities inside of them. When the
speed of the gas approaches the speed of sound, the bulk speed of
the gas molecules is larger than the speed at which pressure can
travel through the medium. The flow properties take on significantly
different relationships and are called compressible flows, sonic
flows, or choked flows.
Therefore, traditional flow controllers relying on pressure drops
employ two pressure sensors and a temperature sensor. Such traditional
flow controllers must also have relatively sophisticated electronics
capable of calculating flow rate by measuring both pressures, calculating
the pressure difference (with custom op amps (analog) or by means
of a programmed digital microprocessor and the needed analog to
digital converters), and most importantly, by calibrating the device
to find out the term K.
With the approach of the present invention, the equation to solve
to obtain flow would look like one the following equations (6) or
Where: K is the calibration factor and is determined during calibration
as will be discussed below. It should be noted that the constant,
R, in Equations (6-10) is not the Universal constant, R, of Equations
(1-4). Rather, it is a gas-dependent constant that varies for laminar
flow or orifice type restrictions.
The case of some gases where non-ideal compressibility must be
taken into account, the following equations (8) and (9) may be used:
Where: Z(P,T) is the compressibility factor that is dependent on
pressure and temperature.
As can be seen by a comparison of equation (5) with any of the
equations (6), (7), (8), or (9), the present invention greatly simplifies
the relationship of mass flow rate to pressure and temperature.
In most cases, compressibility does not need to be obtained directly,
because the controller will be calibrated such that compressibility
is accounted for in the calibration sequence.
At a given temperature and pressure, the gas may already be showing
some non-ideal compressibility that will be inherent in the measurement
taken by the flow standard during calibration. In addition, the
term R is gas specific, so only the gas specific constant needs
to be entered before or during calibration to have a highly accurate
mass flow measurement. The calibration sequence may be implemented
as will be described below with reference to FIG. 5.
After the calibration factor K is calculated using the calibration
sequence, mass flow may be measured using only the following linear
slope equation (10) defining the slope of the plot 60 depicted in
Where: y=mass flow, x=measured pressure, m=K/RT, and b is the zero
With the foregoing basic understanding of the meter system 20 in
mind, the various components of this system will now be described
in further detail below.
II. Mechanical System
Referring now to FIG. 3 of the drawing, depicted in detail therein
is the mechanical system 22 of the exemplary flow meter system 20.
The restrictor 30 of the mechanical system 22 is formed by a main
body assembly 120. The main body assembly 120 defines a main passageway
130 having an inlet 132 and an outlet 134 and defining the restriction
chamber 32. The restriction chamber 32 is arranged between the inlet
132 and the outlet 134.
As generally discussed above, the mass flow meter system 20 measures
the mass flow of fluid that flows through the main passageway 130
from the inlet 132 to the outlet 134 using pressure and temperature
signals generated by the pressure sensor 40 and the temperature
sensor 42. The fluid flowing through the meter system 20 will be
referred to herein as the metered fluid. Seals are formed at the
junctures of the various parts forming the mechanical system 22
such that metered fluid flows only along the paths described herein;
these seals are or may be conventional and thus will not be described
The exemplary main body assembly 120 comprises a main body member
140 and, optionally, a variable orifice assembly 142. The main body
member 140 defines at least a portion of the main passageway 130
the inlet 132 and the outlet 134. The main body member comprises
an inlet section 144 an outlet section 146 and a intermediate
The main body member 140 further defines first and second balancing
ports 150 and 152 located upstream and downstream, respectively,
of the variable orifice assembly 142. The first and second balancing
ports 150 and 152 allow fluid communication between the pressure
balancing system 24 and the inlet and outlet sections 144 and 146
respectively, of the main passageway 130. The first balancing port
150 and second balancing port 152 are connected to input and output
ports 154 and 156 respectively, of the pressure balancing system
The exemplary pressure and temperature sensors 40 and 42 used by
the meter system 20 are arranged to detect the pressure and temperature
of the metered fluid flowing through the main passageway 130. In
particular, the main body member 140 defines first and second test
ports 160 and 162 arranged in the outlet section 146 of the main
body member 140. The test ports 160 and 162 may, however, be arranged
in the inlet and/or intermediate sections 144 or 148 of the body
member 140 in another embodiment of the present invention.
The sensors 40 and 42 are or may be conventional and are inserted
or threaded into the test ports 160 and 162. Seals are conventionally
formed between the sensors 40 and 42 and the test ports 160 and
162. So attached to the main body member 140 the sensors 40 and
42 generate electrical pressure and temperature signals that correspond
to the pressure and temperature of the metered fluid immediately
adjacent to the test ports 160 and 162.
The inlet, outlet, and restriction sections 144 146 and 148 of
the main body member 140 serve different functions and thus have
different geometries. The inlet and outlet sections 144 and 146
are threaded or otherwise adapted to allow a fluid-tight connection
to be made between the main body member 140 and the source 54 and
destination 56 of the metered fluid. The effective cross-sectional
areas of inlet and outlet sections 144 and 146 are not crucial to
any implementation of the present invention except that the flow
of metered fluid to the fluid destination 56 must meet predetermined
system requirements. In the exemplary main body assembly 120 the
inlet and outlet sections 144 and 146 define cylindrical inlet and
outlet internal wall surfaces 170 and 172 and have substantially
the same diameter and effective cross-sectional area.
The intermediate section 148 of the main body member 140 serves
to restrict the flow of metered fluid through the main passageway
130 while still allowing the flow of metered fluid to meet the system
requirements. The effective cross-sectional area of at least a portion
of the intermediate section 148 of the main passageway 130 is thus
smaller than that of the inlet and outlet sections 144 and 146.
In particular, the intermediate section 148 is defined at least
in part by an internal restriction wall 180 of the main body member
140. The restriction wall 180 is substantially cylindrical and has
a diameter smaller than that of the inlet and outlet wall surfaces
170 and 172.
The meter system 20 of the present invention may be manufactured
without the optional variable orifice assembly 142. In this case,
the restriction wall 180 of the main body member 140 defines the
restriction chamber 32. The main body member 40 must be manufactured
to tight tolerances and/or the calibration data may need to be calculated
for each main body member 140 to account for variations in the restriction
portions defined by individual main body members if a variable orifice
assembly is not used.
If a variable orifice assembly 142 is used, the restriction chamber
32 associated with a given main body member 140 may be altered to
calibrate the given main body member 140. Any number of mechanisms
may be used to alter the geometry of the restriction chamber 32.
In the meter system 20 the exemplary variable orifice assembly
142 comprises a tube member 220 having an internal surface 222.
The internal surface 222 of the tube member 220 defines the effective
cross-sectional area of the restriction chamber 32.
In some situations, the tube member 220 may be made of a rigid
material such as some metals or polymers. In this case, the tube
member 220 is made in a plurality of predetermined configurations
each corresponding to a restriction chamber 32 having a different
predetermined cross-sectional area. One of these predetermined configurations
is selected to obtain a desired geometry of the restriction chamber
The exemplary tube member 220 is, however, made of a deformable
material such that, when the tube member 220 is deformed, the effective
cross-sectional area of the restriction chamber 32 is changed. The
exemplary tube member 220 is made of metal, but polymers, natural
rubber, or other materials may be used depending upon the circumstances.
In this respect, the tube member 220 may be made of elastic (e.g.,
polymers or natural rubber) or non-elastic (e.g., metal) material.
The variable orifice assembly 142 used by the exemplary meter system
20 further comprises a compression wedge 224 a compression shim
226 first and second chevron members 228 and 230 and a compression
nut 232 having a threaded surface 234.
To accommodate this variable orifice assembly 142 the intermediate
section 148 of the exemplary main body member 140 comprises the
following interior walls in addition to the restriction wall 180:
a tube seat wall 240 a compression wall 242 a spacing wall 244
and a threaded wall 246. The tube seat wall 240 is located upstream
of the restriction wall 180 described above and is generally cylindrical.
The compression wall 242 is located upstream of the tube seat wall
and is generally conical. The spacing wall 244 is located upstream
of the compression wall and is generally cylindrical. The threaded
wall 246 is located upstream of the spacing wall and is threaded
to mate with the threaded surface 232 of the compression nut 230.
Axial rotation of the compression nut 230 relative to the body
member 140 thus causes the nut 230 to be displaced along a longitudinal
axis A of the body member 140 towards the restriction wall 180.
As the nut 230 moves towards the restriction wall 180 the nut 230
applies a force on the compression wedge 224 through the chevron
members 228 and 230 and compression shim 226. The compression wedge
224 comprises a conical outer surface 250. The outer surface 250
of the compression wedge 224 engages the compression wall 242 such
that the wedge 224 moves radially inwardly towards the longitudinal
axis A. The inward movement of the compression wedge 224 deforms,
as generally described above, the tube member 220 to alter the effective
cross-sectional area of the restriction chamber 32.
III. Electrical System
Referring now to FIG. 4 of the drawing, depicted in detail therein
is one exemplary embodiment of a meter circuit 44 used as part of
the electrical system 24 of the exemplary flow meter system 20.
The meter circuit 44 comprises first, second, and third summing
and scaling systems 320 322 and 324. The first summing and scaling
system 320 combines the calibration factor and raw pressure signal
to obtain a calibrated pressure signal. The second summing and scaling
system 322 combines the raw temperature signal and the gas constant
input to obtain a compensated temperature signal. The third summing
and scaling system 324 combines the calibrated pressure signal and
the compensated temperature signal to obtain the flow output signal.
The design details of the summing and scaling systems 320-324 will
be determined by the specific environment in which the meter system
20 is to be used. Typically, these systems 320-324 will comprise
signal specific components and a summing and scaling amplifier.
The signal specific components convert a raw input signal in either
analog or digital form into a digital or analog conditioned signal
suitable for use by the summing and scaling amplifier associated
with the signal specific components. The summing and scaling amplifier
in turn is designed to generate a scaled signal based on the conditioned
The meter circuit 44 may be implemented using discrete circuit
components, an application specific integrated circuit (ASIC), software
running on an integrated processor such as a general purpose microcomputer
or a digital signal processor, or a combination of these methods.
The exact nature of any given implementation the electrical system
24 will depend upon such factors as manufacturing costs, the designers
background and experience, and the operating environment of the
meter system 20. For example, in an embodiment of the present invention
implemented with a digital signal processor ("DSP"), the
DSP preferably comprises a memory unit having look-up tables that
store calibration conditions including but not limited to the original
calibration conditions for the meter. This data is useful for reference
back to original conditions in the case of pressure and/or temperature
sensor drift. The data is also useful for conducting diagnostic
procedures to determine whether the meter requires calibration or
other service. Additionally, the DSP memory unit preferably has
a look-up table of fluid viscosity vs. temperature for one or more
fluids. This data is useful for use in compensating for changes
in fluid temperature.
Referring now to FIG. 5 depicted therein is one exemplary meter
circuit 44 adapted to generate the flow output signal based on analog
input signals. As shown in FIG. 5 the first summing and scaling
system 320 comprises a signal conditioning module 330 an optional
arithmetic logic unit 332 an optional linearization amplifier 334
and a first and second summing and scaling amplifiers 336 and 338.
The raw pressure signal is initially filtered and amplified by
the signal conditioning module 330. If necessary, the filtered pressure
signal is then applied to one or both of the arithmetic logic unit
332 and linearization amplifier 334 and then to the first summing
and scaling amplifier 336. If the arithmetic logic unit 332 and
linearization amplifier 334 are not required, the filtered pressure
signal is directly passed to the first summing and scaling amplifier
336. The second summing and scaling amplifier 338 generates a calibration
signal based on the calibration factor. The pressure signal and
calibration signal are then applied to the first summing and scaling
amplifier 336 to obtain the processed pressure signal.
The second summing and scaling system 322 comprises a signal conditioning
module 340 a scaling and gain amplifier 342 and a summing and
scaling amplifier 344. The signal conditioning module 340 filters
and amplifies the temperature signal to obtain a filtered temperature
signal. The scaling and gain amplifier 342 generates a gas constant
signal based on the gas constant input. The summing and scaling
amplifier 344 generates the processed temperature signal based on
the filtered temperature signal and the gas constant signal.
The third summing a scaling system 324 comprises a summing and
scaling amplifier 350 and a buffer amplifier 352. The summing and
scaling amplifier generates a flow signal based on the calibrated
pressure signal and the compensated temperature signal as generally
described above. The buffer amplifier 352 generates the flow output
signal based on the flow signal.
IV. Calibration Process
Referring now to FIG. 5 of the drawing, depicted therein at 360
is a flow diagram of one exemplary process for calibrating the meter
system 20 described above. In the following discussion, the particular
meter system 20 being calibrated will be referred to as the DUT.
The first step 362 of the calibration process is to connect the
flow restrictor 30 of the DUT in series with a calibrated meter
system. A negative gauge pressure or vacuum is then applied at step
364 to the outlet of the flow restrictor 30 of the DUT, and the
electronics of the meter system 20 are set to zero.
The next step 366 is to apply pressure upstream of the DUT to create
flow through the DUT. The flow is measured using the calibrated
meter system. The gas specific gas constant input is then applied
at step 368 to the electronic portion 24 using conventional means
such as a digital serial input and/or a set of one or more switches
that can be configured to generate the appropriate gas constant
The maximum flow range is then obtained at step 370 by selecting
an appropriate geometry of the restriction cavity 32 by any one
of the methods described above.
The flow is then decreased at step 372 to ten percent of the maximum
flow setting of the DUT. The pressure and temperature signals associated
with that flow are then read and stored. At step 374 the flow rate
is increased in increments of ten percent up to one hundred percent.
The pressure and temperature signals associated with each incremental
increase in flow rate are measured and stored. The slope of plot
of the pressure signal versus the mass flow rate measured at step
378 by the calibrated meter system is measured and stored as the
calibration factor using conventional means such as a trim pot or
digital serial input.
Referring to FIG. 6 depicted at 380a, 380b, and 380c therein are
exemplary plots of pressure signal versus mass flow rate for several
temperatures. The meter circuit 44 generates the flow signal output
signal based on the pressure/mass flow plots created by the calibration
factor and gas constant input.
Referring now to FIG. 7 depicted therein is a plot 382 of the
pressure signal versus mass flow rate in which the relationship
between the pressure signal and mass flow rate is non-linear. For
example, this relationship may be non-linear in the case of an orifice.
If the pressure/mass flow rate relationship is non-linear, the
filtered pressure signal will be passed through one or both of the
arithmetic logic unit 332 and linearization amplifier 334. The arithmetic
logic unit 332 and linearization amplifier 334 implement a function
that compensates for the non-linearity of the pressure/mass flow
rate relationship. For example, the signal conditioning circuitry
may perform one or both of a "piecewise linearization"
function or a square root function on the filtered pressure signal
to obtain the compensated pressure signal. In particular, referring
back to FIG. 8 depicted at 384 is a curve corresponding to the
inverse of the non-linear curve 382. A curve 386 represents the
midpoint of the curves 382 and 384 and can be used in the linear
slope equation (10) described above.
In practice, the meter circuit 44 is preferably manufactured with
both the arithmetic logic unit 332 and linearization amplifier 334
and, as shown in FIG. 5 switches 390 and 392 configured to allow
either of these circuit elements 332 and 334 to be removed from
the circuit 44. The use of the switches 390 and 392 thus allows
the production of a standard meter circuit 44 that can easily be
customized for a particular environment.
V. Mass Flow Control System
As generally described above, the mass flow meter of the present
invention described above has numerous applications. It can be used
alone simply to measure mass flow rate of a wide variety of fluids
at a wide variety of flow rates. It can be used as part of a larger
system of processing or administering fluids where accurate mass
flow rates are important. It can also be combined with other components
to obtain a more complex stand alone device.
Described in this section with reference to FIG. 9 is an exemplary
mass flow control system 420 that incorporates the exemplary mass
flow meter 20 described above. The mass flow control system 420
is a stand alone device that not only measures mass flow rate but
allows this flow rate to be controlled with a high degree of accuracy
for a wide variety of fluids and flow rates.
The mass flow control system 420 incorporates the flow meter system
20 described above, and the meter portion of the flow control system
420 will not be described again except to the extent necessary for
a complete understanding of the flow control system 420.
In addition to the flow meter system 20 the flow control system
420 comprises a valve control feedback loop system 422 and a flow
controller system 424. The flow controller system 424 is arranged
in series with the flow meter system 20 such that the flow controller
system 424 determines the mass flow of fluid through the flow meter
Preferably, the flow controller system 420 is a mechanical or electro-mechanical
flow controller such as is described in the '849 patent and '708
application cited above. The flow controller system 424 may, however,
be any flow controller system that can increase or decrease the
flow of fluid through the system 420 under electrical or mechanical
In the present invention, the flow signal generated by the summing
and scaling amplifier 350 of the third summing and scaling system
324 is applied to the valve control feedback loop system 422. The
valve control feedback loop system 422 compares the flow signal
with a desired flow rate signal. The desired flow rate signal may
be preset or may be changed as required by the circumstances. For
example, in a medical setting, a doctor may prescribe that a gas
be applied to a patient at a predetermined flow rate. The predetermined
flow rate determined by the doctor would be converted into the desired
flow rate signal.
Based on the difference between the desired flow rate signal and
the flow signal generated by the flow meter system 20 the valve
control feedback loop system 422 generates a flow control signal
that controls the flow controller system 424. If the flow controller
system 424 is a mechanical system, the flow control signal will
be in the form of mechanical movement (rotational, translational)
that operates the flow controller signal to increase or decrease
the fluid flow rate through the system 424. If the system 424 is
an electro-mechanical system, the flow control signal may take the
form of an electrical signal that is converted to mechanical movement
at the system 424.
The combination of the flow controller system 424 and the flow
meter system 20 results in the fluid output of the system 420 being
controllable to a high degree of accuracy.
VI. Alternative Embodiment of Mass Flow Control System
An alternative embodiment of the mass flow control system described
above and in FIG. 9 is shown in FIG. 10. Specifically, the flow
controller 524 comprises either a piezoelectric actuator control
or a solenoid actuator control 526 coupled to a valve 528. The actuator
control 526 delivers a signal 530 to the meter circuit 44. If the
actuator control 526 is a solenoid actuator control then the signal
530 is a current signal is converted to a voltage signal. If the
actuator control 526 is a piezoelectric actuator control, the signal
530 is a voltage signal from an integrated strain gauge. In either
instance, the signal 530 can be identified as V.sub.pm, i.e., Voltage
The signal 530 represents the relationship between the Lorentz
force generated in the actuator control 526 by changes in pressure
in the valve 528. Hence, the signal 530 can be used as an indirect
pressure indicator replacing, augmenting and/or calibrating the
pressure transducer 40 in FIG. 1. For example, in FIG. 10 the pressure
transducer 40 is not present and the signal 530 is used in its stead.
The signal 530 can also be used as a diagnostic indicator to verify
that the value of R for a given flow restriction has not changed.
Preferably, the meter circuit 44 for the mass flow control system
shown in FIG. 10 has at least 128 kilobytes of memory using EEPROM.
The memory for the meter circuit 44 should contain a look up table
of values of V.sub.pm for incremental mass flow rates for various
gases and/or flow restrictions. This look-up table preferably represents
values for the equation: ##EQU6##
Thus, an alternative embodiment using a signal 530 V.sub.pm, to
measure changes in pressure in the system and to control the valve
528 in the mass flow control system is described above.
VII. Additional Considerations
A designer will typically design a particular implementation of
the present invention by initially determining the operating environment
in which the flow meter system is to be used. The operating environment
will include the properties of the fluid itself, the expected range
of fluid input and output pressures, the ambient conditions, the
tolerance for error, and the like. The designer may also consider
commercial factors such as cost.
The properties of many of both the mechanical and electrical components
of the present invention will be changed depending upon the circumstances
to "tune" a specific flow meter system for a particular
For example, the restriction chamber and inlet and outlet openings
may be selected based on the type of fluid, expected inlet pressures,
and desired flow rates.
In addition, the materials used for the various components must
be selected based on the pressures and types of fluids expected.
For example, for air at low pressures, plastic may be used for many
of the components. For caustic fluids and higher pressures, steel
or stainless steel may be used.
The electronics will also be customized for a particular environment.
For example, the implementation details of the various summing and
scaling systems described above will be determined once the particular
operating environment is defined.
Accordingly, the present invention may be embodied in forms other
than those described herein without departing from the spirit or
essential characteristics of the invention. The present embodiments
are therefore to be considered in all respects as illustrative and
not restrictive, the scope the invention being indicated by the
appended claims rather than by the foregoing description; and all
changes which come within the meaning and range of equivalency of
the claims are therefore intended to be embraced therein.