A flow meter having reference and active thermal sensing elements
and a pressure transducer configured to be placed in a conduit of
flowing fluid. Appropriate computation apparatus is employed to
modify the flow velocity reading resulting from normal comparison
of the thermal sensors in accordance with pressure differences reflected
by the pressure transducer.
What is claimed is:
1. A thermal fluid flow sensor for determining mass flow of a fluid
in a conduit, said conduit having a wall, said sensor comprising:
a flow element having a distal end configured to project through
said conduit wall into the conduit;
a reference sensor on said distal end of said flow element, said
reference sensor having an output signal related in value to the
temperature of the flowing fluid;
an active sensor on said distal end of said flow element, said
active sensor being heated and having an output signal related in
value to the velocity of flow of the fluid, resulting in heat dissipation
from said active sensor;
a pressure sensing element coupled to said flow element, said pressure
sensing element having an output signal related to the relative
pressure in said conduit; and
computation and processing means for modifying the compared output
signals of said reference sensor and said active sensor in accordance
with the relative pressure in said conduit.
2. The sensor recited in claim 1 wherein said pressure sensing
element comprises a pressure transducer on said distal end of said
3. The sensor of claim 1 where said pressure sensor element comprises
a pressure transducer coupled to said flow element.
4. The sensor recited in claim 3 wherein said pressure sensing
element comprises a pressure tap opening on said distal end of said
flow element, said pressure tap communicating the pressure at said
distal end to said pressure transducer.
5. The sensor recited in claim 1 and further comprising a turbulence
inducing element secured to said distal end of said flow element
in a manner to cause consistent turbulent fluid flow on said active
6. A method of sensing mass flow of fluid in a conduit, the method
comprising the steps of:
preparing an insertion sensor having a reference sensor element,
a heated active sensor element and a pressure sensor element therein
and electrical connections;
forming an opening in a side wall of the conduit;
inserting the insertion sensor comprising the sensor elements through
the opening and mounting the sensor to the side wall of the conduit
so that the sensor elements are within the conduit;
determining mass flow of the fluid in the conduit by comparing
the signals from the reference sensor and the active sensor; and
modifying the mass flow value in accordance with the relative pressure
change sensed by the pressure sensor element.
7. A thermal fluid flow meter for determining mass flow of a flowing
fluid, the meter comprising:
a reference sensor having an output signal related in value to
the temperature of the flowing fluid;
a heated active sensor having an output signal related in value
to the velocity of flow of the flowing fluid, with the output signal
of the active sensor being based on heat dissipated from the active
a pressure sensing element having an output signal related to a
pressure of the flowing fluid; and
a computation element structured to determine the mass flow of
the flowing fluid based on the output signals of the reference sensor,
the active sensor and the pressure sensing element.
8. The thermal fluid flow sensor according to claim 7 wherein
the output signal of the pressure sensing element related to a static
pressure of the flowing fluid.
9. The thermal fluid flow sensor according to claim 7 wherein
the output signal of the pressure sensing element related to a relative
pressure of the flowing fluid.
10. The thermal fluid flow sensor according to claim 7 wherein
the computation element is structured to determine an uncorrected
mass flow rate based on the output signals of the reference sensor
and the active sensor and to determine a corrected mass flow rate
by modifying the uncorrected mass flow rate based on the difference
between the output signal of the pressure sensing element and a
reference pressure value.
1. Field of the Invention
This invention relates generally to mass flow sensors and more
particularly to a thermal dispersion mass flow meter employing resistance
temperature detection elements coupled with a pressure transducer
for refining the mass flow determined by the thermal sensors in
accordance with relative pressure changes.
2. Discussion of the Related Art
Thermal dispersion mass flow meters are a common choice for flow
metering devices in the commercial and industrial metering markets.
A typical sensor element for use in such meters is the resistance
temperature detector (RTD), the resistance of which is related to
the temperature of the element itself. A typical sensor employs
at least two RTD elements. One of them is referred to as a reference
element and is normally unheated. The active RTD element is heated
and the effect of mass flow on the heated element provides a measure
of the flow velocity of the substance in the conduit being monitored.
The density of the fluid, normally a gas, flowing across the active
RTD is also a factor in the amount of heat dissipated from that
RTD. In some measurement situations, fluid density will be a constant
so it can easily be accounted for in the system.
Two different methods are commonly used in the thermal dispersion
industry to determine the mass flow in a conduit. One is configured
to maintain a constant temperature differential between the reference
RTD and the active RTD. This method measures the voltage or current
required to maintain the active RTD at a constant temperature above
the reference RTD while heat is removed from the active RTD by way
of the physical properties of the flowing fluid. The other method
measures the voltage difference between the active and the reference
RTD's while the active RTD is heated by a constant current or a
constant power heat source. During this measurement, as with the
other method, the active RTD loses heat by way of the physical properties
of the flowing media.
There are many configurations of dispersion mass flow sensors,
and more particularly, of heated RTD type sensors. An early such
flow detector is shown in U.S. Pat. No. 3366942. This patent discloses
a reference sensor, a heated or active sensor, and a separate heating
element located closely adjacent the heated sensor element. The
basic principal of operation of dispersion flow meters is discussed
in this patent. A different configuration of a three-element thermal
dispersion sensor is shown in U.S. Pat. No. 4899584. There are
many other examples of detectors employing differential temperature
sensors, some having three elements as described in the patents
mentioned above, and some having two elements, where the active
sensor has the heater integral therewith. Even a single element
differential temperature sensor may be employed. The single element
sensor works on a time sharing basis where it acts as a reference
sensor part of the time and is then heated to act as the active
sensor in relatively rapid succession.
Most of the known differential temperature sensors are configured
with the reference and heated sensors arranged in parallel. They
are mounted in the fluid conduit and project into the flow path
as an insertion flow sensor. The sensor elements are positioned
to permit unobstructed flow fluid past both the heated sensor and
the reference sensor in such a way that one does not thermally influence
the other. That means that the reference sensor must indeed be a
reference with respect to the fluid being sensed without influence
from the heat of the heated sensor or the fluid heated by the heated
It has been found that at low flow rates, approximately 1.5 feet
per second (fps) or below, pressure changes within the conduit can
make drastic differences in the mass flow rates determined by a
thermal flow meter. For example, tests have shown that for an actual
flow rate of about 1 fps, an increased pressure of 90 psi can cause
an error in the thermal flow meter reading of more than 50%. Since
there are practical instances where flow rates below 1.5 fps at
elevated pressures must be determined with accuracy, conventional
thermal mass flow meters have generally been insufficient for that
SUMMARY OF THE INVENTION
Broadly speaking, the present invention provides a flow meter employing
a differential temperature sensor configuration with pressure compensation
having high levels of sensor accuracy, sensitivity and repeatability
at very low fluid flow rates.
In one exemplary embodiment, the sensor of this invention comprises
a single opening through the flow element adjacent its distal end
which is positioned within the fluid flow path within the conduit.
A reference sensor element projects into the opening from one wall
and an active sensor element projects toward the reference sensor
element from the opposite wall of the opening. The sensors are located
on the longitudinal axis of the flow element. The opening is placed
in the center of fluid flow in the conduit being monitored in order
to have the most representative fluid flow over the sensor elements.
The pressure transducer is mounted on the distal end of the flow
element. However, a remote opening, or pressure tap, could be used
to conduct the pressure from the vicinity of the sensor elements
to the pressure transducer located outside of the conduit.
In a modification of this exemplary embodiment, a flow conditioner
is employed to induce turbulence in a predictable way to enhance
the accuracy of the readings provided by the active sensor element
of the invention.
In an alternative embodiment, a conventional two element thermal
sensor is modified by adding a pressure transducer in any one of
several appropriate configurations.
The common aspect of the invention is that a thermal flow meter
can be made more sensitive and accurate at low flow rates, where
the pressure in the conduit can vary, by adding a pressure transducer
and modifying the flow rate determined by the thermal flow meter
to compensate for pressure changes from a calibrated or reference
pressure. Appropriate computation and processing is accomplished
so that the fluid flow reading from the meter is highly accurate.
BRIEF DESCRIPTION OF THE DRAWING
The objects, advantages and features of this invention will be
more clearly perceived from the following detailed description,
when read in conjunction with the accompanying drawing, in which:
FIG. 1 shows a plan view of a differential temperature sensor with
a pressure transducer for mass flow metering in accordance with
one embodiment of this invention;
FIG. 1A is an alternative embodiment of the pressure sensor of
the invention of FIG. 1
FIG. 1B is an alternative embodiment of the FIG. 1A pressure sensor;
FIG. 2 is a modified enlarged plan view of the sensor head of FIG.
FIG. 3 shows an alternative embodiment of the invention;
FIG. 4 is a modification of the embodiment of FIG. 3;
FIG. 5 is a basic block diagram of the electronics to which the
sensor of the invention is connected and which provide the signal
processing of the sensor outputs;
FIG. 6 is a more detailed block diagram of the sensor head board
of FIG. 5;
FIG. 7 is a more detailed block diagram of the processor board
of FIG. 5; and
FIG. 8 is a graph showing the relationship of changes in heat dissipation
of the active sensor with pressure changes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference now to the drawing, and more particularly to FIGS.
1 and 2 examples of the invention are shown. Probe or flow element
11 is adapted to be connected in the wall of a conduit by appropriate
means, such as threads, adaptors, or couplers, for example. Wall
14 and electronics housing 15 are shown as an example but their
respective shapes, means of connection and locations may be different.
Wires 16 provide external connections as required for signal processing
and for applying power to heat the active or heated sensor. The
length of flow element 11 from wall 14 to distal end 17 can be any
length for the desired use. At the distal end of flow element 11
is pressure transducer 21. In opening 22 through head 23 are reference
sensor 24 and active sensor 25. It is anticipated that the sensor
probe of FIG. 1 will be used in pipes or conduits with a diameter
as small as two inches, in which case the probe would be just slightly
longer than one inch.
Preferred embodiments, at least under high temperature conditions
which are above appropriate temperatures for proper operation of
pressure transducer 21 are shown in FIGS. 1A and 1B. The actual
pressure transducer is located in housing 15 and a pressure tap,
or static pressure opening, is provided on probe 11. In FIG. 1A,
the pressure tap comprises a tube or conduit 28 leading from opening
29 in distal end 17 of the probe to the transducer in housing 15.
In FIG. 1B, the pressure tap comprises a tube or conduit 30 leading
from opening 37 in opening 22. The basic requirement is that the
pressure tap opening (29 37) be positioned perpendicular to the
fluid flow through the conduit in which probe 11 is mounted. If
it were other than perpendicular the fluid flow would have a direct
influence on the detected pressure.
Head 23 of flow element 11 is shown in greater detail in the modified
version of FIG. 2. Reference sensor 24 is connected in end wall
26 of opening 22 by means of tube or mount 27. Active sensor 25
is connected in wall 31 by means of tube or mount 32 in such a way
that the distance D between ends 33 34 of sensors 24 and 25 ranges
between about 0.30 and 1.00 inch. The preferred embodiment is about
0.30 inch, as will be more fully discussed below. For the embodiment
provided as an example here, the length of sensor elements 24 and
25 extending beyond their respective mounts 27 and 32 is about 0.4
inch. The length of mounts 27 32 is about 0.22 inch. Sensor elements
24 and 25 are preferably on the axis of probe 11 so that if the
probe is not mounted with the direction of primary fluid flow exactly
parallel to the walls of opening 22 through head 23 there will
be no degradation of the accuracy of the signals provided by the
sensors. Because they are on the axis, rotation by as much as five
degrees of the sensor head will not adversely affect the accuracy
of the readings provided by the sensors. It can be appreciated that
if the sensor elements were themselves coaxial but not on the sensor
axis, the sides of opening 22 could act as a baffle and affect the
readings. Alternatively, if the sensor elements were not coaxial,
a slight rotation of head 23 could make a substantial difference
in the flow characteristics of the fluid reaching the sensor elements.
With even a few degrees of rotation, the non-coaxial sensor elements
would be in different positions with respect to the flow direction,
causing inaccuracies in the precision of the readings.
It is preferred that the minimum gap D between ends 33 and 34 of
the sensor elements be at least 0.30 inch because if they were closer,
inaccuracies due to convective heating from the heated element to
the reference element could occur, especially at low flow rates
(in the range of one fps, for example). It is also preferred that
the gap between ends 33 and 34 not exceed about one inch because
a larger difference could result in different flow characteristics
over each element, just based upon the large spacing and the fact
that gas flow is not constant over any appreciable cross-section
of the conduit in which flow occurs. Also for dimensional purposes,
since flow element 11 and head 23 can be mounted in a conduit having
a diameter as small as two inches, the dimensions set forth in this
example can accommodate a conduit of that small size. In a two-inch
conduit, it is important that the center of distance D between probe
ends 33 and 34 be as precisely as possible in the center of the
conduit. This is because the center flow and the equal distance
on either side of that flow which can be sensed by sensor elements
with the dimensions given, is most representative of the gas flow
in the conduit. If the sensor head shown in FIG. 2 is mounted in
a larger conduit, for example, ranging between three and ten inches
in diameter (or across, for non-cylindrical conduits), the position
of the center point between sensor ends 33 and 34 could vary by
about 1% from the center for a three-inch duct and as much as about
5% from center in a ten-inch duct. That is to say, the center of
the gap between the sensor elements should be as close as possible
to the center of the fluid flow conduit but some tolerance is permissible.
Further dimensions for the exemplary embodiment of FIG. 2 are that
opening 22 would be approximately 0.5 inch wide and the distance
between end walls 26 and 31 would be approximately 1.54 inch. Again,
this exemplary set of parameters is for the preferred embodiment
of the invention. However, a larger or possibly somewhat smaller
embodiment of head 23 could be structured but the ratios of the
relative length of the sensor elements and their mounts, together
with the gap between them, and the opening in which they are mounted,
should be relatively consistent with the example given. While opening
22 in head 23 is shown as an elongated oval, the actual shape is
a matter of fabrication convenience. For a relatively large conduit,
it is possible that opening 22 could be round rather than elongated,
because the large conduit could accommodate a larger flow element
head. The important thing is that the diameter of head 23 be as
small as possible, that the distance between opposing top and bottom
walls (with reference to the FIG. 2 orientation) of opening 22 be
at least about 0.5 inch and that end walls 26 and 31 be axially
spaced a sufficient distance to enable sensor elements, their mounts,
their mounting means and the gap between the sensor elements to
exist within the relative dimensions given. The body of flow element
11 for the exemplary embodiment described, is preferably about
0.75 inch in diameter, while head 23 is preferably about 0.875 inch
in diameter. The diameter of sensors 24 and 25 is preferably about
0.050 inch and mounts 27 and 32 are about 0.70 inch in diameter.
The preferred embodiment of the sensor of FIG. 2 includes turbulence
inducer 35 which is positioned substantially normal to the axial
orientation of active sensor 25 and is spaced upstream within opening
22 by at least about 0.30 inch and preferably no more than about
0.50 inch. The turbulence inducer is referred to as a flow conditioner.
If it were positioned any closer to active element 25 than about
0.30 inch, conditioned flow would not be able to be developed by
the time the gas encounters the sensor. On the other hand, if the
turbulence inducer is spaced more than about 0.50 inch upstream
from sensor element 25 it could allow turbulence in the line to
creep around the turbulence inducer, thereby reducing its effectiveness
and permitting non-conditioned flow to encounter the sensor element,
thereby degrading the accuracy of the sensor readings. For a sensor
having the dimensions of the examples given above, turbulence inducer
35 could be a wire and should have a cross-section ranging between
about 0.062 and 0.093 inch.
The turbulence inducer should be positioned as close as possible
to the center of the length of active sensor 25 between mount 32
and end 34. The purpose is to condition the flow of the gas being
sensed so that the turbulence is consistent as it passes over the
active sensor. By centering turbulence inducer 35 on the length
of the active sensor, the flow around the turbulence inducer and
then immediately encountering active sensor 25 will be equal and
constant over the entire length of the active sensor. It has been
found that a turbulence inducer element smaller than about 0.062
inch in cross-section would not create the amount of turbulence
necessary to ensure accurate readings by the sensor. On the other
hand, if the cross-section of the turbulence inducer is greater
than about 0.093 inch it would cause too much turbulence and would
effectively block or shade too great a portion of the length of
sensor 25. Not only would the turbulence created by a larger turbulence
inducer element be greater than desired, it could be inconsistent,
and thereby degrade from the desired precision of the sensor readings.
Stated another way, the ratio of the length of sensor 25 to the
diameter of turbulence inducer 35 should be at least about 4:1.
This ensures that the fluid flow is conditioned while at the same
time a minimal amount of the length of the active sensor is shaded
by the turbulence inducer. For larger conduits to be monitored,
where a larger flow element may be used, all of the above dimensions
could be different, but the relative sizes hold true.
In the FIG. 1 embodiment, pressure transducer 21 may be a conventional
device and can be any suitable pressure sensitive element which
can fit on or in the end of flow element 11 without adversely affecting
the flow characteristics of the fluid in the conduit. The transducer
itself may be mounted in housing 15 as explained previously. It
is preferred that the pressure transducer or pressure tap be located
near the thermal sensor elements so it provides accurate indications
of pressure changes in the same general vicinity as the location
of the thermal sensors. In its simplest form, the pressure transducer
comprises a pressure sensitive resistive element configured as one
leg of a Wheatstone bridge. The output of the bridge is zero at
calibration or reference pressure, and changes either plus or minus,
depending upon whether the pressure increases or decreases from
the reference pressure.
The relative effects of pressure changes on the heat dissipated
by the active sensor are shown in FIG. 8. It can be seen that as
pressure increases, the sensor resistance change decreases, which
needs to be compensated for. That is a primary objective of this
Alternative embodiments of the invention are shown in FIGS. 3 and
4. Electronics housing 41 extends outwardly from the fluid flow
conduit in which flow element 42 is mounted. Head 43 may be threaded
into the wall of the conduit, with hex-shaped portion 44 provided
for employing a wrench to mount the unit to the conduit. Extending
into the fluid flow path are reference sensor element 45 active
sensor element 46 and pressure transducer element 47. A pressure
tap, as shown in FIGS. 1A or 1B, could be used here in place of
transducer 47. The reference sensor would normally include a resistance
temperature detector (RTD) or equivalent element, as would the active
sensor. However, the active sensor would also include some means
to heat the sensor element so that there is heat to be dissipated
by the flowing fluid. In this embodiment the pressure transducer
is mounted on a separate probe.
In the FIG. 4 embodiment, reference sensor 51 and active sensor
53 are essentially the same as the sensor elements in FIG. 3. Pressure
transducer or pressure tap 52 may be mounted on or extend toward
housing 41 from surface 52 from which sensors 51 and 53 extend.
For purposes of completeness, appropriate electronic circuitry
appears in FIGS. 5-7 in block diagram form. This circuitry could
be positioned immediately outside the conduit and connected to the
end of the probe which extends through the wall of the conduit.
Final processing would normally be conducted at a central location.
Alternatively, all of the circuitry shown could be at a central,
FIG. 5 shows a block diagram of the general case system electronics.
Power supply 61 is powered by AC or DC input source power through
line 62. Appropriate power and signals to sensor head board 63 are
provided from the power supply through connection 64 and from the
outputs of sensor 11 of FIG. 2. Input/output 66 from the sensor
board supplies information to processor board 67. Line 65 provides
power to the processor board. Pressure transducer 21 from FIGS.
1 and 2 provides an input to sensor head board 63. Bus 68 represents
an input/output bus to the user's control station, normally a computer.
Bus 69 provides an analog output from the processor for another
aspect of user control. A person of ordinary skill in the signal
processing field would readily understand the functions of these
various blocks and how the sensor signals are to be handled to obtain
the useful information for which the sensor is intended. However,
further breakdown of the blocks of FIG. 5 are provided in FIGS.
6 and 7 in order to further detail some of the ways in which the
sensor signals are processed for useful information. It is to be
understood that the invention primarily relates to the improvements
in the flow element structure itself, with the addition of the pressure
transducer, and its ability to obtain accurate signals representative
of flow characteristics, even at very low flow rates, adjusted for
Sensor head board 63 likely mounted in a housing immediately adjacent
the conduit into which sensor 11 projects, such as housing 15 through
conduit wall 14 of FIG. 1 is shown in more detail in FIG. 6. Sensor
11 has current inputs from reference sensor current source 71 and
active sensor current source 72. The sensor outputs are coupled
to signal conditioner 73 and then to multiplexer (MUX) 74 where
the output of temperature sensor 75 is combined with the conditioned
sensor signals. The temperatures recorded are for the inside of
the electronics housing, and temperatures in the conduit or sensor
itself. The sensor signals are then converted to digital form in
A/D converter 76 and processed in processor 77. EE memory 78 stores
set up and calibration data. The output signal from pressure transducer
21 is also input through MUX 74.
Processor board 67 is shown in more detail in FIG. 7. Processor
77 in board 63 and programmable controller 81 in board 67 are interconnected
by interboard (IB) bus 66. RAM 82 and flash memory 83 provide program
memory for processor 84. Operator control and adjustment IR buttons
85 and real time clock 86 provide appropriate control and time information
to processor 84. LCD 87 is a readout for the sensor readings. Optical
isolators 91 provide safety features for board 67. Serial interface
92 provides the interface between the RS-232 and RS-485 buses and
the rest of board 67. The RS buses couple an external computer to
the sensor electronics for communication therebetween. Analog output
93 provides signals for user control purposes.
The boards described above employ conventional off-the-shelf components.
Their functions are straightforward and well known to those of ordinary
skill in the relevant area of technology.
In view of the description above, it is likely that modifications
and improvements will occur to those skilled in the relevant technical
field. It is to be understood that the invention is limited only
by the spirit and scope of the accompanying claims.