Abstrict A mass flow meter for measuring the flow rate of a fluid in a sensing
tube over a wide ambient temperature range and with low sensitivity
to attitude change. A pair of sensor temperature sensitive resistive
wire coils are wound around the outer surface of the tube along
the flow path of the fluid and are encased in open cell foam material.
The sensor elements are heated and the rate of mass flow of the
fluid is directly proportional to the temperature differential between
the sensor elements. The sensor elements can be connected to a bridge
circuit for detecting the temperature differential of the sensor
elements. Tilting the sensor from the initial balance position results
in a change in meter reading of less than 0.5%.
Claims I claim:
1. A mass flow meter for measuring the flow rate of a fluid flowing
in the interior of a sensing conduit, comprising:
a plurality of self-heating sensor element coils comprising two
coils having adjacent ends positioned along the flow path of said
fluid externally of said sensing conduit one of said sensor elements
being closer to one end of said sensing conduit than another of
said sensor elements, said sensor element coils being formed of
temperaturesensitive resistant wire wound around the outer surface
of said sensing conduit and sensing their own temperature modified
by said fluid flow;
means for heating said sensor elements;
means for detecting the temperature differential of said sensor
elements; and
open cell foam material encasing said sensor elements to provide
reduced attitude sensitivity.
2. A mass flow meter in accordance with claim 1 wherein said foam
material is polystyrene.
3. A mass flow meter in accordance with claim 1 wherein said sensor
elements are formed of a center tapped coil wound around the outer
surface of said sensing conduit, the coil material between one end
of said coil and said center tap constituting one of said sensor
elements and the coil material between the other end of said coil
and said center tap constituting the other of said sensing elements.
4. A mass flow meter in accordance with claim 1 wherein said sensor
elements have substantially identical heat generating and sensing
characteristics.
5. A mass flow meter in accordance with claim 1 and wherein said
detecting means includes a bridge circuit to which said sensor elements
are coupled, said bridge circuit including a constant current source
and providing an output signal of the temperature differential of
said sensor elements relatively free from changes in ambient temperature.
Description FIELD OF THE INVENTION
The field of art to which the invention pertains includes the field
of fluid flow sensing, particularly with respect to method an apparatus
for measuring the mass flow of a fluid.
BACKGROUND AND SUMMARY OF THE INVENTION
In conventional mass flow meters, heat is applied to a sensing
tube conducting the fluid to be measured or is directly applied
to a fluid and the temperature of the fluid is measured before and
after the addition. When the upstream temperature is equal to the
unheated stream temperature, mass flow can be measured as inversely
proportional to the temperature difference for a constant heat addition.
A conventional bridge circuit can be used to obtain an electrical
signal versus flow function. In another arrangement, heat is applied
to a sensing tube and the temperature of the tube measured before
and after the heat addition. The upstream temperature of the fluid
is influenced by the heating of the tube and is nearly equal to
the heater temperature at zero flow. The mass flow of the fluid
is proportional to the temperature differential for a constant heat
addition. In a third arrangement, heat is applied to a very small
wire, probe or thermistor in the fluid stream and the cooling effect
of the fluid stream is measured. Cooling of the element is a function
of the mass flow. In still another arrangement, heat is applied
uniformly to a tube by resistance heating and the cooling effect
of the fluid measured with thermocouples to determine mass flow.
In yet another arrangement, described in detail in my earlier application,
referred to above, a pair of temperature sensitive resistance wire
coils are wound around the outer surface of a sensing tube through
which the fluid flows. The coils are heated and the rate of mass
flow of the fluid, which is directly proportional to the temperature
differential of the coils, is measured by a bridge circuit.
Known prior art includes U.S. Pat. Nos. 946886; 1043983; 1193488;
1218717; 1222494; 1254374; 1261086; 1601513; 2594618;
2832018; 2953022; 2972885; 2994222; 3056295; 3181357;
3229522; 3246523; 3251225; 3433068; 3435676; 3500686;
and 3650151; German patent 876484 and British patent 673143.
A publication of interest is "Variable Resistance Sensors Work
Better With Constant Current Excitation", by C. A. Bowes, Instrumentation
Technology (1966).
The last mentioned arrangement, i.e., using a pair of resistance
coils, provides increased sensor efficiency due to reduced loss
of heat in the sensor elements. The spacing and size of the sensor
elements permits fast response to changes in flow. The sensor elements
sense temperatures produced by their own dissipation, thereby keeping
thermal lag to an absolute minimum.
While such a dual coil arrangement enables the measurement of mass
flow of a fluid over a wide temperature range, satisfactory operation
can be obtained substantially only at the attitude at which the
sensing tube is oriented when the bridge circuit is balanced. With
the sensing tube located even in a still air environment, convective
air currents around the coils carry the generated heat up and away.
As the sensing tube is tilted so that one coil is higher than the
other, heat generated from the lower coil is carried to the higher
coil and raises its resistance which, therefore, unbalances the
bridge circuit. The bridge circuit is balanced at no flow and with
the sensing tube horizontal (or with both coils at the same level
if the tube is bent). Therefore, the bridge circuit output is influenced
by the position or attitude of the sensing tube and is not simply
a function of the flow rate of the fluid.
The present invention provides reduced flow meter sensitivity to
changes in attitude and is accomplished by encasing the sensing
tube coils in an open cell foam material. The result is a minimization
of natural convection around the sensor coils and a reduction in
attitude sensitivity from greater than 50%, without the encasement,
to less than 0.5%.
The advantages of this invention, both as to its construction and
mode of operation, will be readily appreciated as the same becomes
better understood by reference to the following detailed description
when considered in connection with the accompanying drawings in
which like reference numerals designate like parts throughout the
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partially schematic circuit diagram of the system of
the present invention;
FIG. 2 is a detailed, schematic view of a mass flow sensor which
can be used in the present system; and
FIG. 3 is an alternative circuit diagram of which is used to temperature-stabilize
the devices of FIGS. 1 and 2.
DETAILED DESCRIPTION
Referring to FIG. 1 there is shown an exemplary embodiment of
the mass flow measurement system made in accordance with the principles
of the invention. The system comprises a bridge circuit 12 which
is coupled to a sensing tube 14. The bridge circuit is of conventional
design and is formed of a first bridge resistor 16 and a second
bridge resistor 18. The bridge circuit further comprises an upstream
sensor element 22 and a downstream sensor element 24. The sensor
elements 22 and 24 are wound around the sensing tube adjacent each
other, with the upstream sensor element 22 closer to the input end
26 of the tube 14 and the downstream sensor element 24 closer to
the output end 28 of the tube 14.
The bridge circuit 12 also comprises a D.C. power supply 30 which
is connected at one side between the junction of the sensor elements
22 and 24. The other side of the power supply is connected through
a switch 32 to the junction of the bridge resistors 16 and 18. Output
signals from the bridge circuit are coupled from a first output
terminal 34 and a second output terminal 36. The first output terminal
34 is connected to the junction of the upstream sensor element 22
and the first bridge resistor 16 and the second output terminal
36 is connected to the junction of the downstream sensor element
24 and the second bridge resistor 18. The upstream sensor element
22 and the downstream sensor element 24 are formed of temperature-sensitive
resistance wire which is wound around the outer diameter of the
tube 14. Such wire can be an iron-nickel alloy, e.g. Balco (a trademark
of the Wilbur-Driver Company). Preferably, the tube 14 is a thin
walled metal member, e.g. of stainless steel or the like, but other
metals can be used to form the tubes.
In accordance with the present invention, insulator material 37
form-fitted to the tube 14 is applied to the tube 14 encasing the
sensor elements 22 and 24. The insulator material must have low
mass so as to minimize its affect on the response time of the sensor,
and must have low thermal conductivity to avoid inordinate reduction
in sensor gain due to heat loss through the insulating material.
These requirements are uniquely met by a foam material of open cell
construction. Suitable materials comprise polystyrene, polyurethane
foam, foamed polyethylene, foamed polypropylene and the like. For
other materials, reference can be made to the Handbook of Foam Plastics
by Rene J. Bender, Lake Publishing Corporation, Libertyville, Ill.
(1956), Modern Plastics Encyclopedia, McGraw Hill, Inc. (1968) and
the Encyclopedia of Polymer Science and Technology, Interscience
Publishers, New York (1970 ), each being incorporated herein by
reference.
In a particular construction, illustrated in FIG. 2 open cell
beaded polystyrene foam, having a density of about 1.0 lb/ft.sup.3
is cut into mating blocks 37a and 37b, form fitted on mating surfaces
to the sensing tube. The blocks are then glued, taped or clamped
to the sensing tube so as to encase the coils.
In operation, when the switch 32 is closed, current flow through
the sensor elements 22 and 24 causing the sensor elements to generate
heat, thereby raising the temperature of the tube 14 adjacent the
elements. Heating of the elements 22 and 24 also raises their resistance.
At zero fluid flow through the tube 14 the temperatures of the
sensor elements 22 and 24 are equal and the bridge is therefore
balanced, producing a zero output voltage across the terminals 34
and 36. As fluid enters the input end 26 of the tube 14 heat generated
by the elements 22 and 24 is carried by the fluid downstream toward
the output end 28 of the tube 14. Thus, a temperature differential
is created between the elements 22 and 24 due to the shifting temperature
profile along the tube 14. As the flow of fluid increases in the
tube, the temperature of the upstream element 22 as well as its
resistance decreases while simultaneously the temperature of the
downstream element 24 as well as its resistance, increases. The
bridge output voltage at terminals 34 and 36 therefor increases
in nearly linear proportion to the flow rate.
Should the flow further increase to a rate several times that of
the usable full scale range of the system, the upstream sensor element
22 temperature approaches the inlet fluid temperature, while the
downstream sensor element 24 levels off and then begins to fall
with the tube temperature as more and heat is carried away by the
fluid flow. The bridge output voltage therefor reaches a peak, then
reverses and approaches zero asymptotically.
The effect of the foam insulation 37 is demonstrated by tests performed
with and without the insulation. In a horizontal position, where
the two coils 22 and 24 are at the same level, there is no appreciable
difference in bridge output. However, when the sensor is tilted
so that one of the coils is vertically positioned over the other,
without the insulation, an attitude sensitivity in excess of 50%
is observed, whereas with the insulation as described an attitude
sensitivity of less than 0.5% is observed. There is some loss in
sensor gain and response time, generally no more than 20%, but the
general efficiency of the present device permits such reductions
without meaningful loss of usefulness and is more than justified
by the gain in accuracy obtained in attitude varying situations.
To measure flow rates of a higher range than measurable with the
sensor system of FIG. 1 one can use a flow splitting technique
as depicted in FIG. 2. The system of FIG. 2 can establish the same
flow rate through the sensing system of FIG. 1 at a desired full
scale flow. In FIG. 2 a portion of the fluid in a main flow tube
52 is coupled into a secondary sensing tube 54 at an inlet end 56
and is returned to the main flow tube 52 at an outlet end 58. For
accurate measurements, the sensing tube 54 should have the same
characteristics of pressure drop versus flow rate as that of the
main flow tube 52 which can be accomplished by known prior art
flow-splitting techniques. By varying the size and effectiveness
of a pressure drop device 60 in the main flow tube 52 an almost
unlimited number of flow meter ranges can be produced.
In the secondary sensing tube 54 an upstream sensor element 62
and a downstream sensor element 64 corresponds to the upstream and
downstream sensor elements 22 and 24 of FIG. 1. The elements 62
and 64 are illustrated as a single coil containing a center tap
66. By utilizing a single coil with a center tap rather than two
separate sensor elements, it is possible to space the coils close
together. Thus, heat loss is reduced, equalization between the upstream
and downstream sensor elements is facilitated, and the gain of the
circuit, (temperature change per unit of flow) is greater. In addition,
the response of the circuit is faster, and the range of useful flow
measurement and the linearity of the circuit is increased. As above
indicated, mating blocks 37a and 37b of open cell beaded polystyrene
foam are secured over the sensing tube 54 encasing the coils 62
and 64.
The following table provides an illustrative example of various
component values for the embodiment of FIG. 2 when connected in
the bridge circuit of FIG. 1 as well as typical output response
for nitrogen (N.sub.2) flowing in the tube 54.
1. Sensing Tube Material = Type 316 Stainless Steel
2. Sensing Tube Length = 3.0 inches
3. Sensing Tube Diameter = 0.014 inches outer diameter
4. Sensing Tube Wall Thickness = 0.002 inches
5. Sensor Element Resistance = 290 ohms each at 32.degree.F (elements
62 and 64)
6. Sensor Element Material = Balco
7. Sensor Element Length = 0.2 inch per element (0.4 inch overall)
8. Foam Insulation Material = 1.0 lb/ft.sup.3 beaded polystyrene
foam
9. Foam Dimension: 0.80 .times. 0.50 .times. 0.12 each block
10. Sensor Element Current = 10.0 milliamps
11. Bridge Output Signal vs. Flow Rate of Nitrogen from 0 to 5.0
SCCM (Standard Cubic Centimeters/ minute) = 100 to 100 millivolts
12. Linearity of Output Signal over Flow Range = .+-.2 millivolts
13. Variation of Output Signal over an ambient temperature range
of 50.degree. to 150.degree.F = .+-.1.0 millivolts at 5.00 SCCM
of N.sub.2 flow
14. Response time (20 to 100% step change in flow) = 12 seconds
to reach 98% of final value
Should the ambient temperature change, the sensor element resistance
and, thus, the dissipation level also changes. The fluid properties,
heat transfer coefficients and relative temperature differences
are also altered. Therefore, a particular bridge ratio, which depends
upon the temperature coefficient of the sensor element material,
must be used to produce a change in output gain with changing sensor
resistance that will approximately cancel the effects caused by
the change in sensor element temperature and other ambient temperature
dependent variables. This ratio will leave the bridge output unaffected
by changes in ambient temperature and responsive only to the flow
rate of the fluid through the tube. Due to complex difinitive heat
transfer equations, and geometrical configuration, the compensating
bridge ratio is best determined empirically by actual test over
the desired operating temperature range. While other circuits may
be used to produce an electrical output for various flow characteristics,
the bridge circuit depicted with the particular bridge ratio produces
excellent accuracy over wide ambient temperature ranges.
Referring now to FIG. 3 there is shown an alternative arrangement
of the flow measurement system of FIG. 1. In FIG. 3 the bridge
resistor 16 and 18 of FIG. 1 are replaced by a pair of constant
current sources 72 and 74 respectively. Therefore, changes in ambient
temperature will have no effect on the temperature stability of
the bridge circuit of FIG. 3 as the constant current sources provides
an infinite bridge ratio.
Due to the complexity of establishing two separate and equal current
sources in the arrangement of FIG. 3 it should be understood that
by making the resistors 16 and 18 of sufficiently high value when
compared to the resistance of the sensor elements 22 and 24 and
inserting a single constant current source in series with the voltage
source 30 of FIG. 1 the current through the sensor elements would
remain substantially constant. Such an arrangement has the advantage
of requiring only one current source. Thus, in practical applications
of flow measuring systems where the ambient temperature is rarely
held constant and the temperature stability of the system is of
utmost importance in achieving good accuracy and repeatability of
the measurements, the above-mentioned alternative arrangements wherein
a constant current source or sources are provided, will normally
overcome the problems associated with ambient temperature changes.
The present system has been described as measuring the flow rate
of a fluid. It should be understood that the term fluid is meant
to encompass a liquid, a vapor or a gas which is flowing in the
sensor tube. It will be appreciated that by insulating the sensor
coils, as illustrated, the mass flow meter can be mounted in positions
which are not permanent, such as in portable or space vehicle flow
measuring equipment. The user is provided more freedom in designing
his system since he does not have to mount the flowmeter in one
particular position to assure proper calibration of the instrument. |