The disclosed thermal mass flow meter has a heat conductive case
having a fine groove and a sensor pipe disposed in said fine groove
so as to carry fluid flow to be measured.
What is claimed is:
1. A thermal mass flow meter, comprising a case whose heat conductivity
is much larger than that of air, a fine groove with an equivalent
diameter of not larger than 4 mm bored through the case, a sensor
pipe disposed in said fine groove so as to carry a fluid flow to
be measured therethrough, and means for supporting said sensor pipe
in said groove in spaced relationship with said case, and a heating
and temperature-sensing means mounted on said sensor pipe.
2. A thermal mass flow meter as set forth in claim 1 wherein said
heating and temperature-sensing means is made of two windings wound
on said sensor pipe and said means for supporting comprises a flange-like
spacer disposed between said two windings.
3. A thermal mass flow meter as set forth in claim 1 wherein said
meter further comprises a thermal insulator stuffed between inner
surface of said groove and said sensor pipe.
4. A thermal mass flow meter as set forth in claim 1 wherein said
heating and temperature-sensing means consists of a pair of windings
which are connected both to a power source and to a voltage-detecting
circuit for heating and detecting the temperature.
5. A thermal mass flow meter as set forth in claim 1 wherein said
sensor pipe is adapted to carry an electric current for heating
and said heating and temperature-sensing means consists of two temperature-sensitive
resistances mounted on said sensor pipe at upstream and downstream
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thermal mass flow meter, and more particularly
the invention relates to an improvement of a thermal mass flow meter
having a sensor pipe defining a passage of gas flow to be measured
and an electric heater heating the sensor pipe, so that the mass
flow rate of the gas in the sensor pipe is determined by measuring
a temperature difference between an upstream point and a downstream
point of the sensor pipe.
2. Description of the Prior Art
A typical thermal mass flow meter of the prior art is schematically
shown in FIG. 1. The gas flow 1 to be measured in a main pipe 2
consists of laminar flow elements 3. The pressure difference across
the upstream and the downstream of the laminar flow elements 3 is
proportional to the volume flow rate Q of the gas flow 1 to be measured.
This pressure difference causes a minor gas flow .DELTA.Q of laminar
type in a sensor pipe 4 which is a branch to the main pipe 2.
Heaters 5 and 6 are wound on the sensor pipe 4. The heaters 5 and
6 are energized by a power source 7 so that the sensor pipe 4 is
heated to a certain temperature, for instance in a range of about
50.degree.-100.degree. C. The heaters 5 and 6 also act as temperature
sensors, and a bridge circuit is formed by the heaters 5 6 and
resistors R.sub.1 and R.sub.2 as shown in the figure. An output
voltage .DELTA.E is produced across the joint between the two heaters
5 6 and the joint between the resistors R.sub.1 R.sub.2. When
the minor flow in the sensor pipe 4 is zero, i.e., .DELTA.Q=0 the
bridge circuit is balanced and its output voltage is also zero,
i.e., .DELTA.E=0. When the minor flow .DELTA.Q assumes a finite
value, the temperature of the upstream heater 5 is reduced while
the temperature of the downstream heater 6 is increased. Accordingly,
the output voltage .DELTA.E also assumes a finite value, which value
is proportional to the flow rate of the gas flowing through the
sensor pipe 4. Since the branching ratio .DELTA.Q/Q is constant,
the mass flow rate in the main pipe 2 can be determined from the
above output voltage .DELTA.E.
The reason why the minor flow .DELTA.Q in the sensor pipe 4 causes
a temperature reduction in the heater 5 and a temperature rise in
the heater 6 is in that the minor flow .DELTA.Q of the gas in the
sensor pipe 4 transmits heat to the downstream. Such transmission
of heat may occur outside the sensor pipe 4. In FIG. 1 if a weak
wind blows from the heater 5 to the heater 6 heat is transmitted
from the left to the right in the outside of the sensor pipe 4
and the heater 5 is cooled while the heater 6 is heated to raise
its temperature, as if a minor flow .DELTA.Q was occurring in the
sensor pipe 4 so as to produce an erroneous output voltage .DELTA.E.
To avoid such erroneous output voltage due to wind, the heater-carrying
portion of the sensor pipe is placed in a sensor housing 8.
Although the sensor housing 8 eliminates the adverse effects of
the wind, it cannot solve the problem of posture error. More particularly,
the initial adjustment of the thermal mass flow meter is effected
while keeping the heaters 5 and 6 horizontally, and if the heater
6 is for instance located at a level higher than the heater 5 during
the usage, heat is convected from the heater 5 to the heater 6
so that a drift in the zero point is caused in the flow meter, as
a posture error. The magnitude of the posture error is maximized
when the heater 6 is positioned immediately above the heater 5 or
when the posture of the flow meter is turned by 90.degree. relative
the horizontal posture. It is undesirable to have a large zero point
drift for a small change of the posture. Especially, in the case
of a flow meter mounted on a car, such zero point drift due to the
posture error is fatal.
As a means to remove the posture error, the Japanese Patent Laying-open
Publication No. 10413/82 teaches the use of an evacuated sensor
housing 8 which encloses the heaters 5 and 6 as shown in FIG. 1.
With the evacuated sensor housing 8 the heat leakage from the heaters
5 and 6 is mostly in the form of radiation to the outside of the
sensor housing 8 and not in the form of convection, so that theoretically
speaking, no posture error can occur. However, such sensor housing
8 must be made airtight even after being evacuated, so that its
structure tends to be costly. Besides, an evacuating step is necessary
during the manufacture. The heat conductivity of air at 1 atmospheric
pressure is kept substantially unchanged even after evacuation unless
the pressure is reduced below 0.01 mmHg, so that the heat conductivity
is unstable unless the vacuum pressure is kept below 0.001 mmHg.
It is very difficult to maintain such a high degree of vacuum for
an extended period of time.
As another means for reducing the posture error due to heat convection,
Japanese Patent Publication No. 23094/81 discloses the use of foamed
thermal insulator 9 such as foamed polyurethane which covers the
heaters 5 and 6 as shown in FIG. 1. The heat from the heaters 5
and 6 leaks to the outside of the thermal insulator 9 by way of
thermal conduction and thermal radiation therethrough. Since thermal
convection has nothing to do with the heat transfer in the thermal
insulator 9 the posture error can be eliminated. Japanese Patent
Laying-open Publication No. 110920/82 discloses a foamed thermal
insulator 9 which also acts as a sensor casing 8.
The method of using the foamed thermal insulator has a shortcoming
in that it has a large time delay in the change of the output voltage
.DELTA.E in response to a step-like change in the minor flow .DELTA.Q.
The reason for the delay in the response is in that the heat conductivity
of the foamed thermal insulator 9 is smaller than that of air. In
short, the thermal capacity of the foamed thermal insulator 9 is
large, so that the delay in the response is caused. A step-like
increase in the minor flow .DELTA.Q will reduce the temperature
of the heater 5 while increasing the temperature of the heater 6
and it takes time before thermal equilibrium is reached at new temperature
distribution. It takes a long time for the temperature change at
the heaters to reach the outer surface of the foamed thermal insulator
9. In general, the foamed thermal insulator 9 made of organic substance
has a low heat resistivity, so that the temperature of the heaters
5 and 6 cannot be raised too high. This is one of the causes for
limiting the sensitivity of the flow sensor or flow meter (output
voltage .DELTA.E/minor flow .DELTA.Q).
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to obviate the
above-mentioned shortcomings of the prior art by providing an improved
thermal mass flow meter.
Another object of the invention is to provide a novel thermal mass
flow meter which has quick response and is free from posture error
caused by change in heat convection at different postures, without
using any vacuum sensor case and any thermal insulator.
To fulfil the above-mentioned object, a thermal mass flow meter
according to the present invention comprises a sensor pipe carrying
heating means mounted thereon, a temperature detector detecting
a temperature difference across said heating means, and a case having
a groove with an equivalent diameter of less than 4 mm, said sensor
pipe being disposed in said groove of the case.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference is made
to the accompanying drawings, in which:
FIG. 1 is a partially cutaway schematic view of a conventional
thermal masss flow meter; and
FIG. 2 is a partially schematic sectional view of an essential
portion of a thermal mass flow meter according to the present invention.
Like parts are designated by like numerals and symbols throughout
different views of the drawings.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 2 showing a preferred embodiment of the invention,
a case 10 has a groove 11 with an inside diameter D, and a sensor
pipe 4 having heaters 5 and 6 wound thereon is disposed in the groove
11. Electricity to the heaters 5 and 6 is applied through relaying
terminals 12 and 13. In the illustrated embodiment, a thin annular
or flange-like spacer 14 is mounted on the sensor pipe 4 at the
joint between the heaters 5 and 6 so as to hold the sensor pipe
4 substantially at the center of the groove 11. The spacer 14 is
made of a material which has a high heat resistivity and a small
heat conductivity, such as polyimide. O rings 15 facilitate the
connection of the sensor pipe 4 to a main pipe 2 (FIG. 1).
The sensor pipe 4 of FIG. 2 branches the flow of fluid being measured
in the main pipe 2 in the same manner as the flow meter of FIG.
1. The heaters 5 and 6 of the flow meter of the invention are also
connected to resistors R.sub.1 and R.sub.2 so as to form a similar
bridge circuit as that of FIG. 1 so as to produce an output voltage
.DELTA.E as described in the foregoing by referring to FIG. 1. Since
the branching of the main pipe 2 to the sensor pipe 4 and the connection
for the bridge circuit do not constitute essential portion of the
invention, details thereof will not be dealt with here.
The inventors have found through experiments that, the smaller
the inside diameter D of the groove 11 is, the less the occurrence
of the convection in the groove 11 is and the less the posture error
becomes. The spacer 14 acts not only as a holder of the sensor pipe
4 in the groove 11 but also as a separator for preventing the heat
transmission between the heaters 5 and 6.
A test model of the mass flow meter of the invention as shown in
FIG. 2 was prepared with the materials and dimensions listed below,
and the following operating characteristics were determined by actual
measurement on the test model.
1. Sensor pipe 4: material, 316 stainless steel; inside diameter
of 0.25 mm, outside diameter of 0.35 mm, a wall thickness of 0.05
2. Heaters 5 6 (temperature-sensitive resistors): Ni(70%)-Fe(30%)
alloy wire with a diameter of 0.01525 mm; temperature coefficient
of about 0.4%; each heater wire being wound with turns in tight
contact over a width of 6 mm, a gap of 0.5 mm between heaters 5
and 6; each heater with a resistance of 360 .OMEGA. at room temperature
3. Power source 7: 13 V; sensor pipe temperature rise of about
94.degree. C. at the spacer 14
4. Output: 0-110 mV for 0-5 SCCM(Standard Cubic Centimeter per
Minute) of N.sub.2 gas, non-linearity error of less than 2%
5. Response: time constant of 2.0 sec, in terms of 63.2% response
time for step-like change in the flow rate
6. Posture error: 0.15% of posture error by 90.degree. turn, with
a groove diameter D=2 mm
The above posture error was determined by carrying out the adjustments
of both the zero point and the measuring span while keeping the
two heaters 5 and 6 on the same horizontal level, turning the sensor
pipe 4 by 90.degree. so as to place the heater 6 immediately above
the heater 5 measuring the shift of the output voltage, and calculating
the posture error being defined as a quotient of the thus measured
shift over the measuring span output. The above value of the posture
error 0.15% does not cause any difficulty for all practical purposes.
The posture error was found to increase with the increase of the
inside diameter D of the groove 11 as shown by the test result
of the following table 1.
TABLE 1 ______________________________________ Groove Groove inside
Spacer Posture error (%) 11 diameter (mm) 14 after 90.degree. turn
______________________________________ none -- none 80 used 6 none
34 used 4 none 7 used 2 none 0.5 used 2 used 0.15 ______________________________________
As can be seen from Table 1 the posture error is rapidly reduced
when the inside diameter D of the groove 11 used becomes smaller
than about 4 mm. The posture error is reduced with the reduction
of the inside diameter D of the groove 11 but when the inside diameter
D is smaller than 1 mm, it becomes difficult to hold the sensor
pipe 4 exactly at the center of the groove 11 and the assembling
operation becomes cumbersome. Accordingly, there is a limit in reducing
the inside diameter of the groove 11.
As to the time constant T of the variation of the output in response
to the step-like change of the minor flow .DELTA.Q in the sensor
pipe 4 there is the following relationship.
H: per-unit-length thermal capacity of the sensor pipe, including
the heaters [JK.sup.-1 m.sup.-1 ],
C: per-unit-length cooling constant of the sensor pipe [JK.sup.-1
m.sup.-1 S.sup.-1 ].
When the foamed thermal insulator is used, the thermal capacity
in the numerator of the above equation increases by an amount corresponding
to that of the thermal insulator, and the time constant increases
accordingly. On the other hand, in the case of the present invention,
only the heat capacity of the air in the groove 1 is added to the
per-unit-length heat capacity H, and the value of this heat capacity
of the air is much smaller than that of the foamed thermal insulator.
In the case of the present invention, the gap between the heaters
5 6 and the case 10 is small, so that considerable heat leakage
to the case 10 occurs through heat conduction. Accordingly, the
cooling constant in the denominator of the equation (1) is large,
and the time constant becomes small. In addition to the heat conduction,
the heat radiation assists the heat leakage to the case 10 so that
it is preferable to roughen and blacken the inner surface of the
groove. As to the material of the case 10 it is preferable to render
as large a heat conductivity as possible. The inventors have found
that a case 10 made by die casting of an aluminium alloy proved
the best, while a case 10 made of shaped plastics resulted in an
It is noted here that the heaters 5 and 6 of FIG. 1 and FIG. 2
act both as heater elements and as temperature sensor elements.
In certain flow meter systems of the prior art, single-purpose heaters
are wound on a sensor pipe for heating it with a constant power
and separate temperature-sensitive resistances are wound on the
sensor pipe at upstream and downstream positions. The present invention
can be used with such systems having separate heaters and temperature
In other flow meter systems, an electric current is directly applied
to the sensor pipe 4 and temperature difference at a central portion
thereof across an upstream position and a downstream position is
measured by a temperature-sensitive resistance means or a thermocouple
means. The present invention can be also directly applied to such
systems which heat the sensor pipe by an electric current therethrough.
In the foregoing, the groove 11 of round shape is implied, but
it is apparent to those skilled in the art that similar effects
can be achieved by using grooves of other shapes such as rectangular
or triangular shape. What is important is to prevent heat convection
by disposing the sensor pipe 4 and the case 10 as close as possible.
As an extreme example, the similar effects as those of the invention
can be achieved by disposing the sensor pipe between parallel boards
which are disposed very close to each other.
To further reduce the heat convection, inorganic thermal insulator
such as quartz fibers may be stuffed very lightly in the sensor
pipe 11. Since the inside diameter of the sensor pipe 11 is very
small to minimize the heat convection, the stuffing of a very small
amount of the thermal insulator therein substantially eliminates
the heat convection. The increase of thermal capacity by the addition
of such thermal insulator is so small that the time constant is
In the above-mentioned embodiment, a constant voltage is implied
for energizing the heaters, but such heating can be effected on
a constant current basis. Although the measurement of gas flow has
been described in the embodiment, the meter of the invention can
be also used for measuring flow rate of liquid.
To facilitate assembly, the case 10 may be formed of two parts
of matching shapes, and such two parts may be assembled while inserting
the sensor pipe 4 in a manner similar to coupling a plug with a
socket in electric appliances.
As described in detail in the foregoing, a thermal mass flow meter
according to the present invention simultaneously achieved the reduction
of the posture error and the improvement of the response by disposing
the sensor pipe in a groove of specific equivalent diameter formed
in a case. Besides, the thermal mass flow meter of the invention
does not use any vacuum element or any massive thermal insulator,
so that it is free from deterioration of the vacuum pressure and
the thermal insulation due to aging, so that a very high reliability
can be provided easily. Thus, the invention contributes greatly
to the industry.