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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a thermal type flowmeter and, more
particularly, to a thermal type flowmeter which has a high resolution of
power in sensing a small amount of flow of a liquid and which is also
capable of sensing a reverse flow.
While a fluid to be measured flows as a laminar or turbulent current
through a stream pipe, a boundary layer of flow is formed along the inner
wall surface of the stream pipe. When the stream pipe is heated by a
heat-sensitive resistance wire wound thereon, the heat is carried over
into the fluid by heat conduction through the stream pipe wall and through
the boundary layer of the fluid's flow. The heat conductivity of the
stream pipe is determined as a variable value depending upon the
composition of its material and the amount of heat transferred through its
surface, and the heat conductivity of the boundary layer of the fluid's
flow is determined as a value relating to the specific amount of heat in
the fluid to be measured. Accordingly, the heat conductivity of the stream
pipe is calculated from such data as its make-up (i.e. the material it is
made of), the pipe's diameter etc. On the other hand, the heat
conductivity of the fluid, is determined by its density and flow rate. The
thermal-type flowmeter can be used as a simple mass flowmeter which, being
based upon the above-mentioned operating principle, can determine the mass
flow of a fluid of a known kind without hindering the fluid from flowing.
A conventional thermal type flowmeter comprises a heat-conductive stream
pipe provided with an upstream side and a downstream side
temperature-sensing element having resistance wires wound round the pipe
at a certain distance there-between to assure that there will be no
thermal influence with each other, and a control means to control a bridge
circuit including a ground point at one end and a connecting point at the
other end of each of the temperature-sensing elements so as to keep the
temperature difference between the upstream side and the downstream side
temperature-sensing elements at a constant level, and which determines a
mass flow of the fluid from the voltage measured at the connecting point
of the downstream side temperature-sensing element.
However the conventional thermal type flowmeter as mentioned above has a
non-linear relationship between the flow rate and the sensor output, which
show a small inclination and an inflection point in the small range of
flow wherein the flow rate approaches zero thereby indicating the
decreased sensitivity of the sensor output in the small range.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a thermal type
flowmeter which is highly sensitive even in the range of a small
measurement of flow and that is capable of obtaining a high sensor output
through the full range of the measurement of flow with no inflection
point.
It is another object of the present invention to provide a thermal type
flowmeter which is possible to detect a reverse flow by sensing a change
in the sensor output signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a construction view of a conventional thermal type flowmeter.
FIG. 2 shows a schematic circuit diagram of the conventional thermal type
flowmeter shown in FIG. 1.
FIG. 3 shows a flow output characteristic curve of a conventional thermal
type flowmeter.
FIG. 4 is a construction view of a thermal type flowmeter embodied in the
present invention.
FIG. 5 is a schematic circuit diagram of the thermal type flowmeter shown
in FIG. 4.
FIG. 6 shows a flow output characteristic curve of a thermal type flowmeter
embodied in the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a construction view of a conventional thermal type flowmeter. In
FIG. 1, the numeral 11 indicates a heat-conductive stream pipe of a small
diameter and usually is made of stainless steel through which a fluid, to
be measured, flows in the direction indicated by an arrow. Said stream
pipe 11 has an upstream side temperature-sensing element 21 and a
downstream side temperature-sensing element 31 which are heat-sensitive
resistance wires S.sub.11 and S.sub.12 made of nickel alloy or platinum.
Both the resistance wires are wound round the stream pipe and spaced at a
certain distance from each other so that no thermal influence may occur
between them. The resistance S.sub.11 of the upstream side
temperature-sensing element 21 has a value about one order larger than
that of the resistance S.sub.12 of the downstream side temperature-sensing
element 21. Therefore, a larger current for heating the fluid is fed to
the downstream side temperature-sensing element 31.
FIG. 2 is a schematic circuit diagram of the above-mentioned conventional
thermal type flowmeter. The resistors S.sub.11 and S.sub.12 are active
resistors forming two arms of a bridge circuit having connecting portions
19, 19.sub.1 and a round end 17.sub.2. The bridge circuit includes further
resistors R.sub.11, R.sub.12 and R.sub.13. The resistor R.sub.13 has a
value for causing the downstream side temperature-sensing element 31 to
have a certain temperature difference in relation to the upstream side
temperature-sensing element 21. The bridge circuit is composed of a
combination of serially connected resistors R.sub.11, R.sub.13, S.sub.11
and serially connected resistors R.sub.12, S.sub.12. The connecting
portion 19 and 19.sub.1 are connected respectively to an inverting input
and a non-inverting input of an amplifier 15. The numeral 16 designates an
NPN type transistor which is connected with its collector to the terminal
17, and at its emitter to the power source side connecting point 17.sub.1
of the bridge circuit and at its base to the output side of the amplifier
15. A sensor signal, proportional to the mass flow rate is formed on the
basis of the voltage at the connecting point 19 and sent out from a
terminal 19.sub.3. The above-mentioned circuit is intended to feed back a
part of the signal to the amplifier 15 and the transistor 16 so that the
potential difference between the connecting portions 19 and 19.sub.1
becomes about zero, thereby the temperature difference between the
upstream side temperature-sensing element 21 and the downstream side
temperature-sensing element 31 may be kept at a constant value
corresponding to the resistor R.sub.13. Since the voltage drop of the
resistor S.sub.12 is proportional to the heat used for heating the fluid's
flow to maintain a constant value to the difference in temperature, the
sensor output at the terminal 19.sub.3 corresponds to the mass flow of the
fluid.
FIG. 3 is a characteristic curve showing a non-linear relationship between
the rate of flow and the sensor output for a conventional thermal type
flowmeter.
Referring to FIG. 3, there is shown a characteristic curve indicating the
rate of flow vs. sensor output relationship, which has a small inclination
and an inflection point in the small range of flow wherein the flow rate
approaches zero thereby indicating the decreased sensitivity of the sensor
output in said range. This can be explained as follows: Since current is
fed to the downstream side temperature-sensing element 31 so as to keep
the temperature difference with respect to the upstream side
temperature-sensing element 21 at a constant value, the heat carried away
by the fluid's flow decreases as the fluid's flow rate decreases,
resulting in rising the temperature at the downstream side and thereby the
heating current passing through the resistor S.sub.12 increases and, at
the same time, the temperature at the upstream side of the stream pipe is
also increased by heat conduction from the downstream side
temperature-sensing element 31. Consequently, the fluid flowing into the
pipe portion, heated by the downstream side temperature-sensing element
31, has been affected by heat conduction and the heating power of the
downstream side temperature-sensing element 31 is decreased resulting in
decreasing of the temperature change of the downstream side
temperature-sensing element 31, due to the heat transfer from said element
31 to the fluid's flow.
In view of the foregoing, the present invention was made to provide a
thermal type flowmeter which is highly sensitive even in the range of a
small measurement of flow and that is capable of obtaining a high sensor
output through the full range of the measurement of flow with no
inflection point, and which comprises a heat-conductive stream pipe
provided with an upstream side and a downstream side temperature-sensing
element having resistance wires wound round the pipe at a certain distance
there-between to assure that there will be no thermal influence with each
other, and a control means to control a bridge circuit including a ground
point at one end and a connecting point at the other end of each of the
temperature-sensing elements so as to keep the temperature difference
between the upstream side and the downstream side temperature-sensing
elements at a constant level, and which will determine a mass flow of the
fluid from the voltage measured at the connecting point of the downstream
side temperature-sensing element and which has an auxiliary heater with an
adjustable temperature and placed on the stream pipe from the downstream
side near the downstream side temperature-sensing element.
FIG. 4 is a construction view of a thermal type flowmeter embodying the
present invention. In FIG. 4, numeral 1 indicates a heat-conductive stream
pipe which usually is a small diameter pipe made of stainless steel,
numeral 2 indicates an upstream side temperature-sensing element which is
a heat-sensitive slender wire made of nickel alloy or platinum acting as a
resistor S.sub.1 and wound round the upstream side of the stream pipe 1.
Numeral 3 designates a downstream side temperature-sensing element which
is a heat-sensitive slender wire made of nickel alloy or platinum acting
as a resistor S.sub.2 and wound round the downstream side of the stream
pipe 1 being separated from the upstream side temperature-sensing element
2 by enough distance to assure that there is no thermal influence on said
upstream side element 2. Numeral 4 is an auxiliary heater formed of a
heat-sensitive slender wire acting as a resistor H wound round the stream
pipe in the downstream side near the downstream side temperature-sensing
element 3. The resistor S.sub.1 of the upstream side temperature-sensing
element 2 is about one order larger than the resistor S.sub.2 of the
downstream side temperature-sensing element 2, therefore most of the
electric current for heating the fluid can flow toward the downstream side
of the temperature-sensing element 3.
Referring now to FIG. 5, there is shown a schematic circuit diagram of the
thermal type flowmeter according to the present invention, in which the
resistors S.sub.1 and S.sub.2 are connected respectively at one end to a
ground point 72 and at the other end to a series of resistors R.sub.3,
R.sub.1 (for resistor S.sub.1) and a resistor R.sub.2 (for resistor
S.sub.1), thereby a bridge circuit is formed, in the arms of which
resistors R.sub.1, R.sub.3 +S.sub.1, R.sub.2 and S.sub.2 are included, and
resistors S.sub.1 and S.sub.2 are used as active resistors. These
resistors serve to give a constant temperature difference to the
downstream side temperature-sensing element 3 in relation to the upstream
side temperature-sensing element 2. An amplifier 5 has two input
connections 9 and 9.sub.1 which represent an inverse input and a
non-inverse input respectively. NPN type transistor 6 has a collector
connected to a power supply terminal 7, an emitter connected to a
connecting point 7.sub.1 of the bridge circuit power source, and a base
connected to the output side of the amplifier 5. A rheostat VR connected
to a heating power supply terminal 8 is used for adjusting the heating
current to be supplied to the auxiliary heating device 4. A sensor output
terminal 10 is provided for sensing a value at the connecting portion 9,
which represents the mass flow being proportional to the electric power
for heating the downstream side temperature-sensing element. As is
apparent from the above-mentioned circuit diagram, a temperature
difference between the upstream side temperature-sensing element 2 and the
downstream side temperature-sensing element 3 is kept at a constant value
corresponding to the resistor R.sub.3 by controlling the transistor in
such a way that the input voltages of the amplifier 5 through the
connecting points 9 and 9.sub.1 of the bridge circuit may be almost equal
to each other, and furthermore when the flow rate is small and near zero,
i.e. in a small range of the flow's measurement, the rheostat may be used
to adjust the heating power of the auxiliary heater 4 so that the sensor
output is decreased to a minimum. Heating the auxiliary heater 4 decreases
the electric power to be supplied to the downstream side
temperature-sensing element 3, i.e. the upstream portion of the downstream
side temperature-sensing element 3 on the stream pipe is not heated
thereby the possibility of the temperature rising at the element 3 due to
heat being transferred from the upstream side at a small rate of flow is
eliminated and on the contrary the temperature of said element 3 is
lowered because of the heat absorption of the flowing fluid. In such a
condition the bridge circuit operates to increase the electric power to be
supplied, thereby increasing the sensor output.
FIG. 6 shows the relationship between the flow and sensor output of a
thermal type flowmeter according to the present invention. The
characteristic curve obtained shows that the output evenly and
continuously (with no inflection point) increases as the flow of the fluid
increases.
By using an auxiliary heater 4, according to the present invention, it
becomes also possible to detect a reverse in the flow of the fluid.
Namely, if the fluid flows in a reverse direction, the heat generated by
the auxiliary heater 4, shown in FIG. 1, is transferred to the downstream
side temperature-sensing element 3, thereby the temperature of said
element 3 rises and the input voltage of the bridge circuit is decreased
by the balancing action of said circuit. Thereby the sensor output is
reduced. The reverse flow of the fluid can be detected by sensing a change
in the sensor output.
As is apparent from the foregoing description, the thermal type flowmeter,
according to the present invention, is adapted to attain an output that is
characteristically continuously increasing through the range from a
smaller to a larger flow, with a remarkable improvement in the small
flow's range. Consequently, the measurement of the flow's range can be
effectively widened and the correction for linearity simplified. In the
thermal type flowmeter, according to the present invention, the adoption
of such simple means as an auxiliary heater of wire wound round a stream
pipe also makes it possible to detect a reverse flow by sensing a change
in the sensor output signal.
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