Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2000-05-22
2003-07-08
Williams, Hezron (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06588268
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention belongs to a fluid flow rate detection technology, and particularly relates to a flow rate sensor and flow rate detecting apparatus for detecting the flow rate of fluid flowing in a pipe line, and to a temperature sensor for detecting temperature of fluid when detecting the flow rate thereof. The flow rate sensor of the present invention is suitably used to accurately measure the flow rate of fluid under various temperature conditions and to make it easy to fabricate the flow rate sensor.
Further, the present invention particularly intends to improve the measurement accuracy of the flow rate sensor, flow rate detecting apparatus and temperature sensor.
(2) Description of Related Art
Various types of sensors have been hitherto used as a flow rate sensor (or flow velocity sensor) for measuring the flow rate (or flow velocity) of various fluid, particularly liquid, and a so-called thermal (particularly indirectly heated type) flow rate sensor is used because the cost can be easily reduced.
A sensor in which a thin-film heating element and a thin-film temperature sensing element are laminated through an insulating layer on a substrate and the substrate is secured to a pipe line is used as an indirectly heated type flow rate sensor, so that the substrate and the fluid in the pipe line are thermally contacted to each other. By passing current through the heating element, the temperature sensing element is heated to vary the electrical characteristic of the temperature sensing element such as the value of the electrical resistance of the temperature sensing element. The electrical resistance value (varied on the basis of the temperature increase of the temperature sensing element) is varied in accordance with the flow rate (flow velocity) of fluid flowing in the pipe line. This is because a part of the heating value of the heating element is transferred through the substrate into the fluid, the heating value diffusing into the fluid is varied in accordance with the flow rate (flow velocity) of the fluid, and the heating value to be supplied to the temperature sensing element is varied in accordance with the variation of the heating value diffusing into the fluid, so that the electrical resistance value of the temperature sensing element is varied. The variation of the electrical resistance value of the temperature sensing element is also varied in accordance with the temperature of the fluid. Therefore, a temperature sensing device for temperature compensation is installed in an electrical circuit for measuring the variation of the electrical resistance value of the temperature sensing element to suppress the variation of the flow-rate measurement value due to the temperature of the fluid at maximum.
An indirectly heated type flow rate sensor using thin film elements as described above is disclosed in JP-08-146026(A), for example.
The conventional indirectly heated type flow rate sensor has a metallic pipe line to be connected external pipe lines. The fluid flows in the pipe line, which is exposed to the outside. Since the metallic pipe line has high thermal conductivity, the temperature variation of the environmental atmosphere is easily transmitted to the fluid in the pipe line, especially to the fluid at the vicinity of inner wall of the pipe line, resulting in lowering the accuracy of detection of the flow rate by the thermal flow rate sensor, especially in case of small amount of flow rate. Such a problem is significant when the difference between the temperature of the fluid flowing through the pipe line and the environmental temperature is great.
The conventional indirectly heated type flow rate sensor is attached to the external pipe line so that the substrate of a flow rate detector or a casing which is thermally connected to the substrate is exposed from the wall surface of the pipe line to the fluid.
For example, the indirectly heated type flow rate sensor disclosed in the above JP-08-146026(A) as the sensor of high thermal response, high measuring accuracy, small size and producibilty with low cost has the following construction:
As shown in
FIGS. 31A and 31B
, a flow rate sensor
501
is composed of a thin film heating element
503
, thin film temperature sensing element
504
laminated via an insulating layer
505
on a substrate
502
, and attached to an appropriate portion of a pipe line
506
as shown in
FIG. 32
in application.
In the flow rate sensor
501
, the temperature sensing element
504
is heated by supplying electric power to the heating element
503
, and the change of the electric resistance value in the temperature sensing element is detected. The flow rate sensor
501
is disposed on the pipe line
506
, and therefore a part of the heating value of the heating element
503
is transferred through the substrate
502
into the fluid flowing through the pipe line. The heating value transferred to the temperature sensing element
504
amounts to the heating value generated by the heating element subtracted with the heating value diffusing into the fluid, which is varied in accordance with the flow rate of the fluid. Therefore, the flow rate of the fluid flowing through the pipe line
506
can be detected by detecting the electrical resistance value of the temperature sensing element which is varied in accordance with the heating value to be supplied to the temperature sensing element
504
.
The dispersing heating value is also varied in accordance with the temperature of the fluid, and therefore as shown in
FIG. 32
, a temperature sensor
507
is arranged on an appropriate portion of the pipe line
506
, and electrical resistance value of the temperature sensing element is also varied in accordance with the temperature of the fluid. Therefore, a temperature sensing device for temperature compensation is installed in an electrical circuit for measuring the variation of the electrical resistance value of the temperature sensing element to suppress the variation of the flow-rate measurement value due to the temperature of the fluid at maximum.
However, since the conventional flow rate sensor
501
is directly connected to the metallic pipe line
506
which is exposed to the outside, the heating value posessed by the fluid is dissipated to the outside or the heating value is supplied to the fluid through the metallic pipe line
506
having high thermal conductivity, resulting in that the detection accuracy of the flow rate sensor
501
is lowered. The influence of such heat dissipation on the detection accuracy of the flow rate sensor is significant when the flow rate of the fluid is very small, and more significant when the specific heat of the fluid is small.
When the fluid is viscous fluid, particularly viscous fluid having relatively high viscosity, particularly liquid, the flow-velocity distribution on the section perpendicular to the flow of the fluid in the pipe line
506
is more remarkable so as to show a parabolic curve having an extreme value at the central portion, that is, the flow-velocity at the central part greatly didders from the flow velocity at the vicinity of the wall of pipe line. In the case of the conventional sensor in which the substrate
502
or the casing
508
connected to the substrate is merely exposed to the fluid at the wall of the pipe line, the flow-velocity distribution has a great effect on the precision of the flow-rate measurement. This is because the flow velocity of the fluid flowing at the center portion on the section of the pipe line is not taken into consideration, but only the flow velocity of the fluid in the neighborhood of the wall of the pipe line is taken into consideration. As described above, the conventional flow rate sensor has such a problem that it is difficult to measure the flow rate of fluid accurately when the fluid is viscous fluid having relatively high viscosity.
Even when fluid has low viscosity at room temperature, it induces a problem connected to the above viscosity problem because the viscosity of the fluid increa
Inoue Shin'ichi
Kawanishi Toshiaki
Koike Atsushi
Tomonari Kenji
Yamagishi Kiyoshi
Baker & Daniels
Mack Corey D.
Mitsui Mining & Smelting Co. Ltd.
Williams Hezron
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