Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2001-12-20
2004-01-06
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06672154
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a fluid flow rate detecting technique, and particularly to a flowmeter for determining the flow rate or integrated flow rate of fluid such as gas, liquid or the like and a flow rate sensor unit for use in the same, and a flow rate sensor for detecting the flow rate of fluid flowing in a pipe.
BACKGROUND TECHNIQUE
There have been hitherto used various types of flow rate sensors (or flow velocity sensors) for determining the flow rate (or flow velocity) of various kinds of fluid, particularly liquid, and a so-called thermal type (particularly, indirectly heated type) flow rate sensor has been used because the price thereof can be reduced.
There is used an indirectly heated type flow rate sensor in which a sensor chip having a thin film heater and a thin film temperature sensor laminated on a substrate through an insulating film by using the thin film technique is disposed so that heat can be transferred between the sensor chip and fluid in a pipe. By supplying current to the heater, the temperature sensor is heated to vary the electrical characteristic of the temperature sensor, for example, the value of the electrical resistance. The variation of electrical resistance value (based on temperature increase of the temperature sensor) is varied in accordance of the flow rate (flow velocity) of fluid flowing in the pipe. This is because a part of the heating value of the heater is transferred into the fluid, the heating value dispersed into the fluid is varied in accordance with the flow rate (flow velocity) of the fluid, and thus the heating value supplied to the temperature sensor is finally varied in accordance with the flow rate (flow velocity) of the fluid, so that the electrical resistance value of the temperature sensor is varied in accordance with the flow rate (flow velocity) of the fluid. The variation of the electrical resistance value of the temperature sensor is also different in accordance with the temperature of the fluid, and thus a temperature sensor for temperature compensation is installed in an electrical circuit for detecting the variation of the electrical resistance value of the temperature sensor to suppress the variation of the flow rate measurement value due to the temperature of the fluid as much as possible.
For example, an indirectly heated type flow rate sensor using a thin film element disclosed in JP(A)-8-146026, which is estimated to be excellent in thermal response, high in measurement precision, compact in size and low in cost, has the following construction.
That is, as shown in
FIGS. 24A and 24B
, a flow rate sensor
501
has a thin film heater
503
and a thin film temperature sensor
504
which are laminated on a substrate
502
through an insulating layer
505
by using the thin film technique, and it is used while disposed at a proper position of a pipe
506
as shown in FIG.
25
.
In the flow rate sensor
501
, the temperature sensor
504
is heated by supplying current to the heater
503
to detect the variation of the electrical resistance value of the temperature sensor
504
. Since the flow rate sensor
501
is disposed in the pipe
506
, a part of the heating value of the heater
503
is dispersed through the substrate
502
to the fluid flowing in the pipe, and thus the heating value transferred to the temperature sensor
504
corresponds to the value achieved by subtracting the dispersed heating value from the heating value of the heater
503
. Further, since the dispersed heating value is varied in accordance with the flow rate of the fluid, the flow rate of the fluid flowing in the pipe
506
can be determined by detecting the variation of the electrical resistance value of the temperature sensor
504
which varies in accordance with the heating amount being supplied thereto.
Furthermore, since the dispersed heating value is also varied in accordance with temperature, a temperature sensor
507
is disposed at a proper position of the pipe
506
as shown in
FIG. 25
, and a temperature compensating circuit is added in the flow rate detecting circuit for detecting the variation of the electrical resistance value of the temperature sensor
504
to reduce the error of the flow rate measurement value due to the temperature of the fluid at maximum.
However, the conventional flow rate sensor
501
is directly mounted on the metal pipe
506
, and also the metal pipe
506
is exposed to the outside air. Therefore, the heating value of fluid itself is dispersed to the outside air through the metal pipe having high thermal conductivity, or the heating value of the outside air is liable to be supplied to the fluid, resulting in reduction in the measurement precision of the flow rate sensor
501
. Particularly when the flow rate of fluid is very low, it has a great effect on the measurement precision, and thus when the temperature difference between the fluid and the outside air is large or when the specific heat of the fluid is small, the effect is more remarkable.
When the fluid is viscous fluid, particularly viscous fluid having relatively high-viscosity, particularly liquid, the flow velocity in the cross-section perpendicular to the flow direction of the fluid in the pipe
506
is largely different between the portion in the neighborhood of the pipe wall and the center portion, and the flow velocity vector exhibits a substantially parabolic distribution having the extreme value at the center portion. That is, the non-uniformity of the flow velocity distribution is remarkable.
In the case where the substrate
502
or a casing
508
connected to the substrate
502
is merely mounted on the pipe wall and exposed to the fluid to detect the flow velocity at only the portion in the neighborhood of the pipe wall as described above, the flow velocity distribution has a large effect on the precision of the flow rate measurement. This is because no consideration is given to the flow velocity of fluid flowing at the central portion in the cross-section of the pipe and consideration is given to only the flow velocity of fluid flowing in the neighborhood of the pipe wall of the pipe. As described above, when the fluid is viscous fluid having relatively high viscosity, the conventional flow rate sensor has a problem that it is difficult to accurately determine the flow rate. Even when the fluid has low viscosity at normal temperature, the viscosity increases as the temperature is reduced, so that the problem associated with the fluid viscosity as described above occurs. Particularly, the above problem based on the viscosity is more remarkable when the flow rate per unit time is relatively small than when the flow rate per unit time is large.
Further, the flow rate sensor
501
is used under various different environments such as geographical conditions, indoors/outdoors, etc., and various other factors such as season conditions, day
ight, etc. are also added particularly outdoors, so that consideration must be given to temperature variation due to external environments. However, the conventional flow rate sensor
501
is designed to be likely influenced by such external environmental temperature, so that the measurement value of the flow rate has a large error. Therefore, a flow rate sensor that can determine the flow rate with high precision under broad external environmental temperature has been required.
In order to solve this problem, a flow rate sensor as shown in
FIG. 26
has been proposed. This is substantially the same as disclosed in JP(A)-11-118566, for example.
In
FIG. 26
, a flow rate detector
306
having a thin film heater and a thin film temperature sensor which are laminated on a substrate
302
through an insulating layer is mounted on a horizontal portion
307
a
of a fin plate
307
which is bent in an L-shape, thereby forming a flow rate sensor
301
. In a casing
308
, glass
310
is sealingly filled between the vertical portion
307
b
of the fin plate
307
and the opening portion of a flow pipe
309
, and the flow rate detector
306
and the overall horizontal portion
307
a
o
Inoue Shin'ichi
Takahata Takayuki
Tomonari Kenji
Yamagishi Kiyoshi
Lefkowitz Edward
Pitney Hardin Kipp & Szuch LLP
Thompson Jewel
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