Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1999-03-24
2001-05-15
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06230560
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flow measuring device which is used in the technical field of metering such as in a gas meter, flow meter, and so forth, and, in particular, to a heat sensing type flow measuring device and flow measuring method which measures a flow velocity using a change of a resistance value of a heating resistor due to a temperature change resulting from fluid flowing.
2. Description of the Related Art
Recently, a flow meter using a fluidic flow control device is being studied as a next generation gas meter which will substitute for a conventional integrating-type film-type meter. However, the fluidic flow control device cannot measure a low flow rate equal to or less than 150 liters/hour (hereinafter, referred to as ′L/H′). Therefore, as a flow measuring device for a low flow rate range, a flow measuring device referred to as a flow sensor is used which is of a heat sensing type wherein a heating resistor is arranged on a micro bridge and is exposed to a fluid.
As a flow measuring device (flow sensor), various systems have been proposed.
Japanese Utility-Model Publication No.7-51618, Japanese Utility-Model Publication No. 7-117436, and Japanese Laid-Open Patent Application No. 5-312616 disclose flow measuring devices of a system (first system) in which a balanced bridge circuit is used and a heating resistor is driven to a fixed temperature. An example of such a flow measuring device in the prior art will now be described with reference to FIG.
1
. The flow measuring device
1
uses a balanced bridge circuit
2
. The balanced bridge circuit
2
has a resistance thermometer bulb
3
and a heating resistor
4
. These resistors
3
and
4
have the same large temperature coefficient of resistance. A temperature setting resistor
5
which has a small temperature coefficient of resistance is connected to the resistance thermometer bulb
3
. The resistors
3
and
5
form resistance thermometer bulb portion
6
. Another resistor
7
is connected to the resistance thermometer bulb portion
6
, and another resistor
8
is connected to the heating resistor
4
. The resistors
7
and
8
have the same small temperature coefficient of resistance. As a result of the resistors
7
and
8
being connected, the balanced bridge circuit
2
is formed.
The point or node
9
at which the resistors
3
and
4
are connected to one another is grounded. To the point
10
at which the resistors
7
and
8
are connected to one another, a direct-current power source
12
is connected via a control transistor
11
. A differential amplifier
15
is connected to the connection point
13
of the resistance thermometer bulb portion
6
and resistor
7
and the connection point
14
of the resistors
4
and
8
. The differential amplifier
15
is connected to the control transistor
11
so as to provide feedback.
In the flow measuring device
1
, the resistance thermometer bulb
3
and the heating resistor
4
are placed in a flow path, and feedback control is performed so that the balance of the balanced bridge circuit
2
is maintained. In the state in which the balanced bridge circuit
2
balances, power consumed by the heating resistor corresponds to the flow rate of the fluid. Therefore, the flow rate of the fluid can be measured by measuring the output of the differential amplifier
15
and the voltage of the connection point
14
. According to the flow measuring device
1
, feedback control is performed on the balance of the balanced bridge circuit
2
which changes according to change of the flow rate of the fluid. Thereby, the flow rate of the fluid can be measured. However, in the flow measuring device
1
, when the temperature of the fluid changes, an error occurs in the measurement result. Therefore, it is necessary to correct the flow measurement result according to change of the temperature of the fluid.
For this purpose, in a flow measuring device disclosed in Japanese Laid-Open Patent Application No. 4-204119, a resistance thermometer bulb is detachable from a balanced bridge circuit. The resistance thermometer bulb detached from the balanced bridge circuit is driven by a constant current and the voltage drop is measured. From the measurement result, the temperature of the fluid is measured. In a flow measuring device disclosed in Japanese Laid-Open Patent Application No. 5-164583, a heating resistor which is driven by a constant voltage and a resistance thermometer bulb which is driven by a constant current are provided separately. The flow-rate measurement error is corrected as a result of multiplying flow rate data obtained from the heating resistor by fluid temperature data obtained from the resistance thermometer bulb. In a flow measuring device disclosed in Japanese Laid-Open Patent Application No. 2-120621, a resistance thermometer bulb of a pair of resistance thermometer bulbs is placed upstream of a heating resistor and another resistance thermometer bulb of the pair of resistance thermometer bulbs is placed downstream of the heating resistor. A balanced bridge circuit is formed including these resistance thermometer bulbs. Data corresponding to the temperature difference of the pair of resistance thermometer bulbs is detected from a balance change of the balanced bridge circuit. Data corresponding to the heating temperature of the heating resistor is detected from the resistance values of the pair of resistance thermometer bulbs. By dividing the temperature difference data by the heating temperature data, data corresponding to the fluid flow rate is obtained.
A second system is provided by a constant-current driving type system, in which a heating resistor is placed upstream of a flow path and another heating resistor is placed downstream of the flow path. Each of the heating resistors is heated by a constant current. The flow rate of the fluid is measured based on change of the resistance values of the heating resistors.
An example of such a flow measuring device in the prior art disclosed in Japanese Patent Publication No. 3-52028, for example, will now be described with reference to
FIG. 2. A
flow measuring device
21
which will now be described has a first heating resistor
22
and a second heating resistor
23
, each of which has a large temperature coefficient of resistance. These heating resistors
22
and
23
are formed, for example, by printed wiring on a surface of a wiring substrate (not shown in the figure). To the first and second heating resistors
22
and
23
, first and second operational amplifiers
24
and
25
, which act as first and second measuring means, are connected, respectively. Thereby, feedback loops
26
and
27
are formed.
A potentiometer
29
is connected to a pair of input terminals
28
. To the potentiometer
29
, the feedback loops
26
and
27
are connected via resistors
30
and
31
. Thereby, a portion from the input terminals
28
to the feedback loops
26
and
27
acts as first and second power supply means which supply variable-voltage and constant-current power to the first and second heating resistors
22
and
23
. To the feedback loops
26
and
26
, resistors
32
,
33
,
34
and
35
and a third operational amplifier
36
are connected. The third operational amplifier
36
and a ground terminal
37
are connected to a pair of output terminals
38
, respectively.
The flow measuring device
21
can measure the flow rate of a fluid such as gas. In this case, the wiring substrate is appropriately placed in the flow path of the fluid. Thereby, the first and second heating resistors
22
and
23
are positioned in the stated order in the fluid flow direction. When power is supplied to the feedback loops
26
and
27
via the input terminals
28
in this state, variable-voltage and constant-current power is supplied to the first and second heating resistors which are then heated.
When the fluid flows in this state, a heat quantity moves from the upstream first heating resistor
22
to the downstream second heating resistor
23
. Thereby, the res
Fuller Benjamin R.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
Thompson Jewel V.
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