JPS60135821A - Heat-sensitive flow detector - Google Patents

Abstract

PURPOSE:To make accurate the temperature compensation for the temperature of a fluid and thereby to enable the accurate detection of a flow rate, by detecting the flow rate of the fluid from an output signal proportional to the quantity of heat in a heating resistor. CONSTITUTION:A heating resistor 3 and a resistor 4 for fluid temperature compensation are disposed in a pipe line 10 through which a fluid flows in the direction of an arrow, resistances 3 and 4 are connected in series to fixed resistances 1 and 5 respectively, the temperature coefficients of the resistances 3 and 4 are made equal to each other, and the value of the resistance 4 is made 50 to 100 times larger than the value of the resistance 3. Moreover, a point of connection between the resistances 1 and 3 and a point of connection between 2 and 5 are connected to the inversion input terminal and the noninversion input terminal of an operational amplifier 8, while the output terminal thereof is connected to the base of a transistor 7 which supplies a current to a bridge circuit. Then, an output voltage of a subtractor 11 and a voltage in the resistor 3 are multiplied by a multiplier 12, and thereby an output voltage 9 is obtained. As a result, the voltage 9 turns to be a function of the rate of the fluid alone, and thus a flow rate can be detected with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a flow rate detector that detects the flow rate of a fluid using heat transfer between a heating element and the fluid.

[Prior art]

Conventional methods include disposing a heating resistor whose electrical resistance is a function of temperature in a flow path (pipeline) and electrically measuring the flow rate from the relationship between the amount of heat generated by the heating resistor and the flow velocity of the fluid. It is commonly used since. Conventionally, there has been a device as shown in FIG. 1 as this type of device.

In the figure, l, 2, and 5 are fixed resistors, 3 is a heating resistor connected in series with resistor l, 4 is a fluid temperature compensation resistor connected in series with resistor 5, and Together, they form a bridge circuit.

The connection point between resistors 1 and 3 becomes an inverting input terminal of operational amplifier 8, and the connection point between resistors 2 and 5 becomes a non-inverting input terminal of operational amplifier 8. Reference numeral 6 denotes a DC power supply, and 7 a transistor for supplying current to the bridge circuit, the base of which is connected to the output terminal of the operational amplifier 8. 9 is the output voltage at the connection point between the transistor 7 and the resistor l. A heating resistor 3 and a fluid temperature compensating resistor 4 are disposed in the conduit 10, and fluid flows in the direction of the arrow in the conduit.

Next, the operation of the above device will be explained.

The heating resistor 3 and the temperature compensating resistor 4 have the same resistance temperature coefficient α, and are each expressed by the following equation.

R,=R,. (1+α'rs) ... (1) R4=
R40(1+αT4) −(2) However, R, Io,
R4° is the resistance value of the heat generating resistor 3 and the fluid temperature compensating resistor 4 at 0°C, and -Ts and T4 are the respective temperatures of the heat generating resistor 3 and the temperature compensating resistor 4.

In addition, the equilibrium condition of the bridge circuit is R3・(Rs + 14) = Rt・R8...(
3), the temperature difference ΔT between the heating resistor 3 and the fluid temperature compensating resistor 4 is calculated from equations (1) to (3) as follows:
). Here, R1・Rso=RI”R
By selecting the circuit constants to be 4°, the above temperature difference ΔT is given by the following equation, ΔT=! 3-=c<c is a constant) (5) αRφ and 4T becomes a constant value.

Incidentally, the following relational expression generally holds between the calorific value Q of the cylindrical heat generating resistor 3 disposed in the flow path (pipe line) and the flow velocity (2) of the fluid.

Q = I” ・Rs= (a 10bV+)ΔT −(
6 However, h is the current flowing through the heating resistor 3.

Further, % a and b are determined by the shape or size of the heating resistor 3 and the physical properties of the fluid, and can be considered to be almost constant within a limited temperature range. Therefore, if the respective temperatures of the heating resistor 3 and the fluid are constant, the flow velocity can be increased by detecting the amount of heat generated.

According to equation (6), the stain construction is given by, and the output voltage 9 is expressed as shown in the following equation.

Conventional heat-sensitive flow rate detectors use the voltage value in the bridge circuit as the output signal, as described above, so even when the fluid flow rate is constant, the temperature of the fluid may change. Changes in the resistance value R directly result in measurement errors.Therefore, there was a drawback that accurate temperature compensation for the measured fluid temperature was impossible to achieve. This was done to eliminate the drawbacks of the conventional device.It detects the voltage proportional to the current flowing through the heating resistor and the voltage between both ends of the heating resistor, and By multiplying the voltage value of the fluid by calculating the fluid flow velocity from a value that is substantially equivalent to the calorific value calculated as follows, it is possible to detect the fluid flow velocity without being affected by temperature changes in the fluid. It is an object of the present invention to provide a heat-sensitive flow rate detector having a certain high degree of accuracy.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 2 shows an embodiment of the present invention.

In the figure, 1 ``'', ``2'' and 5 are fixed resistors, 3 is a heating resistor connected in series with the resistor l,
4 is a fluid temperature compensating resistor connected in series with the resistor 5. The electrical resistance of resistors 3 and 4 is a function of temperature, and the value of resistor 4 is about 50 to 100 times the value of resistor 3,
In addition, the temperature coefficients of both resistances are selected to be the same.

Further, the connection point between the resistors 1 and 3 becomes an inverting input terminal of the operational amplifier 8, and the connection point between the resistors 2 and 5 becomes a non-inverting input terminal of the operational amplifier 8. Reference numeral 6 denotes a DC power supply, and 7 a transistor for supplying current to the bridge circuit.

A heating resistor 3 and a fluid temperature/compensating resistor 4 are disposed within the conduit 10, and fluid flows in the direction of the arrow in the conduit 1O. 11 is a subtracter, and the voltage between both ends of the resistor l appears as an output. 12 is the above subtractor 1
This is a multiplier that multiplies the output voltage at 1 by the voltage at heating resistor 3, and 9 is its output voltage.

In this embodiment, the sum of resistors 2, 5, and 4 is selected to be sufficiently large compared to the sum of resistors 1 and 3. At this time, the equilibrium condition of the bridge circuit is R3・(Rs + R
4) = Rt·R3 (9).

Here, substituting equations (1) 2 and (2) into equation (9),
Temperature difference ΔT between the heating resistor 3 and the fluid temperature compensation resistor 4
When you add , R3Rsol Rs &Rs.

ΔT= (1--)Ts+H"C""-I), -(1
0) R+ R40. Here, by setting each circuit constant such that &Rso=R+R2O is satisfied, the temperature difference ΔT becomes constant, and is expressed by the following.

By the way, the following relational expression generally holds between the calorific value Q of the heat generating resistor 30 disposed in the flow path (pipe line) and the flow velocity V of the fluid.

Q = I'Rs = (a + bV4) △T ・(1, where ■ is the current flowing through the heating resistor 3 and is % a.

, b are determined by the shape or size of the heating resistor 3 and the physical properties of the fluid, and can be considered to be almost constant within a limited temperature range.

According to the α4 formula, the current ■ is, is given by

In the figure, the input impedance of subtracter 11 and multiplier 12 is sufficiently large compared to resistors 1 and 2, and resistor 1
The current flowing through is equal to the above current IK. Therefore, the voltage value at the connection point between resistor l and transistor 7 is (R+
+Rs ) I, and the voltage value at the connection point between the resistor l and the heating resistor 3 is R3I, so the output voltage v1. is, VH=I (R1+Rs)
IRs=I&''Q4).

In the multiplier 2, the voltages R and I are multiplied by the voltage V11, and the output voltage 9 is given by the following equation.

Vo= R1'&'I' = R+ ・(a+b
V') ΔT - (i!19 Here, since the resistance value R5 and the temperature difference ΔT are constant regardless of changes in fluid temperature, the output voltage 9 is a function only of the fluid flow velocity V.

FIG. 3 shows another embodiment of the invention. The heating resistor 3, the fluid temperature compensation resistor 4, and the resistors 1, 2, and 5, which have the same temperature coefficient, constitute a bridge circuit, so that the middle end potential in the bridge circuit is brought to zero.
When the voltage applied to the bridge circuit is controlled by the operational amplifier 8 and the transistor 7, the above equation (13) holds true between the current flowing through the heating resistor 3 and the flow velocity V of the fluid.

In FIG. 3, similarly to FIG. 2, the voltage applied to the bridge circuit and the voltage applied to the heating resistor 3 are outputted from the bridge circuit, converted into digital values by the converter 13, and converted to digital values by the microprocessor 14. (14) 2 and (1) calculations are performed. The calculation results are output as a digital signal to the output signal 9.

〔Effect of the invention〕

As described above, according to the present invention, by detecting the flow rate of the fluid from the output signal proportional to the amount of heat generated by the heating resistor, accurate temperature compensation for the fluid temperature can be realized, and an extremely highly accurate device can be realized. It has the effect of being able to get the benefit of others.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing the circuit of a conventional heat-sensitive flow rate detector.
FIGS. 2 and 3 are circuit diagrams showing circuits of a heat-sensitive flow rate detector according to an embodiment of the present invention. 1.2.5...Fixed resistance, 3...Heating resistor, 4.
... Resistor for fluid temperature compensation, 6... DC power supply, 7...
- Transistor, 8... Operational amplifier, 9... Output signal, IO... Pipe line, 11... Subtractor, 12... Multiplier, 13... A/() converter, 14 ...Mike arithmetic processor. In addition, in the figures, the same reference numerals indicate the same or equivalent parts. Agent Masuo Oiwa Figures 1, 2 and 3

Claims (1)

[Claims] (In a heat-sensitive multiplication flow rate detector consisting of a bridge circuit disposed in a υ pipe and including a heating resistor whose electric resistance is a function of temperature and a fluid temperature compensation resistor, the output from the bridge circuit is A voltage value proportional to the amount of heat generated in the heating resistor calculated from a voltage value applied between both ends of the heating resistor and a voltage value proportional to the current flowing through the heating resistor, or a voltage value substantially equal to the voltage value. A heat-sensitive flow rate detector characterized in that the flow rate is detected by an output signal equivalent to .