JPH01233324A - Mass flowmeter - Google Patents

Abstract

PURPOSE:To measure a mass flow rate of a fluid linearly, by a method wherein a difference between electric resistors wound round on a conduit is adjusted to be proportional precisely to the mass flow rate of the fluid flowing through the conduit. CONSTITUTION:A differential amplifier 20 outputs a potential difference between a potential at the node of a resistance 1 and a resistance 2 and a potential at the node of two resistors A and B. An output E of the differential amplifier 20 is expressed as E=c[(RA-RB)i/2]=c(DELTARi/2) [(c) is an amplification constant] and is proportional to a difference AR in electric resistance between the two resistors A and B. Thus, the output E of the differential amplifier 20 is proportional to the difference DELTAR, and by adjusting a variable resistance Rk of a proportional multiplication circuit 21, an output E+(kE)<2> of a multiplier 22 can be made proportional to flow rate Q. By adjusting the variable resistance Rk beforehand, therefore, the mass flow rate Q of a fluid F flowing through a conduit T can be measured linearly with excellent precision.

Description

[Detailed Description of the Invention] [Industrial Application Field 1] The present invention relates to a flowmeter for measuring the mass flow rate of fluid flowing in a conduit, and in particular to a flowmeter for linearizing a measured electrical signal with respect to the flow rate. This invention relates to a flowmeter with an improved linearization circuit.

[Prior art 1] A flowmeter for measuring the mass flow rate of fluid flowing in a conduit is a system in which two identical resistors whose electrical resistance changes depending on the temperature are connected in series in a conduit through which the fluid flows. Wind it around the
There is a flowmeter that uses a constant current circuit to maintain a constant current through both resistors, measures the difference in electrical resistance between the resistors, and measures the mass flow rate of fluid in a conduit based on this value.

In this flowmeter, as shown in Figure 2, the mass flow rate Q of the fluid is
When is O, the temperature of the upstream resistor A and the temperature of the downstream resistor B are equal, but as the flow rate Q increases, the temperature of the upstream resistor A decreases almost linearly. Later, it asymptotes to the initial temperature of the fluid. The other downstream resistor B
In this case, the temperature of resistor B initially rises due to the inflow of fluid heated by resistor A on the upstream side, but as the flow rate Q increases, the fluid is sufficiently heated by resistor A on the upstream side. Since the temperature of the resistor B is no longer heated, the temperature of the resistor B begins to decrease and then asymptotically approaches the initial temperature of the fluid. Therefore, the electrical resistances of both resistors also show the same behavior as the temperature changes.

Therefore, the difference ΔR=RΔRa between the electrical resistance RA of the upstream resistor A and the electrical resistance RB of the downstream resistor B initially increases, then decreases and becomes 0. Asymptotes to . In other words, in an area where the flow rate Q is relatively small (the area indicated by the dotted line in Figure 3), the difference in electrical resistance ΔR increases with the flow rate Q, so this can be used as a flow meter, but even in this area Since ΔR is not proportional to Q and has an error of 3 to 5% of the full scale, in order to control the flow rate using this flowmeter, a linearization circuit that outputs an output proportional to the flow rate Q must be installed. need to be added.

Conventionally, such a linearization circuit divides the above-mentioned section, that is, the maximum measured flow rate, into several parts, for example, 25.
Set break points at 50.75% and 100%, approximate each divided region by a straight line, and then calculate the flow rate Q over the entire measurement region.
A polygonal line correction was performed to configure the circuit so that the output was proportional to .

[Problems to be Solved by the Invention] Firstly, in the polygonal line correction in the conventional flowmeter described above, even if divided at break points, the difference in electrical resistance Δ within each divided region is
Since R is not a linear function of the flow rate Q, there is still a problem that the error cannot be eliminated.

Second, the more you try to improve the accuracy of polygonal line correction, the more
There was a problem in that the number of breakpoints increased and the circuit configuration became complicated. Thirdly, it is essential to make circuit adjustments to prevent discontinuities in the polygonal lines at each breakpoint, and the more breakpoints there are, the more time and effort it takes to make adjustments.

[Means and effects for solving the problem] In order to solve the above problem, the present inventor has conducted repeated research, and by adjusting the constant as shown in FIG.
We found that it is possible to make it proportional to .

Here, since the correction amount bQ2 is a smaller value than ΔR, and ΔR is approximately proportional to Q, bQ2 can be approximated by aΔR2.

By adjusting the constant a, ΔR+aΔR2 can be made proportional to the flow rate Q.

Therefore, in the present invention, two identical resistors whose electrical resistance changes depending on the temperature are wound in series in a conduit through which a fluid flows, and the current passing through both resistors is kept constant by a constant current circuit. In a flowmeter that measures a difference ΔR in electrical resistance between the two resistors, and measures a mass flow rate of fluid in the conduit based on the difference ΔR in electrical resistance, further This is a mass flowmeter characterized by adding a circuit for counting ΔR+aΔR2 (where a is a constant).

The gist of the present invention is a flowmeter that utilizes the fact that the flow rate Q is proportional to ΔR + aΔR2, and a mere difference in proportionality constant is not a problem. Therefore, for example, when a value proportional to ΔR is E, the flow rate Q A flowmeter configured so that is proportional to E+(kE)2 (k is a constant) will end up with ΔR+ a
This is nothing but counting ΔR2 and falls within the scope of the present invention.

[Example] An example of a flowmeter according to the present invention will be described with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing the same embodiment. In the figure, T is a conduit, and fluid F flows in the conduit T in the direction of the arrow. A is a resistor wound around the outer periphery of the upstream side of the conduit T, and is made of a material with a large temperature coefficient such as an iron-nickel alloy.

B is the same resistor as the resistor A, which is wound around the outer periphery of the conduit T on the downstream side in contact with the resistor A.

Both resistors have a circuit configuration as follows. That is, one end of resistor B is connected to one end of resistor A, and one end of resistor B is connected to one end of resistor A.
A resistor 1 is connected to the other end, a resistor 2 which is the same as the resistor 1 is connected to the other end of the resistor 1, and the other end of the resistor B is connected to the other end of the resistor 2. . One end of a resistor 3 is connected to the connection point between the resistor A and the resistor 1, and the other end of the resistor 3 is grounded.

10 is a constant current circuit in which the potential at one end of the resistor 3 is
is input to the differential amplifier 12, and the differential amplifier 12 compares this potential V with a reference potential defined by the Zener diode 11, and when the former V is lower than the reference potential, turns on the transistor 13 and turns on the resistor B. A current is passed through the connection point between the resistor 2 and the resistor 2, and thus the potential V at one end of the resistor 3 is kept constant.
Therefore, the current i passing through the resistor 3 is kept constant, since the electrical resistance between the resistors 2 and 3 is much larger than the electrical resistances RA and RB between the resistors A and B.
Almost all of the constant current i passing through resistor 3 flows through both resistors A,
The current flowing through B and therefore through both resistors has a constant value i
and the potential at the connection point between resistor B and resistor 2 is V+
(RA+Ra)i.

20 is the potential at the connection point between resistor l and resistor 2, and both resistors A
, B is a differential amplifier that outputs the potential difference with the potential at the connection point of B. Therefore, the potential at the connection point between resistor 1 and resistor 2 is V+(RA+RB)i/2 because both resistors 1.2 are equal, and the potential at the connection point between both resistors A and H is V+RAi. The output E of the differential amplifier 20 is E = c[(RΔ
RB)i/2] = c(ΔRi/2) (C is a constant of the amplifier), and is proportional to the difference ΔR between the electrical resistances of both resistors A and H.

21 and 22 are circuits added according to the present invention, 21 is a constant multiplier circuit including a variable resistor Rk, and 22 is a multiplier which inputs the output E of the differential amplifier 20. The circuit 21 outputs kE, and the multiplier 22 which inputs the output kE and the output E of the differential amplifier 20 outputs the output kE (kE)2.
Output.

This embodiment has the above configuration and operation, and the differential amplifier 20
The output E of the multiplier 22 is proportional to the difference ΔR between the electrical resistances of the resistors A and H, and by adjusting the variable resistance Rk of the constant multiplier circuit 21, the output E)(k, E)2 of the multiplier 22 is proportional to the flow rate Q. Since it can be made proportional, by adjusting the variable resistance Rk in advance, the mass flow rate Q of the fluid F flowing inside the conduit T can be measured linearly with high accuracy.

That is, with the configuration of this embodiment, the linearity of the output E of the differential amplifier 20 with respect to the flow rate Q has a 3 to 5% full scale error, but the linearity of the output of the multiplier 22 with respect to the flow rate Q is to 5% full scale or less.

[Effects of the Invention] With the mass flowmeter according to the present invention, the difference in electrical resistance between the two resistors wound around the conduit can be adjusted to be accurately proportional to the mass flow rate of the fluid flowing inside the conduit. , the mass flow rate of the fluid can be measured linearly.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of a mass flowmeter according to the present invention, FIG. 2 is a diagram showing the relationship between the temperature and electrical resistance of both resistors in the above embodiment, and the flow rate. The figure shows the relationship between the difference in electrical resistance between the two resistors and the flow rate. T... Conduit F... Fluid A, B...
・Resistor 1.2.3...Resistor 10...Constant current circuit 20...Differential amplifier 21...Constant multiplier circuit 22
... Multiplier agent Patent attorney Katsuhiko Inokuma Figure 2 Figure 3

Claims (1)

[Claims] Electrical resistance changes depending on temperature in a conduit through which fluid flows2
Two identical resistors are wound in series, the current passing through both resistors is kept constant by a constant current circuit, and the difference ΔR between the electrical resistances of the two resistors is measured. In a flowmeter that measures the mass flow rate of fluid in the conduit based on the difference ΔR, an additional difference Δ
A mass flowmeter characterized in that a circuit for counting R+aΔR^2 (where a is a constant) is added.