JPH1038652A - Thermal mass flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mass flowmeter which can be manufactured at a low cost and is constituted so that stability and high sensitivity can be secured. SOLUTION: A thermal mass flowmeter measures the mass flow rate of a fluid flowing through a by-pass conduit 10 by using a pair of heating and temperature measuring sensors 26 and 27 installed to a measuring conduit 11 through which the fluid is made to flow by dividing the fluid at a prescribed dividing ratio. Each sensor 26 and 27 is constituted by forming a prescribed resistance pattern on an insulating film backed with a metallic film and the sensors 26 and 27 are stuck to the conduit 11 with an adhesive.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a mass flow rate of a fluid by using a pair of heating and temperature measuring sensors provided in a measuring pipe which divides a fluid such as a gas flowing in a bypass pipe at a predetermined split ratio. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal mass flowmeter for measuring, and more particularly, to a thermal mass flowmeter improved so that it can be manufactured at a low cost and secure stability and high sensitivity.

[0002]

2. Description of the Related Art FIG. 7 is a configuration diagram showing a configuration of a conventional thermal mass flow meter. A measurement pipe 11 that divides the flow rate Q of the gas flowing through the bypass pipe 10 and flows it as a measurement flow rate Q ₁ is fixed to the bypass pipe 10 in a U-shape, and the remaining flow rate Q ₂
Is discharged through the throttle mechanism 12.

[0003] The inner diameter of the measuring pipe 11 is 0.2 mm to
Since the pressure is extremely small as 0.3 mm, a large pressure loss occurs. Therefore, a throttle mechanism 12 is inserted into the bypass pipe 10 in order to keep the branch flow ratio (Q ₁ / Q ₂ ) constant. Balanced with losses.

[0004] A pair of heating and temperature measuring sensors 13 and 14 are wound and fixed in a coil shape on the measuring pipe 11.
These heating temperature sensors 13 and 14 are provided with resistors 15 and 16
And constitute a bridge.

A series circuit of the heating and temperature measuring sensors 13 and 14 and a series circuit of the resistors 15 and 16 are connected in parallel with each other, and these connection terminals serve as a power supply terminal of a bridge. A DC voltage is applied from the power supply 18 via the.

The connection point between the heating and temperature measuring sensors 13 and 14 and the connection end between the resistors 15 and 16 are configured as output terminals 19 and 20 of the bridge. _A voltage difference appears between the heating temperature sensors 13 and 14 due to the mass flow rate ρQ ₁ (ρ: density of fluid) of ₁ .

[0007] The upstream heating temperature sensor 13 is deprived of heat and its temperature is lowered, and conversely, the downstream heating temperature sensor 14 is heated and its temperature rises. At this time, since a certain relationship is established between the temperature difference and the mass flow rate of the fluid,
Using this relationship, the mass flow can be detected by the bridge circuit.

By the way, the heating temperature measuring sensors 13 and 14
Is as shown in FIG. FIG.
Indicates the overall configuration of the heating temperature measurement sensors 13 and 14.
Hereinafter, this will be described.

The upstream heating temperature sensor 13 and the downstream heating temperature sensor 14 are arranged at a predetermined distance L ₁ from each other. The thin wires 22 and 23 functioning as resistors are wound around the wire.

The heating temperature sensors 13 and 14 are connected to the measuring line 1
1 is required to be good and stable, and the electrical characteristics and the thermal characteristics of the upstream heating temperature sensor 13 and the downstream heating temperature sensor 14 are equal.

For this reason, the fine wires 22 and 23 are made of wires having a very small diameter of several μm so that a predetermined resistance value is ensured in a limited space, and furthermore, the fine wires are used to secure the degree of adhesion. 22 and 23 are uniformly wound around the measuring pipe 11 while applying an appropriate tension.

In particular, the non-insulating fine wires 22 and 23 are densely wound while ensuring insulation between the wires, and an insulating layer 21 is provided around the measuring pipe 11 to ensure heat conduction. It is applied thinly and uniformly.

FIG. 9 shows another configuration of a thin wire functioning as a resistor. In this case, the thin wire 25 on which the insulating material 24 is applied is wound directly on the measurement pipeline 11.

[0014]

However, such a thermal mass flow meter as described above is not suitable for making a heating temperature measuring sensor.
The work is difficult since a fine wire of several μm is uniformly wound in a limited space while applying an appropriate tension so as to have a predetermined resistance value. Further, in the case of a non-insulating fine wire, it takes a lot of man-hours to densely wind the wire while ensuring insulation between the wires, and it is not easy to apply the insulating layer thinly and uniformly.

In the case of a type in which a thin wire coated with an insulating material as shown in FIG. 9 is wound directly on a measuring pipe, it is difficult to apply a thin insulating material to the thin wire, and furthermore, a spacefiber is required. There is also the problem that actors get worse.

[0016]

According to the present invention, as a main structure for solving the above-mentioned problems, a pair of measurement pipes provided in a measurement pipe which divides a fluid flowing in a bypass pipe at a predetermined split ratio and flows the same. In a thermal mass flow meter that measures the mass flow rate of a fluid using a heating temperature sensor, a predetermined resistance pattern is formed on an insulating film having a metal film on the back surface as the heating temperature sensor. It is designed to be attached to the measurement pipe via an adhesive.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a longitudinal sectional view showing the configuration of one embodiment of the present invention. Although only the configuration corresponding to the measurement pipeline and the bypass pipeline shown in FIG. 7 is shown, the portions having the same functions are denoted by the same reference numerals.

A measurement pipe 11 which divides the flow rate Q of the gas flowing through the bypass pipe 10 and flows it as the measurement flow rate Q ₁ is fixed to the U-shaped bypass pipe 10, and the flow rate Q of the remaining gas is Q.
₂ flows out through the throttle mechanism 12.

Since a pair of heating temperature measuring sensors 26 and 27 which are supplied with constant electric power and generate heat are provided in the measuring pipe 11, these heating temperature measuring sensors 26 and 27 are provided.
By detecting that the heat flow transmitted from the to the measurement fluid changes according to the flow rate, the flow rate flowing through the measurement pipeline 11 can be detected.

In this case, the heat flow transmitted to the fluid is not the volume flow rate Q ₁ , but is proportional to (ρcQ ₁ ) if the density of the fluid is ρ and the specific heat is c, and if the specific heat c is constant, the mass flow rate is It is proportional to ρQ ₁ . Therefore, if the flow is laminar, the split flow ratio (Q ₁ / Q ₂ ) is constant, and the total mass flow ρQ can be determined by measuring the mass flow ρQ ₁ .

Incidentally, the heating temperature measuring sensors 26 and 27
Is as shown in FIG. FIG.
3 shows an overall configuration of the heating temperature measurement sensors 26 and 27, and FIG. 3 is a perspective view showing a specific configuration of the heating temperature measurement sensors 26 and 27.

[0022] heating temperature measuring sensor 26 upstream, the heating temperature measuring sensor 27 are spaced apart by a predetermined distance L ₁ from each other on the downstream side (FIG. 2). These heating temperature measuring sensors 26
(27) denotes a resistor 26a (27a) and electrodes 26b, 26c (2) connecting these resistors 26a (27a).
7b, 27c) and an insulating film 28 which is a polyimide insulator on which these are mounted (FIG. 3).

The material of the resistor 26a (27a) is nickel having a good temperature coefficient linearity, and is formed in a thin film on the insulating film 28 by a vapor deposition method or the like, and a metal film 29 is formed on the back surface thereof. Have been.

In this case, the heating temperature sensors 26 and 27 are
It is necessary that the heat transfer behaves well and stably, and that the heat conductance between the resistors 26a and 27a of the heating temperature sensors 26 and 27 and the fluid be large and stable.

For this purpose, the insulating film 28 needs to be thin and uniform because of its low thermal conductivity.
Further, it is necessary that the adhesion between the resistors 26a and 27a, the insulating film 28, and the measurement pipe 11 be good and the secular change be small.

In addition, the measuring circuit constitutes each side of the bridge by the resistors 26a and 27a of the heating temperature sensors 26 and 27 and the resistors 15 and 16, but since the bridge needs to keep its balance, the heating measurement is performed. It is necessary that the electrical characteristics and the thermal characteristics of the temperature sensors 26 and 27 are equal to each other and stable.

FIG. 4 is a sectional view showing a state in which the heating temperature measuring sensor 26 shown in FIG. 3 is installed in the measuring pipe 11 made of stainless steel or the like. Since the conductive paste 30 containing a metal filler such as silver is used for installation in the measurement pipe 11, heat conduction is good, and variation in the thickness of the paste has little effect on heat transfer.

Since the thermal conductivity of the polyimide film of the insulating film 28 is about three orders of magnitude smaller than that of a metal, the thickness of the film is preferably as small as possible in terms of thermal response. If the thickness becomes thinner, bending occurs alone and handling becomes difficult.

However, since the metal film 29 is provided on the back surface of the insulating film 28 made of polyimide film,
This rigidity can cover a bend, and the metal film 29 also facilitates bonding with the conductive paste 30.

The material of the resistors 26a and 27a can achieve the object if the temperature coefficient is constant. The pattern of the resistors 26a and 27a can be formed not only by vapor deposition but also by sputtering. Can also be formed.

As described above, as shown in FIGS.
According to the configuration, the resistor 26a (27a) is formed by an evaporation method or the like.
Therefore, the pair of resistors 26a and 27a
Characteristics are small, and the resistors upstream and downstream
Distance L between 26a and 27a ₁Can also be set accurately
it can.

Since a uniform and highly insulating film is used as the insulating film 28, uniform heat transfer characteristics can be obtained and good insulating properties can be ensured. Further, the film is provided on the back surface of the insulating film 28. Since the metal film 29 and the conductive paste 30 containing a filler such as silver are fixed to the measurement pipe 11, the heat transfer characteristics are improved by the adhesion.

FIG. 5 is a block diagram showing another embodiment of the present invention. This embodiment has a configuration in which the aperture mechanism 12 shown in FIG. 1 is omitted. In the configuration shown in FIG. 1, since the inside diameter of the measurement pipe 11 is extremely thin, that is, 0.2 mm to 0.3 mm, and a large pressure loss occurs, the throttle mechanism 12 is provided in the bypass pipe 10 to divide the flow into the measurement pipe 11. .

For this reason, if a small amount of dust is mixed in the fluid, the dust adheres to the gap of the throttle mechanism 12 or the bypass line 10 to block the flow path, and to perform accurate flow rate measurement. Has become a major obstacle. Further, the shape and size of the throttle mechanism 12 that determines the flow dividing ratio are inevitably required to require a large production cost to maintain the accuracy.

Therefore, in the configuration shown in FIG.
2 and the sheet-shaped resistors 26a and 27a manufactured by the printing method shown in FIG.
By using this, the straight pipe method is effectively realized.

In FIG. 5, the ratio between the flow rates Q ₄ and Q ₃ flowing through the bypass line 31 and the measurement line 32 is determined by the pressure loss in each line. Here, the flow rate Q
₃ [Delta] P ₃ a pressure loss occurring in the measuring tube 32 by, when the pressure loss generated in the bypass line 31 and [Delta] P _4, [Delta] P ₃
= ΔP ₄ so that the flow rates Q ₄ and Q ₃ are split and flow.

Assuming that the flow is laminar, the pressure loss is mainly determined by the pipe friction, and λ ₃ (L ₃ / D ₃ ) (V ₃ ²
/ 2g). Here, λ ₃ is the coefficient of friction of the pipe in the measurement pipe 32 λ ₃ = 64 / _Re , L ₃ is the length of the measurement pipe 32, D ₃ is the inner diameter of the measurement pipe 32, and V ₃ is the measurement pipe. 32.

Therefore, the pressure loss occurring in the measuring line 32
Loss ΔP_ThreeIs ΔP_Three∝V_ThreeL_ThreeQ_Three/ D_Three ^FourAnd the measuring pipeline
32 inner diameter D_ThreeIs inversely proportional to the fourth power of. Where the inner diameter D_ThreeTo
If it is 1 mm, it should be 0.25 mm in the case of FIG.
Therefore, in the case of FIG. 5, the pressure loss is (0.25 / 1)^Four≒
Reduce to 0.004.

Therefore, even if the throttle mechanism is not inserted in the bypass pipe line 31 to secure a large pressure loss, it is possible to sufficiently realize the pipe friction alone, and to change the span (change the split ratio). This can be done by changing the inner diameter of the bypass conduit 31.

However, since the inner diameter D ₃ of the measuring pipe 32 is selected to be larger than that in the case shown in FIG. 1, the heat generated by the heating temperature measuring sensors 26 and 27 is difficult to efficiently transmit to the fluid, and the measuring pipe is difficult to measure. The heat capacity of the passage 32 is increased, and there is a possibility that it is not possible to secure a rise in the inner wall temperature of the tube required to obtain a sufficient signal level.

Therefore, the heating temperature measuring sensors 26 and 27 shown in FIGS. 2 and 3 are made of a thin polyimide film in which the insulating film 28 can withstand high temperatures, and the resistors 26a and 27 are used.
a is heated to a high temperature and heat transfer is performed well.

Further, if λ ₀ is the thermal conductivity, S is the contact area with the heating element, L is the length of the heating element, and ΔT is the temperature difference between the heating element and the fluid, the amount of heat conduction q is q = (Λ ₀ S / L) Δ
Since it is indicated by T, the heating elements, that is, the resistors 26a and 26
It is necessary to increase the contact area S between 7a and the measurement pipeline 32 in order to improve the sensitivity.

To this end, a metal film 29 for preventing bending due to thinning of this polyimide film and maintaining good heat conduction is disposed on the back surface of the insulating film 28, and a conductive paste 30 having good heat conduction is used. Measuring pipe 3
2 and closely adhered thereto.

FIG. 6 shows the heating temperature sensor 26 shown in FIG.
It is a longitudinal cross-sectional view which shows the detail of the part of FIG. Measurement pipeline 3
The heating and temperature measurement sensors 26 and 27 are fixed on the top of the heating element 0 by using a conductive paste 30 and are covered with a heat insulating material 33 to prevent heat loss due to convection.

As described above, members having different thermal conductivities (i =
1, 2,...) Overlap, the heat conduction amount q is q = SΔ
Since it is represented by T / [Σ (L _i / λ _i )], the heat conductivity is large and thin to increase the heat flow due to conduction.

[0046]

As described above, according to the first aspect of the present invention, as specifically described with the embodiments of the present invention,
As a pair of heating temperature sensors, a resistance pattern is formed on the insulating film and attached to the measurement pipe, so that variations in the characteristics of the resistance patterns on the upstream and downstream sides can be kept small, and the characteristics are uniform. A thermal mass flowmeter can be obtained.

According to the first aspect of the present invention, the distance between the upstream and downstream resistance patterns can be accurately set as a pair of heating temperature measurement sensors.
A thermal mass flow meter with less variation in sensitivity can be obtained.

Further, according to the first aspect of the present invention, an insulating film having good uniformity is used as a substrate, a metal film is applied to the back surface of the insulating film, and the insulating film is fixed to a measuring pipe via an adhesive. As a result, good heat transfer characteristics can be maintained.

Next, according to the second aspect of the invention, the sensitivity can be expected to be improved by the configuration of the first aspect. Can be easily selected and a wider range of span change can be handled.

According to the third aspect of the present invention, the throttle mechanism is not disposed in the bypass pipe.
It is possible to reduce the trouble of blocking the bypass pipeline due to the adhesion of dust and the like.

According to the fourth aspect of the present invention, nickel is used as the resistance pattern, and the conductive paste is used as the adhesive. Therefore, the linearity with respect to temperature is good and the heat transfer characteristic is good. A thermal mass flow meter can be obtained.

According to the fifth aspect of the invention, since the polyimide is used as the insulating film, high insulation can be ensured and the current flowing through the resistance pattern can be increased. As a result, a high-sensitivity thermal mass flowmeter can be obtained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing one embodiment of the present invention.

FIG. 2 is a perspective view showing the entire configuration of the heating and temperature measuring sensor shown in FIG.

FIG. 3 is a perspective view showing a specific configuration of the heating temperature measuring sensor shown in FIG. 2;

FIG. 4 is a longitudinal sectional view when the heating temperature measuring sensor shown in FIG. 3 is installed in a measuring pipe.

FIG. 5 is a longitudinal sectional view showing another embodiment of the present invention.

FIG. 6 is a longitudinal sectional view when the heating and temperature measuring sensor shown in FIG. 5 is installed in a measurement pipeline.

FIG. 7 is a configuration diagram showing a configuration of a conventional thermal mass flow meter.

8 is a configuration diagram specifically showing the heating temperature measuring sensor shown in FIG. 7;

9 is a configuration diagram showing another configuration of the resistor of the heating temperature measuring sensor shown in FIG. 7;

[Explanation of symbols]

 10, 31 Bypass line 11, 32 Measurement line 12 Throttle mechanism 13, 14, 26, 27 Heating and temperature measuring sensor 21 Insulating layer 26a, 27a Resistor 26b, 26c, 27b, 27c Electrode 28 Insulating film 29 Metal film 30 Conduction Paste 33 Insulation material

Claims (5)

[Claims] 1. A thermal mass flow meter for measuring a mass flow rate of a fluid using a pair of heating temperature measuring sensors provided in a measurement pipe which divides and flows a fluid flowing in a bypass pipe at a predetermined split ratio. A thermal mass flowmeter as the heating temperature measuring sensor, wherein a predetermined resistance pattern is formed on an insulating film having a metal film on a back surface, and the resistance pattern is attached to the measuring pipe via an adhesive. 2. The thermal mass flowmeter according to claim 1, wherein a throttle mechanism is arranged in the bypass line to secure a flow rate flowing through the measurement line to a predetermined value. 3. The thermal mass flowmeter according to claim 1, wherein a flow rate flowing through said measurement pipe is maintained at a predetermined value without disposing a throttle mechanism in said bypass pipe. 4. The thermal mass flowmeter according to claim 1, wherein nickel is used as said resistance pattern and a conductive paste is used as said adhesive. 5. A thermal mass flowmeter according to claim 1, wherein said insulating film is made of polyimide.