JPH0145853B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hot wire flowmeter, and particularly to the structure of a lead wire that connects a hot wire and an electric circuit.

Examples of conventional hot wire flowmeters include those shown in FIGS. 1 to 4.

To explain this, for example, a retaining ring 2 disposed in an intake air passage 1 of an internal combustion engine has four holes formed in its wall, two of which are connected to an electric circuit. A small ring 5 sealed (filled with glass powder and baked) with glass 4 near the tips of the lead wires 3 is fitted and fixed (soldered), and the tips of these lead wires 3 are positioned inside the retaining ring 2. I've let it happen. Furthermore, in place of the lead wires 3, small rings 5 having hooks 6 sealed with glass 4 are fitted and fixed in the other two holes, and these hooks 6 are also located within the retaining ring 2. Inside the retaining ring 2, a hot wire (generally a platinum wire) 7 is stretched in a U-shape by being engaged with two hooks 6 midway between the tips of a pair of lead wires 3.

By connecting the lead wire 3 and the electric circuit, the hot wire 7 is connected to the electric circuit as shown in FIG.
(Rh) and resistors R ₁ to _{R 3} (R ₁ is a temperature compensation resistor, R ₂ is a flow rate adjustment resistor, and R ₃ is a reference resistor) constitute a bridge circuit. Note that R1 ₁ and R1 ₂ in the figure indicate the resistance of the lead wire 3. Further, the current supplied to this bridge circuit is controlled via the differential amplifier 8 and transistors 9 and 10 based on the terminal voltage of the resistor R ₂ and the terminal voltage of the resistor R ₃ . For example, as the flow rate increases, the hot wire Rh
As it cools further, its resistance value decreases, but at this time the terminal voltage of resistor _R3 increases,
The output of the differential amplifier 8 decreases, and the current supplied to the bridge circuit via the transistors 9 and 10 increases.
Keep the resistance value constant {Rh=R ₁・R ₃ /R ₂ −(R1 ₁ +R
1 ₂ )}, and the resistance
The flow velocity is read from the terminal voltage Um of _R3 .

By the way, the lead wire 3 should have such rigidity that no slack will occur in the hot wire 7 after the hot wire 7 is installed.
The material is selected on the condition that it has a coefficient of linear expansion that is almost the same as that of the hot wire 7.

For this reason, conventionally, Fe--Ni alloy or Ni wire has been used as a material that satisfies the above conditions.

However, when these materials are used for the lead wire 3, the resistance value of Fe-Ni is 2.99.
mΩ/mm, resistance temperature coefficient is 3577ppm/℃, resistance value is 0.763mΩ/mm, resistance temperature coefficient is 0.763mΩ/mm for Ni.
4702ppm/℃, which is not only a large resistance value, but also _a large resistance temperature coefficient. , R1 ₂ )
changes greatly, and based on this, the resistance value (Rh) of the hot wire 7, that is, the current supplied to the hot wire 7 changes. The problem was that it took a long time to change and reach a stable state.

The present invention was made in view of such conventional problems, and the part from the part held by the lead wire holding ring to the hot wire mounting part side is made of the same material as in the past, but the part connected to the electric circuit is made of the same material as in the past. are made of a different material, that is, a material whose resistance value changes little with changes in current value, in other words, a material with a small resistance value or a small resistance temperature coefficient, and the parts made of each material are connected together. By using it, stable characteristics can be obtained immediately after the start of energization while supporting the hot wire reliably.

The present invention will be explained below based on an embodiment shown in FIG.

The lead wire 11 on the side where the hot wire is attached is made of the same material as the conventional material, that is, a material that has sufficient rigidity to reliably support the hot wire and has a coefficient of linear expansion almost equal to that of the hot wire, specifically Fe. - It is made of Ni alloy or Ni wire, and is sealed with glass 4 through which the small ring 5 passes. Then, a lead wire 12 on the connection portion side with an electric circuit is connected to this lead wire 11.

The lead wire 12 on the connection side with the electric circuit is made of a material with a low resistance value and temperature coefficient of resistance, such as Cu wire (resistance value 0.068 mΩ/mm, temperature coefficient of resistance 1040 ppm/
℃).

In this way, most of the portion between the hot wire and the electric circuit is constituted by the lead wire 12 having a small resistance value and resistance temperature coefficient, so that the R1 ₁ ,
Not only is R1 ₂ small, but the temperature change therein is also small, so the change in the value of Rh is also small, and the output change after the start of energization is small. Therefore, stable measurement accuracy can be obtained immediately after the start of energization.

Furthermore, since the hot wire attachment portion is constructed of the lead wire 11 having sufficient rigidity, it is possible to prevent the hot wire from sagging.

As explained above, according to the present invention, among the lead wires that connect the hot wire and the electric circuit, the side where the hot wire is attached is made of a relatively rigid material, and electricity flows through the side where the hot wire is connected to the electric circuit. Since it is made of a material that has a small change in resistance value, it is possible to suppress changes in the output characteristics immediately after the start due to the influence of the lead wires and obtain stable characteristics. This has the effect of preventing such problems.

[Brief explanation of drawings]

Fig. 1 is a perspective view of the main parts of a conventional hot wire flowmeter, Fig. 2 is a sectional view of the same as above, Fig. 3 is a perspective view of the lead wire in Fig. 1, and Fig. 4 is a circuit of the hot wire flowmeter. 5 is a diagram showing how the output characteristics change after the start of energization in the conventional case, and FIG. 6 is a perspective view of a lead wire showing an embodiment of the present invention. 1...Intake air passage, 2...Retaining ring, 4...
...Glass, 5...Small ring, 6...Hook, 7,
Rh...Heat wire, 11...Lead wire on the side where the heat wire is attached, 12...Lead wire on the side where it connects to the electric circuit.

Claims (1)

[Claims] 1 A hot wire made by holding a pair of lead wires connected to an electric circuit by penetrating the wall of a retaining ring disposed in a fluid passage, and attaching a hot wire between the pair of lead wires inside the retaining ring. In the flowmeter, the part of the lead wire held by the retaining ring to the hot wire attachment part is made of a relatively rigid material, and the part connected to the electric circuit is made of a relatively rigid material, and the part of the lead wire that is connected to the electric circuit is made of a material that is resistant to changes in current value. A hot wire flowmeter characterized by being made of a material with little change in resistance value.