JPS6126823A - Thermal type flow rate detecting sensor - Google Patents

Abstract

PURPOSE:To improve the sensitivety of the sensor and increase its accuracy by setting the both-end distance of a spiral groove formed by laser beam machining to >=90% of the length of an insulating pipe. CONSTITUTION:A temperature dependent resistance film is vapor-deposited on the external surface of the cylindrical insulating pipe 1 made of ceramic including its flanl. Then, the spiral groove 3 is formed uniformly by laser beam machining. In this case, the both-end distance of the spiral groove 3 is set to >=90% of the length of the insulating pipe 1. Consequently, the sensitivity of the sensor is improved, the accuracy is high, and superior responsibility is obtained.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a heat-sensitive flow rate detection sensor that detects a flow rate using heat transfer between a temperature-dependent resistive film and a fluid8 [Prior Art] FIG. 1 is a perspective view showing a conventional heat-sensitive flow rate detection sensor disclosed in, for example, Japanese Patent Publication No. 56-106159. In the figure, (1) is an alumina cylindrical insulation tube; 2) Temperature-dependent resistive film coated on the outer surface of the insulating tube [11, for example, platinum resistive film, +3) LD platinum resistive film (2) Disposed spirally with uniform width using a K laser trimmer. The grooves are formed by laser machining so that no platinum resistive film remains in the grooves, and the heat-sensitive flow rate detection sensor is formed to have a desired resistance value. (4) is an electrode terminal inserted and fixed from both ends of the insulating tube il+, (5) is a glass coated film, +61
This is the lead wire connected to the above-mentioned terminal. Here, the portion where the groove (3) is provided is at least 70 times the length of the insulating tube (1).

The heat-sensitive flow rate detection sensor configured as described above is installed in the flow path so as to be perpendicular to the flow direction.

Next, the operation will be explained. It is generally known that the following relationship exists between the amount of heat transfer between a heating resistor placed in a flow and a fluid and the mass flow rate Q.

x=A(C!l+c2rq)”(TH-Ta)
filHere, A: Area of contact between the heat generating resistor and the fluid TH: Temperature of the heat generating resistor Ta: Temperature of the fluid C1, (!2: Constant Also, the amount of heat generated by the heat generating resistor Pkep=Ru■2f21 Here, , RH The resistance value of the heating resistor is given by the heating current of the heating resistor: When the amount of heat generated by the heating resistor and the amount of heat transfer are in equilibrium, the following relationship holds.

H is detected as a function only of the flow tQvx [In Equation (2), it is desirable that the temperature difference between Tu-Ta, that is, the heating resistor and the surrounding fluid, be constant; such a constant temperature difference measurement method requires a time constant. It has the advantage of being small and is widely used today.

As shown in Fig. 1, the platinum resistive film forming the heat generating resistor has its central part laser-trimmed, so it has a resistance distribution in the direction of the central axis, and the resistance value at the electrode terminal part is at the center s.
Very small compared to Vc. Therefore, the amount of heat generated by the equation (2) is considered to be the amount of heat generated in the trimmed portion, and the contact area between the heating resistor and the fluid given by the equation (1) is equal to the amount of heat generated in the trimmed portion. It can be approximated to the contact area with the fluid.

In conventional heat-sensitive flow rate detection sensors, the distance between both ends of the laser-trimmed groove is 70 inches to 90% of the length of the Me tube, and the effective contact area is small compared to the size of the temperature-dependent resistive film. For example, the structure had a low sensitivity to changes in flow velocity near the city pole.

When there is a sudden increase in flow rate, the heat generated at the trimming part of the temperature-dependent resistive film is directly transferred to the fluid and also flows toward the fluid through the electrode terminal, so the heat capacity of the electrode terminal increases. There is a time delay corresponding to .

Since the conventional heat-sensitive flow rate detection sensor is constructed as described above, the effective contact area with the fluid is small, resulting in drawbacks such as low sensitivity and large time constant.

[Summary of the invention]

This release E! This was done in order to eliminate the drawbacks of the conventional ones as described above, and by making the distance between both ends of the spiral groove created by laser processing at least 90% of the length of the insulating tube, high The present invention provides a 7C heat-sensitive flow rate detection sensor with excellent accuracy and responsiveness.

[Example of invention]

Hereinafter, one embodiment of the present invention will be explained with reference to the drawings.
In the figure, fi+ has an inner diameter of 0.3 mm and an outer diameter of 0.8 mm.
n, a cylindrical insulating tube made of ceramic with a length of 3 mm, (
21Vi A temperature-dependent resistance film made of platinum formed by vapor deposition on the outer surface including the side surface of the insulating tube; (3) is a laser-processed groove with a groove width of several pm;
It is uniformly set in a spiral shape from the other end to the vicinity of the part 1 mm from the other end. (6) is a lead wire made of stainless steel with an outer diameter of 0.2 to 0.3 mm, which is inserted through both openings of the insulating tube (11).

(7) is connected to the lead wire (6) by platinum resistive film +21 I'i by sintering with a conductive paste such as platinum paste.

(5) A glass coated film with a thickness of several 71 m is to be tested on the platinum resistive film (2). The resistance value of such a heating resistor is 10 to 20Ω at room temperature, and the temperature coefficient is 0.35%/°C.

The heat-generating resistor constructed as described above is incorporated in one side of the bridge circuit, a temperature-sensitive resistor of the same type is placed on the other side, and the voltage applied to the bridge is fully controlled so that the bridge balance is maintained.

The flow rate can be measured by keeping the resistance value RH of the heat generating resistor constant, that is, the temperature of the heat generating resistor, and detecting the heating leakage.

Flow rate measurement using such a constant temperature difference method generally has better responsiveness than the constant current method, and the time constant is approximated by the value obtained by dividing the time constant of the temperature sensing element itself by the feedback rate. This approximation is true when there is no thermal insulation between the platinum resistive film and the support, and when Joule heat is uniformly distributed over the entire surface of the platinum resistive film.

FIG. 3 shows the change in heating current over time when there is a sudden increase in flow rate. In the figure, B is the transient characteristic of the conventional product, and B is the transient characteristic of an embodiment of the present invention. In the above two embodiments, the thermal resistance between the platinum resistive film and the lead is sufficiently larger than the thermal resistance with the fluid, and the amount of heat generated is evenly distributed up to the vicinity of the connection between the resistive film and the lead. most of which is transmitted directly in the fluid direction. Therefore, as mentioned above, the time constant is approximated by the value divided by the feedback rate of the control system, and the time delay corresponding to the heat capacity of the untrimmed portion of the resistive film, which was not found in conventional products, can be ignored. In order to obtain the characteristics of response immediately f′ and K
became.

Next, FIG. 4 shows the relationship between the trimming rate (the ratio of the distance between both ends of the groove to the length of the insulating tube) and the time constant when measured by the constant temperature difference method. From the figure, the higher the trimming rate, the more
The time constant becomes small and changes almost linearly when the trimming rate is around 90%. Therefore, the trimming rate of the temperature sensing element is 9
By setting it to 0% or more, the HFC heat-sensitive type flow rate detection device with quick response can become a real problem.

In the above embodiments, all the temperature-dependent resistance films were formed by vapor deposition of platinum, but those coated with a paste containing platinum powder were also used, and nickel and copper were also used. It can be anything.

In the above embodiment, the case of a heat generating resistor was explained, but the same effect as in the above practical example can be obtained also in the case of a temperature sensitive resistor for temperature compensation.

〔Effect of the invention〕

As mentioned above, since the distance between both ends of the spiral groove created by laser processing is 90% or more of the length of the insulating tube, the sensitivity is improved and the accuracy is improved. , and also has the effect of providing excellent responsiveness.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing a conventional heat-sensitive flow rate detection sensor;
FIG. 2 is a perspective view showing a heat-sensitive flow rate detection sensor according to an embodiment of the present invention, FIG. FIG. 4 is a characteristic diagram showing the relationship between the trimming rate and the time constant of a heat-sensitive flow rate detection sensor according to an embodiment of the present invention. il+...Insulating tube, (2)...Temperature-dependent resistance film,
(3) Groove In the drawings, the same reference numeral (B) -1 or equivalent parts are shown.

Claims (1)

[Claims] A temperature-dependent resistive film is formed on the outer surface of the insulating tube, and a uniform spiral groove is created in the temperature-dependent resistive film by laser processing so that the temperature-dependent resistive film does not remain in the groove. A heat-sensitive flow rate detection sensor formed to have a desired resistance value, characterized in that the distance between both ends of the spiral groove created by laser processing is 90% or more of the length of the insulating tube. A heat-sensitive flow rate detection sensor.