Thermal gas-flow measuring instrument

Abstract

A thermal gas-flow measuring instrument has a first heating resistor placed in the gas to be measured. A temperature sensing resistor is positioned upstream or downstream of the first heating resistor and generates signals relating to the flow rate. Between the first heating resistor and the support for supporting the first heating resistor is a second heating resistor that is electrically insulated from the first heating resistor. The second heating resistor suppresses the heat being transferred from the first heating resistor to the support. A control circuit controls the temperatures of the first heating resistor and the second heating resistor so that the operational temperature range of the second heating resistor or the temperature range at the joint section between the first heating resistor and the second heating resistor is at or above the temperature at which water droplets evaporates to disappear by film boiling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the principle of the present invention.

FIG. 2 shows an output circuit diagram for the detected temperature in an embodiment of the present invention.

FIG. 3 shows a control circuit diagram in an embodiment of the present invention.

FIG. 4 illustrates causes of temperature variations at the joint section of the first heating resistor and the second heating resistor.

FIG. 5 is a graph showing the effect of the present invention.

FIG. 6 illustrates the principle of a comparative example different from the present invention.

FIG. 7 shows a temperature distribution in a range from the heating resistor to the support in the example of FIG. 6.

FIGS. 8A to 8C illustrate the cause of pollution occurrence while heating the heating element.

FIG. 9 is a graph showing the relation between the heating temperature of the heating resistor and the evaporation rate of engine oil.

FIG. 10 is a graph showing the relation between the surface temperature and the life of water droplet until it disappears when it is dropped dropwise onto a high temperature plane.

FIGS. 11A and 11B illustrate an example of the structure of a sensor according to an embodiment of the present invention.

FIGS. 12A and 12B illustrate another example of the structure of a sensor according to an embodiment of the present invention.

FIGS. 13A and 13B illustrate the output error cause which may appear during the progress of the pollution.

FIG. 14 illustrates the effect of suppressing the output error cause during the progress of pollution according to the present invention.

Claims

1. A thermal gas-flow measuring instrument comprising: a heating resistor placed in a gas to be measured;a temperature sensing resistor positioned at upstream side or downstream side of the heating resistor, thus measuring the flow rate of the target gas; andmeans to suppress the heat transfer from the heating resistor to a support, being positioned between the heating resistor and the support for supporting the heating resistor.

2. A thermal gas-flow measuring instrument according to claim 1, wherein the means to suppress the heat transfer is a second heating resistor being positioned between a first heating resistor which is the heating resistor according to claim 1 and the support for supporting the first heating resistor, while the second heating resistor is electrically insulated from the first heating resistor.

3. A thermal gas-flow measuring instrument according to claim 2, wherein the temperature range of the second heating resistor is set to at or above the temperature at which a water droplet contacting with the surface of the first heating resistor is evaporated to disappear by film boiling.

4. A thermal gas-flow measuring instrument according to claim 2, wherein the temperature of the first heating resistor and the temperature of the second heating resistor are set so as the temperature range in a zone between the first heating resistor and the second heating resistor to become at or above the temperature at which a water droplet contacting with the surface of the zone is evaporated to disappear by film boiling.

5. A thermal gas-flow measuring instrument according to claim 2, wherein the temperatures of the first heating resistor and the temperature of the second heating resistor are controlled so as the heat transfer rate among the first heating resistor, the temperature sensing resistor, and the second heating resistor to become a constant value independent of the temperature variations in the gas.

6. A thermal gas-flow measuring instrument according to claim 2, wherein there is applied control so as the heat transfer rate between the first heating resistor and the temperature sensing resistor to be kept constant independent of the temperature variations in the gas, and so as the temperature of the second heating resistor to be kept constant.

7. A thermal gas-flow measuring instrument according to claim 2, further comprising a control means in which the first heating resistor is connected with a first temperature sensing resistor in parallel with each other to form a first bridge circuit, the second heating resistor is connected with a second temperature sensing resistor in parallel with each other to form a second bridge circuit, thereby controlling the first heating resistor to become a first constant temperature, while controlling the second heating resistor to become a second constant temperature.

8. A thermal gas-flow measuring instrument according to claim 2, wherein the first heating resistor is positioned at center of surface of a first cylindrical member, the second heating resistor is positioned at surface of a second cylindrical member which is positioned at each end of surface of the first cylindrical member, while the support of the first heating resistor is connected to near each end of a hollow section of the cylindrical member.

9. A thermal gas-flow measuring instrument according to claim 2, wherein the first heating resistor is positioned at center of surface of a cylindrical member, the second heating resistor is positioned at surface of each end of the cylindrical member, while the support of the first heating resistor is connected to near each end of the cylindrical member.

10. A thermal gas-flow measuring instrument according to claim 8, wherein the first heating resistor and the second heating resistor are protected by insulation with a glass film.