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A heating resistor flow rate measuring instrument, having a heating resistor and a thermally sensitive resistor disposed in a sub-passage of a main passage. The heating resistor is disposed in a portion of the sub-passage which has an upstream-side opening substantially perpendicular to the forward

A heating resistor flow rate measuring instrument, having a heating resistor and a thermally sensitive resistor disposed in a sub-passage of a main passage. The heating resistor is disposed in a portion of the sub-passage which has an upstream-side opening substantially perpendicular to the forward direction of a fluid flow and a downstream-side opening substantially parallel to the reverse flaw direction, and the thermally sensitive resistor is disposed in a second sub-passage portion having an upstream-side opening substantially parallel to the forward flow direction and a downstream-side opening substantially perpendicular to the reverse flow direction. Based on changes in resistor temperatures when the flow direction reverses, a precise flow rate may be determined even when a pulsation occurs in the fluid, as both a plus effect caused by a backward flow is eliminated and a value corresponding to the backward flow may be subtracted to obtain accurate flow information.

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The invention claimed is: 1. A heating resistor flow rate measuring instrument comprising a heating resistor and a thermally sensitive resistor both disposed in a main passage, and measuring the flow rate of a fluid passing through said main passage, said instrument including: a first sub-passage e

The invention claimed is: 1. A heating resistor flow rate measuring instrument comprising a heating resistor and a thermally sensitive resistor both disposed in a main passage, and measuring the flow rate of a fluid passing through said main passage, said instrument including: a first sub-passage exposed to the fluid flowing in one direction within said main passage in a larger amount than the fluid flowing in a direction opposite to the one direction; and a second sub-passage exposed to the fluid flowing in the opposite direction within said main passage in a larger amount than the fluid flowing in the one direction, wherein said heating resistor is disposed at said first sub-passage, said thermally sensitive resistor is disposed at said second sub-passage, the flow rate of the fluid passing through said main passage is measured based on amounts of heat radiated from said heating resistor and said thermally sensitive resistor, and the shape of said first sub-passage is different from that of said second sub-passage. 2. The heating resistor flow rate measuring instrument according to claim 1, wherein said heating resistor is heated to be higher than a fluid temperature in said main passage by a first predetermined temperature, and said thermally sensitive resistor is heated to be higher than the fluid temperature in said main passage by a second predetermined temperature. 3. The heating resistor flow rate measuring instrument according to claim 1 or 2, wherein said first sub-passage has a first opening that faces substantially perpendicular to the fluid flowing in the one direction, and a second opening that faces substantially parallel to the fluid flowing in the opposite direction, and said second sub-passage has a third opening that faces substantially perpendicular to the fluid flowing in the opposite direction, and a fourth opening that faces substantially parallel to the fluid flowing in the one direction. 4. The heating resistor flow rate measuring instrument according to claim 1 or 2, wherein said first sub-passage has a first opening that faces substantially perpendicular to the fluid flowing in the one direction, and a second opening that faces substantially parallel to the fluid flowing in the opposite direction, and a wall portion having a surface substantially perpendicular to the lengthwise direction of said main passage is formed in the one-direction side of said second sub-passage. 5. The heating resistor flow rate measuring instrument according to claim 1 or 2, wherein said first location is provided by a first sub-passage having a first opening that faces substantially perpendicular to the fluid flowing in the one direction, and a second opening that faces substantially perpendicular to the fluid flowing in the opposite direction and has a smaller opening area than said first opening, and said second location is provided by a second sub-passage having a third opening that faces substantially perpendicular to the fluid flowing in the one direction, and a fourth opening that faces substantially perpendicular to the fluid flowing in the opposite direction and has a larger opening area than said third opening. 6. The heating resistor flow rate measuring instrument according to claim 1 or 2, wherein said thermally sensitive resistor is heated to a temperature 20째 C.-40째 C. higher than the fluid temperature in said main passage. 7. The heating resistor flow rate measuring instrument according to claim 1, wherein said first sub-passage includes a curved portion. 8. The heating resistor flow rate measuring instrument according to claim 1, wherein a partition extending in the lengthwise direction of the second sub-passage is formed in the second sub-passage. 9. The heating resistor flow rate measuring instrument according to claim 8, wherein said partition has a taper portion which defines a tubular path gradually widening toward the downstream side of the second sub-passage. 10. The heating resistor flow rate measuring instrument according to claim 1, wherein said second sub-passage includes a simple tubular path in the upstream side, and a tubular path gradually widening toward the downstream side, the downstream end of said simple tubular path narrowing abruptly. 11. The heating resistor flow rate measuring instrument according to claim 1, wherein a projection-shaped flow resistor is formed in the tubular path of said second sub-passage and the thermally sensitive resister is arranged in a position shielded by the flow sensor when viewed from the upstream side. 12. A heating resistor flow rate measuring instrument for measuring the flow rate of a fluid passing through a passage, said instrument comprising: a first heating resistor radiating a larger amount of heat to the fluid flowing in one direction within said passage than to the fluid flowing in a direction opposite to the one direction; and a second heating resistor radiating a larger amount of heat to the fluid flowing in the opposite direction than to the fluid flowing in the one direction; wherein a bridge circuit including said first heating resistor and said second heating resistor is formed, said first heating resistor is heated to be higher than said second heating resistor by a predetermined temperature, and the flow rate of the fluid passing through said passage is measured based on the amount of heat radiated from said first heating resistor. 13. A heating resistor flow rate measuring instrument comprising a heating resistor and a thermally sensitive resistor both disposed in a main passage, and measuring the flow rate of a fluid passing through said main passage, wherein: said instrument includes a thermally-sensitive-resistor arrangement sub-passage in which said thermally sensitive resistor is disposed and radiates a larger amount of heat when the fluid flows in a direction opposite to one direction within said main passage than when the fluid flows in the one direction; said heating resistor is heated to be higher than a fluid temperature in said passage by a first predetermined temperature; said thermally sensitive resistor is heated to be higher than the fluid temperature in said passage by a second predetermined temperature; and the flow rate of the fluid passing through said passage is measured based on amounts of heat radiated from said heating resistor and said thermally sensitive resistor. 14. The heating resistor flow rate measuring instrument according to claim 13, wherein said heating resistor is disposed at a location in which said heating resistor radiates a larger amount of heat when the fluid flows in the one direction within said main passage than when the fluid flows in the opposite direction.