Patent ID: 7251995

Claim:
A flow sensor, comprising: a substrate located in a gas fluid; a heat generating resistance portion including a first heat generating resistor, a second heat generating resistor, a third heat generating resistor and a fourth heat generating resistor formed on the substrate, each of the four resistors having same properties and a resistance value changing with a temperature thereof, wherein the first and the third heat generating resistors are formed in an upstream side with respect to a flow direction of the gas fluid, and the second and the fourth heat generating resistors are formed in the downstream side with respect to the flow direction of the gas fluid; a first bridge circuit including the first to the fourth heat generating resistors, wherein the first and the second heat generating resistors are serially connected and the third and the fourth heat generating resistors are serially connected and wherein the two serial circuits of the heat generating resistors are connected in parallel so that the first and the fourth heat generating resistors are positioned in a first side of a power supply source and the second and the third heat generating resistors are positioned in a second side of the power supply source; a second bridge circuit including: a thermometer resistor a resistance value of which changes with a surrounding temperature thereof; and a first resistor and a second resistor, each having a predetermined resistance value, wherein the second bridge circuit is formed of the first bridge circuit, the thermometer resistor, the first resistor and the second resistor, and wherein a resistance value of each of the first and the second resistor is adjusted so that when the second bridge circuit is balanced, a temperature of each of the first to the fourth heat generating resistor is higher by a predetermined value than a temperature in the gas fluid; a current adjusting section for monitoring an electric potential difference between two intermediate terminals in the second bridge circuit to adjust the current flowing in the second bridge circuit based upon the electric potential difference so that the second bridge circuit is balanced; and a computer section connected to two intermediate terminals in the first bridge circuit for calculating a flow quantity and a flow direction of the gas fluid based upon an electric potential difference between the two intermediate terminals.