Gas flow measuring apparatus

A gas flow measuring apparatus comprising:

• an electric heater disposed in a gas tube;
• first and second temperature dependent resistors respectively disposed downstream and upstream of said electric heater in said gas tube, said second temperature dependent resistor being spaced from said electric heater so that heat generated by said electric heater does not influence said second resistor;
• a reference resistor circuit including first and second reference resistors connected in series;
• said first and second temperature dependent resistors and said first and second reference being connected to form a bridge circuit, one pair of diagonal points include a junction between said first and second reference resistors and a junction between said first and second temperature dependent resistors, said one pair of diagonal points producing therebetween a potential difference (.DELTA.V) as functions of flow rate (G) of gas in the gas passage tube, power (P) supplied to said electric heater and voltage (V) applied to the other pair of diagonal points of said bridge circuit;
• a voltage generating circuit for generating first and second reference voltages;
• an oscillator circuit for producing a pulse signal;
• a first adding circuit connected to one of said one pair of diagonal points of said bridge circuit and connected to said voltage generating circuit to receive said second reference voltage (Vr.sub.2) for producing a first sum voltage (V.sub.5) of a potential (Va) of said one point of said one pair of diagonal points of said bridge circuit and said second reference voltage (Vr.sub.2);
• a chopper circuit connected to receive said potential difference (.DELTA.V) from said bridge circuit, connected to said oscillator circuit, and connected to said first adder circuit to receive said first sum voltage (V.sub.5) as a hypothetical ground potential at output terminal through a resistor, said chopper circuit converting under control of said pulse signal from said oscillator circuit said potential difference (.DELTA.V) into a square wave signal having a center reference level corresponding to said first sum voltage (V.sub.5);
• an amplifier circuit connected to said chopper circuit and said first adder circuit for producing an amplified rectangular wave signal (V.sub.1) having a center reference corresponding to said first sum voltage (V.sub.5);
• a rectifier circuit connected to said amplifier circuit and said first adder circuit for half-wave rectifying said amplified rectangular wave signal;
• a sample-hold circuit connected to said rectifier circuit and said oscillator circuit for producing a voltage signal (V.sub.3) having a magnitude varied depending on said potential difference (.DELTA.V);
• a second adder circuit connected to said voltage generating circuit and said first adder circuit for producing a second sum voltage (V.sub.6) of said first sum voltage (V.sub.5) and said first reference voltage (Vr.sub.1);
• a differential amplifier connected to said sample-hold circuit and said second adder circuit for producing an output voltage (V.sub.4) proportional to a difference between said voltage signal (V.sub.3) from said sample-hold circuit and said second sum voltage (V.sub.6) from said second adder circuit;
• and
• a power amplifier circuit connected to said differential amplifier circuit for applying to said bridge circuit an amplified voltage of said output voltage (V.sub.4) from said differential amplifier, said amplified voltage being increased with increased flow rate of the intake air due to a decrease in said potential difference (.DELTA.V) between said one pair of diagonal points of said bridge circuit thereby to increase the heat generated by said electric heater resulting in an increase of said potential difference (.DELTA.V) to maintain said potential difference (.DELTA.V) at a predetermined constant value, whereas said amplified voltage being reduced with decreased flow rate due to an increase in said potential difference (.DELTA.V) thereby to reduce the heat generated by said electric heater resulting in a decrease of said potential difference (.DELTA.V) to maintain said potential difference constant to achieve a stabilized state of control of said bridge circuit, a value of current flowing through said electric heater being used to determine the gas flow rate.