Hot wire anemometer gas flow sensor having improved operation and compensation

A method for measuring the flow rate of a gas passing a sensing resistor provided with current to maintain the resistance of the resistor at a constant value, said method comprising the steps of:

• supplying a voltage drop signal from the resistor to a first input of a differential amplifier, said differential amplifier being driven into a saturated state by a differential input signal of a predetermined magnitude;
• providing a bias signal to a second input of the differential amplifier to form, with the voltage drop signal, the differential input signal to the differential amplifier;
• establishing the magnitude of the bias signal at a level that limits the differential input signal to not greater than the predetermined magnitude over a given range of the voltage drop signal;
• and
• changing the magnitude of the bias signal as the magnitude of the voltage drop signal varies so that the differential input signal does not exceed the predetermined magnitude,
• an output signal of said differential amplifier providing an indication of the gas flow rate of the gas.

A method of compensating a bridge circuit gas flow rate sensor when replacing a first hot wire sensing resistor with a second hot wire sensing resistor, said method comprising the steps of:

• (a) at zero gas flow, operating the bridge circuit containing the first sensing resistor (401) to establish a balanced condition in the bridge circuit at a desired current through the first sensing resistor;
• (b) determining the voltage drop (Vw1(0)) across the first sensing resistor at such conditions;
• (c) at zero gas flow, operating the bridge circuit containing the second sensing resistor (402) to establish a balanced condition in the bridge circuit with the desired current through said second sensing resistor;
• (d) determining the voltage drop (Vw2(0)) across the second sensing resistor at such conditions;
• and
• (e) establishing a ratio of the voltage drops so determined across the first and second sensing resistors which, when applied to the flow component Vw2(f) of the voltage drop across said second sensing resistor when sensing gas flow, compensates the sensor for the replacement of said first sensing resistor with said second sensing resistor.

A method of compensating a gas flow rate sensor for changes in the composition of gas, the flow rate of which is being measured, said gas sensor having a hot wire sensing resistor in a bridge circuit, said method comprising the steps of:

• (a) at zero gas flow, operating the bridge circuit with the sensing resistor exposed to a first gas of a first composition to establish a balanced condition in the bridge circuit at a desired current through the sensing resistor;
• (b) determining the voltage drop (Vw′(0)) across the sensing resistor at such conditions for the first gas;
• (c) at zero gas flow, operating the bridge circuit with the sensing resistor exposed to a second gas of a second composition;
• (d) determining the voltage drop (Vw″(0)) across the sensing resistor at such conditions for the second gas;
• and
• (e) establishing a ratio of the voltage drops so determined across the sensing resistor which, when applied to the flow component Vw″(f) of the voltage drop across said sensing resistor when sensing the flow of the second gas, compensates the sensor for the change in the composition of the gas, the flow rate of which is being measured.