Flow sensor calibration

A method of calibrating a flow sensor composed of two heat sensors each having an electrical impedance parameter which varies as a defined function of the temperature of the heat sensor, means for supplying electric power at a selected level to the heat sensors, means for causing a fluid to flow in thermal communication with the heat sensors such that the relation between heat flow to or from one sensor and heat flow to or from the other sensor varies with the rate of fluid flow, and output means connected to produce an output signal representative of that heat flow relation, said method comprising:

• establishing a known flow rate for the fluid;
• electrically connecting one of the heat sensors to a circuit element which alters the magnitude of current flowing through the one heat sensor;
• and
• determining, in a first determining step, the output signal value while the one heat sensor is electrically connected to the circuit element, wherein the circuit element is a current source and said step of electrically connecting connects the current source across the one of the sensors.

In apparatus for calibrating a flow sensor composed of two heat sensors each having an electrical impedance parameter which varies as a defined function of the temperature of the heat sensor, means for supplying electric power at a selected level to the heat sensors, means for causing a fluid to flow in thermal communication with the heat sensors such that the relation between heat flow to or from one sensor and heat flow to or from the other sensor varies with the rate of fluid flow, and output means connected to produce an output signal representative of that heat flow relation, the improvement comprising:

• at least one circuit element capable of altering the magnitude of current flowing through a heat sensor;
• means for electrically connecting one of said heat sensors to said circuit element;
• and
• means for determining the output signal value while said one heat sensor is electrically connected to said circuit element and the fluid has a known flow rate,
• wherein said at least one circuit element includes two resistors and said means for electrically connecting comprise two switches each connected in series with a respective resistor.