Method for determining a flow rate of a fluid independent of the thermal properties of the fluid

A method for determining a flow rate of a fluid independent of the physical properties of the fluid comprising:

• placing a thermal flow sensor in thermal contact with a fluid flow;
• measuring a flow rate;
• placing a thermal property sensor in a measurement cavity in fluidic contact with the fluid flow;
• receiving a portion of the fluid in the measurement cavity of the thermal property sensor, in such a way, that the portion of the fluid is essentially stationary in the measurement cavity, wherein the measurement cavity is fully open to the fluid flow;
• measuring at least one thermal property (κ, ρcp) of the fluid, or a combination of thermal properties (κ, ρcp) of the fluid;
• and
• compensating the measured flow rate for the at least one measured thermal property.

The method for determining a thermal conductivity (κ) and a heat capacity (cp) of a fluid, whose flow is to be determined, comprising:

• placing a thermal flow sensor in thermal contact with a fluid flow;
• measuring flow rate;
• placing a thermal property sensor in a measurement cavity in fluid connection with the fluid flow, wherein the measurement cavity is fully open to the fluid flow;
• receiving the portion of the fluid in the measurement cavity of the thermal property sensor, in such a way, that the portion of the fluid is essentially stationary in the measurement cavity,
• heating a heating wire of the thermal property sensor with a constant current (DC) or very low frequency alternating current (AC), for heating the portion of the fluid, and
• measuring the voltage of the heating wire during the heating of the portion of the fluid with voltage measurement means connected to the heating wire and relating the measured voltage to a thermal conductivity;
• heating the heating wire of the thermal property sensor with an alternating current (AC);
• and
• measuring a phase and amplitude of the third harmonic of the measured alternating current (AC) voltage of the heating wire during the heating of the portion of the fluid with voltage measurement means connected to the heating wire and relating the measured phase and amplitude of the third harmonic of the measured AC voltage to a heat capacity.

A thermal property sensor for use with a thermal flow sensor, wherein fluid, of which a flow rate is to be determined, flows through the thermal flow sensor during use, comprising:

• a heating wire configured for being placed in a measurement cavity for receiving a portion of the fluid, in such a way, that the portion of the fluid is essentially stationary in the measurement cavity, the heating wire further being configured for being heated with at least one of:
• a constant current (DC) or very low frequency alternating current (AC), for heating the portion of the fluid, wherein the voltage of the heating wire during the heating of the portion of the fluid is measured with voltage measurement means connected to the heating wire, wherein the measured voltage is related to a thermal conductivity;
• and
• an alternating current (AC), wherein a phase and amplitude of the third harmonic of the alternating current (AC) voltage of the heating wire during the heating of the portion of the fluid are measured with voltage measurement means connected to the heating wire, wherein the measured phase and amplitude of the third harmonic of the measured AC voltage are related to a heat capacity.