A system configured to measure an average temperature of mixed fluid in an enclosed chamber, the system comprising: a processor; the enclosed chamber; a primary pipe disposed in the enclosed chamber, wherein both ends of the primary pipe reside within the enclosed chamber and the primary pipe contains air as a medium and the medium is isolated from surroundings in a sense that is not affected by the mixed fluid present in the enclosed chamber or by any upward fumes that may be present in the enclosed chamber; a secondary pipe fluidly connected to the primary pipe at a perpendicular angle wherein the secondary pipe fluidly connects the primary pipe to the outside of the enclosed chamber, thereby maintaining a constant atmospheric pressure in the primary pipe; an airflow meter attached to the secondary pipe for measuring the atmospheric pressure in the primary pipe; at least one ultrasound transducer comprising at least one ultrasound transmitter and at least one ultrasound receiver, wherein the at least one ultrasound transmitter and the at least one ultrasound receiver are placed at the two extreme ends of the primary pipe, and wherein the at least one ultrasound transmitter is configured to produce a flow of an acoustic wave into the primary pipe in a frequency sweep mode, wherein the frequency sweep mode is an operation that excites a particular frequency to an air coupled ultrasound transducer for a specified time and then changes to a different excitation frequency at specified frequency steps, and the sweep operation starts from a specified start frequency and stops at a specified end frequency; and wherein the at least one ultrasound receiver is configured to receive the transmitted acoustic wave at a receiving end of the primary pipe; a frequency computing module configured to calculate the temperature of the primary pipe based on a measurement of the velocity of the acoustic wave using a Swept Frequency Acoustic Interferometry (SFAI) measurement principle; a resistivity computing module configured to compute resistivity of the primary pipe using a pre-configured Wheatstone bridge arrangement, wherein the resistivity of the primary pipe increases with increase in temperature; and a temperature measuring module configured to compute a temperature T" from the resistivity of the primary pipe, and to determine an average temperature of a mixed fluid in the enclosed chamber from a weighted average of the temperatures T and T". the enclosed chamber; a primary pipe disposed in the enclosed chamber, wherein both ends of the primary pipe reside within the enclosed chamber and the primary pipe contains air as a medium and the medium is isolated from surroundings in a sense that is not affected by the mixed fluid present in the enclosed chamber or by any upward fumes that may be present in the enclosed chamber; a secondary pipe fluidly connected to the primary pipe at a perpendicular angle wherein the secondary pipe fluidly connects the primary pipe to the outside of the enclosed chamber, thereby maintaining a constant atmospheric pressure in the primary pipe; an airflow meter attached to the secondary pipe for measuring the atmospheric pressure in the primary pipe; at least one ultrasound transducer comprising at least one ultrasound transmitter and at least one ultrasound receiver, wherein the at least one ultrasound transmitter and the at least one ultrasound receiver are placed at the two extreme ends of the primary pipe, and wherein the at least one ultrasound transmitter is configured to produce a flow of an acoustic wave into the primary pipe in a frequency sweep mode, wherein the frequency sweep mode is an operation that excites a particular frequency to an air coupled ultrasound transducer for a specified time and then changes to a different excitation frequency at specified frequency steps, and the sweep operation starts from a specified start frequency and stops at a specified end frequency; and wherein the at least one ultrasound receiver is configured to receive the transmitted acoustic wave at a receiving end of the primary pipe; a frequency computing module configured to calculate the temperature of the primary pipe based on a measurement of the velocity of the acoustic wave using a Swept Frequency Acoustic Interferometry (SFAI) measurement principle; a resistivity computing module configured to compute resistivity of the primary pipe using a pre-configured Wheatstone bridge arrangement, wherein the resistivity of the primary pipe increases with increase in temperature; and a temperature measuring module configured to compute a temperature T" from the resistivity of the primary pipe, and to determine an average temperature of a mixed fluid in the enclosed chamber from a weighted average of the temperatures T and T".