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A sensor for measuring the speed of sound in a gas. The sensor has a helical coil, through which the gas flows before entering an inner chamber. Flow through the coil brings the gas into thermal equilibrium with the test chamber body. After the gas enters the chamber, a transducer produces an ultras

A sensor for measuring the speed of sound in a gas. The sensor has a helical coil, through which the gas flows before entering an inner chamber. Flow through the coil brings the gas into thermal equilibrium with the test chamber body. After the gas enters the chamber, a transducer produces an ultrasonic pulse, which is reflected from each of two faces of a target. The time difference between the two reflected signals is used to determine the speed of sound in the gas.

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1. A transit time speed of sound sensor for gaseous media, comprising:a helical channel having a generally cylindrical shape, such that it encloses an inner chamber;end pieces at each end of the inner chamber, such that the inner chamber may contain the gas to be tested;a gas input port operable to

1. A transit time speed of sound sensor for gaseous media, comprising:a helical channel having a generally cylindrical shape, such that it encloses an inner chamber;end pieces at each end of the inner chamber, such that the inner chamber may contain the gas to be tested;a gas input port operable to receive a flow of gas into the helical channel;wherein the helical channel is arranged such that gas may enter the helical channel, flow through the helical channel, and enter the inner chamber;a dual-faced target at one end of the chamber;a transducer at the opposing end of the chamber, the transducer operable to produce an ultrasonic pulse in the gas and to receive a reflection of the pulse from each face of the target; anda gas output port operable to exhaust gas from the chamber. 2. The sensor of claim 1, further comprising a porous diffuser sleeve enclosing the inner chamber volume of the sensor, such that the gas may exit the helical channel, enter a space between the inside wall of the chamber and the diffuser sleeve, and enter the inner chamber through the diffuser sleeve. 3. The sensor of claim 1, wherein the dual-face target is made from a single piece of material. 4. The sensor of claim 1, wherein the target is made from a material having a low thermal expansion. 5. The sensor of claim 1, further comprising a temperature sensor within the chamber. 6. The sensor of claim 1, further comprising a pressure sensor within the chamber. 7. The sensor of claim 1, wherein the transducer has an operating resonance frequency in the range of 300-500 kilohertz. 8. The sensor of claim 7, wherein the transducer has a bandwidth of between 75-100 percent of its operating resonance frequency. 9. The sensor of claim 1, further comprising an outer sleeve enclosing the helical channel. 10. The sensor of claim 1, further comprising a processing unit, operable to receive signals representing pulses reflected from each face of the target, and to cross correlate the reflected pulses. 11. A transit time speed of sound sensor, comprising:a helical channel having a generally cylindrical shape;a gas input port operable to receive a flow of gas into the helical channel;a porous diffuser sleeve enclosing an inner volume of the gas test chamber of the sensor;end pieces at each end of the inner chamber, such that the inner chamber may contain the gas;wherein the helical channel and diffuser sleeve are arranged such that gas may enter the helical channel, flow through the helical channel, enter a space between the inside wall of the chamber and the diffuser sleeve, and enter the chamber through the diffuser sleeve;a dual faced target at one end of the chamber;a transducer at the opposing end of the chamber, the transducer operable to deliver an ultrasonic pulse and to receive a reflection of the pulse from each face of the target; anda gas output port operable to exhaust gas from the sensor. 12. A method of measuring the speed of sound of a gas in a chamber, comprising the steps of:receiving the gas into a helical channel surrounding the chamber;flowing the gas through the helical channel until it reaches a temperature substantially the same as that inside the chamber;receiving the gas into the chamber;transmitting an ultrasonic pulse from one end of the chamber, directed toward the opposing end of the chamber;reflecting the pulse from a first face of a target;reflecting the pulse from a second face of a target, the second face being a known distance farther from the transducer than the first face; andreceiving the reflected pulses. 13. The method of claim 12, wherein the step of receiving the gas into the chamber is performed by diffusing the gas through a porous material separating the helical flow channel coil and the inner volume of the chamber. 14. The method of claim 12, wherein the target is fabricated from a single piece of material. 15. The method of claim 12, wherein the target is made from a low thermal expansion material. 16. The method of cla im 12, further comprising the step of cross correlating the reflected pulses. 17. The method of claim 12, wherein the transducer has an operating resonance in the range of 300-500 kilohertz. 18. The method of claim 17, wherein the transducer has a bandwidth of between 75-100 percent of its operating resonance frequency.