Acoustic thermometry is an arcane, elegant technique that measures temperature using the temperature-dependent transit time of sound in a medium (references 1 through 4). The medium can be a solid, a liquid, or a gas. Acoustic thermometers function in environments, including extreme temperatures, destructive physical abuse, and nuclear reactors, that conventional sensors cannot tolerate. Sonic speed in air varies predictably as the square root of temperature. The sonic transit time in a gas-path thermometer is almost entirely insensitive to pressure and humidity. Gas-path acoustic thermometers respond quickly to temperature changes. They have essentially no thermal mass or lag.
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