standard as in the example of the ultrasonic interferometer developed by Harmon Plumb and George Cataland of the National Institute of Standards and Technology. A fluidic oscillator in which the resonant frequency of the cavity varies with temperature can be employed as a high-temperature pyrometer. Ultrasonic thermometers are based on the effect of the temperature on the velocity of sound waves in the medium transmitting the sound waves. Temperature measurements can be made not only in gases, but also in liquids or solids. The relationship between sound velocity and temperature for various materials is shown in Figure 4.15b. The acoustic velocity can be detected by immersing a rod or wire into the fluid or by using the medium itself as an acoustic conductor. The sensor rod can measure the temperature at a point or, by means of a series of constrictions or indents, can profile or average the temperature within the  medium. This is done by measuring the time lags of the sound waves as they are reflected from the consecutive indents (see Figure 4.15c). When the medium is used as the conductor of the ultrasonic pulse, the transducer can also be located within the vessel, but is usually placed on its external shell. This latter configuration is useful when measuring the temperature of solids or extremely hot or corrosive materials such as molten sodium. The acoustic pulse can be generated by a piezoelectriccrystal cut to resonate at a frequency ranging from 0.5 to  3 MHz or by means of magnetostrictive materials.The thin wire sensor (Figure 4.15c) is installed like aand its temperature range. Reflection from the beginning and the end of the thin wire provides the time lapse information for temperature determination. The selection of materials is
more flexible than for TCs as only one material is involved. Boiler/Furnace Applications An acoustic sound source and a sound receiver can be located on the outside of opposing walls of a boiler, cement kiln, or

furnace. Such noninvasive applications employ a transmitting transducer to send acoustic energy through the process medium. Receiving transducers detect the energy, and time delay is measured to determine the velocity of sound and therefore the temperature of the process. A nonintrusive, dual-path ultrasonic thermometer employing pneumatically require calibration; measures average temperature across two paths; has no moving parts other than the air supply valve; is linear; and can be provided with data display, logging, setup, and diagnostics either locally on a personal computer or remotely, using a modem connection. generated sound waves is shown in Figure 4.15d.