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Application Note:
Ultrasonic Thermometry for Liquids





New possibilities in non-intrusive mass flow and temperature measurement are enabled by current ultrasonic instrumentation and signal processing.  It is now feasible to measure variations in ultrasonic time-of-flight (ToF) with precision in the picosecond range.  For example, the local temperature of a fluid flowing in a channel can be measured with sensors attached to an exterior surface, as shown in Figure 1.  Figure 2 compares the variation in temperature as measured by a thermocouple to the ToF within a JP-10-filled internal channel.  The ToF variations track the thermocouple temperature and provide a means for measuring temperature non-intrusively. 

Diagram
Figure 1  Internal channel insonified by an externally mounted sensor

Measured ToF
Figure 2  Comparison of measured ToF in channel with temperature measured by thermocouple

The ultrasonic velocity is dependent upon B , the bulk modulus and r, the density.  In general both of these parameters are a function of temperature. 
Equation 1
Variations in temperature can be measured by measuring precise variations in ToF using sensors mounted external to the channel.  Figure 3 shows the measured ultrasonic velocity variation of a) water and b) JP-10 aviation fuel as a function of temperature.

a)

b)
Figure 3 Temperature variation of ultrasonic velocity for a) water and b) JP-10