Ultrasonic, radar, and magnetostrictive level measuring instruments use the principle of time-of-flight to determine the level of a process substance in a vessel.
A critical factor for the accuracy of any time-of-flight measurement technology is the velocity of propagation for the wave in question, through the substance(s) that wave must travel.
Level Measuring Instruments
1. Examine each of these illustrations and then determine which of the velocities of propagation (v) matter and which do not. Be prepared to explain why, in each case!
2. Next, identify physical variables eﬀecting the velocity of propagation for each of the waves in question.
3. Which of these level-sensing technologies do you suspect enjoys the greatest immunity from calibration error resulting from changing process conditions?
4. In each case, identify factors inﬂuencing the strength of the received signal
- Ultrasonic level, bottom-mounted: vwater matters, vair does not
- GWR level: vair matters, vwater does not
The velocity of propagation for sound waves varies with the density of the medium and also its bulk modulus. The velocity of propagation for radio waves varies with permittivity.
In both cases, changes in density (typically caused by changes in pressure and/or temperature) aﬀect these factors, thereby aﬀecting the velocity of propagation.
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Credits: Tony R. Kuphaldt