Ultrasonic Level Sensors - Main Features

Key features of ultrasonic level measurement devices are listed below:

• These sensors use frequencies in the tens of kilohertz range; transit times are ~6 ms/m. The speed of sound (340 m/s in air at 15°C (1115 fps at 60°F) depends on the mixture of gases in the headspace and their temperature.
• The speed of sound waves traveling via the medium which is normally air is prone to get affected by changes in the working temperature. In order to compensate for these changes in temperature and resulting changes in sound wave speed, the level measurement system must include a temperature sensing device. This will help in correct distance calculations and hence accurate level detection results.
• In cases where heavy foam is found on the surface of the process fluid, the use of ultrasonic level measurement techniques are usually avoided since this foam work as a sound absorbent. Consequently, the sound wave will get scattered resulting in non reception of the exact signal by the sensor. This will cause improper functioning of the measurement system.
• Excessive surface turbulence of the fluid can result in wide fluctuations in level measurement results. To avoid this issue, one may employ a damping correction or a response delay with the device.
• Good level measurement requires that the reflected echo from the fluid surface returns back in a straight line to the sensor. Besides, it calls for proper installation of ultrasonic transmitter over the tank. The transmitter should be mounted in such a way that the inner composition of the vessel or tank doesn’t get in the way of the signal.
• In level measurement fields where sound waves get influenced by factors like foam and vapor etc., one can connect a beam guide to the sensor for improving performance of these devices.
• Ultrasonic level measurement technique proves to be quite costly when employed for point level measurement applications.
• In case of fluids which are less viscous, one can execute point level measurement via a technique called ultrasonic gap technique. In this method, a transmit crystal is activated on one side of a “measurement gap” and a receive crystal listens on the opposite side. The signal from the receive crystal is analyzed for the presence or absence of tank contents in the measurement gap.
• To overcome barricades encountered in the vessel or tanks, a technique called tank mapping has been introduced. Tank mapping lets the operator take a "sonic snapshot" of an empty vessel. The transducer transmits a sound burst and the echo is recorded as a signature of the tank. Any obstructions in the vessel will send an echo and create a profile. Later on, this signature or profile is locked into the ultrasonic unit’s memory so it will not respond to echoes created by these obstructions.