An ultrasonic level transmitter is mounted on the top of the tank and transmits an ultrasonic pulse down into the tank. This pulse, travelling at the speed of sound, is reflected back to the transmitter from the liquid surface. The transmitter measures the time delay between the transmitted and received echo signal and the on-board microprocessor calculates the distance to the liquid surface using the formula.
Distance = (Speed of light x time delay) / 2
Once the transmitter is programmed with the bottom reference of the application – usually the bottom of the tank – the liquid level is calculated by the microprocessor.The basic equation for calculating the tank level is
Level = Tank Height – Distance
Basic Concept and Elements of the Ultrasonic Level Measurement
Minimum measuring distance (Xm): (also known as the “Dead Band”) is a feature common to all ultrasonic level meters. This is a short range in front of the sensor within which the ultrasonic device can not measure.
Maximum measuring distance (XM): The longest range under ideal condition within which the device can measure. No measurement is possible beyond this distance.
- Ultrasonic transmitters are easy to install on empty tanks or on tanks containing liquid.
- Set-up is simple and those devices with on-board programming capability can be configured in minutes.
- As there is no contact with the media and no moving parts, the devices are virtually maintenance free. Wetted materials are usually an inert fluoropolymer, and resistant to corrosion from condensing vapors.
- Because the device is non-contacting, the level measurement is unaffected by changes in the liquid density, dielectric, or viscosity, and performs well on aqueous liquids and many chemicals.
- Changes in process temperature will change the speed of the ultrasonic pulse through the space above the liquid, but built-in temperature compensation automatically corrects this.
- Changes in process pressure do not affect the measurement.
- Ultrasonic transmitters rely on the pulse being unaffected during its flight time. Liquids which form heavy vapors, steam or vapor layers should be avoided (use a Radar transmitter in these instances). As the pulse needs air to travel through, vacuum applications are not possible.
- Materials of construction generally limit the process temperature to around 158 °F (70 °C) and pressure to 43 psig (3 bar).
- The condition of the liquid surface is also important. Some turbulence can be tolerated but foaming will often damp out the return echo.
- Obstructions in the tank, such as pipes, strengthening bars and agitators, will cause false echoes, but most transmitters have sophisticated software algorithms to allow masking or ignoring of these echoes.
- Ultrasonic transmitters can be used on silos containing dry products such as pellets, grains or powders, but these are more difficult to commission. Factors such as surface angle of repose, dusting and long ranges must be taken into account. A Guided Wave Radar transmitter is better suited to dry product applications.