Ultrasonic level sensors are available in two basic types which are explained below:
Non-Contact Ultrasonic Sensors
This category of ultrasonic level sensors consists of:
- An analog signal processor
- A microprocessor
- Binary coded decimal i.e. BCD range switches
- An output driver circuit
In these devices, the microprocessor generates the gate signal and pulses and directs them to the ultrasonic sensor via the analog signal processor. The sensor then transmits a beam of ultrasonic waves to the surface of the fluid. It also receives the reflected echoes from the fluid surface and sends them back to the microprocessor. The microprocessor keeps on receiving echoes of sound waves and performs calculations to determine distance between the sensor and the fluid surface and hence detects the fluid level.
Contact Ultrasonic Sensors
These types of ultrasonic sensors are primarily used to detect fluid level at a specific point only. These level measurement devices consume less energy and basically include:
- A sensor installed in the process field and
- An integrated solid state amplifier
They consist of no movable parts and hence do not need any calibration. “Typically, they are equipped with terminal blocks for connection of a power source and external control devices. The ultrasonic signal crosses a one-half inch gap in the sensor, controlling relay switches when the gap contains liquid. The sensing level is midway along the gap for horizontally mounted sensors, at the top of the gap for vertically mounted sensors.” When the level of the fluid drops below the sensing level, the strength of the ultrasonic signal gets reduced. This eventually brings the relay to its former position.
These ultrasonic sensors find their application in vessels or pipes where they are used for automatic action of pumps, solenoid valves, and high or low level alarms. In these areas, two sensors need to be employed: one for filling and emptying tanks and the other one for measuring fluid volumes. They are suitable for use with mostly all kinds of fluids. Their performance does not get easily influenced by coatings, clinging droplets, foam or vapor etc but sometimes, highly aerated or viscous fluids can create trouble by choking the sensor gap.