Beginner's guide to ultrasonic level transmitter

Sonic is the sound we can hear. Ultrasonic is the sound above the human hearing range. A human can hear a maximum up to a frequency of 20 KHz. Ultrasonic frequencies are above 20 KHz. Ultrasonic waves are used to measure the level of liquids and solid objects in industries. Ultrasonic level measurement is the contactless principle and most suitable for level measurements of hot, corrosive and boiling liquids. The normal frequency range used for ultrasonic level measurements is within a range of 40 ­ 200 KHz.

1. What is the principle of ultrasonic level measurement?

Ultrasonic waves detect an object in the same way as Radar does it. Ultrasonic uses sound waves, and Radar uses radio waves. When the ultrasonic pulse signal is targeted towards an object, it is reflected by the object and echo returns to the sender. The time traveled by the ultrasonic pulse is calculated, and the distance of the object is found. Bats use a well-known method to measure the distance while traveling. The ultrasonic level measurement principle is also used to find out fish positions in the ocean, locate submarines below water level, also the position of a scuba diver in the sea.

We will refer to Fig-1 and make an effort to understand the technicalities of the ultrasonic level transmitter. An ultrasonic level transmitter is fixed at the top of a tank half filled with liquid. The reference level for all measurements is the bottom of the tank. Level to be detected is marked as “C”, and “B” is the distance of the ultrasonic sensor from the liquid level. Ultrasonic pulse signals are transmitted from the transmitter, and it is reflected back to the sensor. The travel time of the ultrasonic pulse from the sensor to target and back is calculated. Level “C” can be found by multiplying half of this time with the speed of sound in air. The measuring unit final result can be centimeters, feet, inches, etc.

2. Practical system design problems of Ultrasonic Level Transmitter:

The above principle of measurement looks quite straightforward and true only in theory. In practice, there are some technical difficulties that are to be taken care to get correct level reading.

• The velocity of sound changes due to the variation of air temperature. An integrated temperature sensor is used to compensate for changes in the velocity of sound due to temperature variations.
• There are some interference echoes developed by the edges, welded joints, etc. This is taken care of by the software of the transmitter and called interference echo suppression.
• The calibration of the transmitter is crucial. The accuracy of the measurement depends on the accuracy of the calibration. The Empty distance “A” and measurement span “D” is to be ascertained correctly for inclusion in the calibration of the transmitter.
• The transient characteristics of the sensor will develop a Blocking distance as shown in Fig-1. Span “D” should never extend to the blocking distance.

3. The basic structure of an Ultrasonic Transducer (Refer: Fig-2):

Ultrasonic Sensor is the heart of the ultrasonic level Transmitter instrument. This sensor will translate electrical energy into ultrasound waves. Piezoelectric crystals are used for this conversion process. Piezoelectric crystals will oscillate at high frequencies when electric energy is applied to it. The reverse is also true. These piezoelectric crystals will generate electrical signals on receipt of ultrasound. These sensors are capable of sending an ultrasound to an object and receive the echo developed by the object. The echo is converted into electrical energy for onward processing by the control circuit.

4. Functional block diagram of a typical Ultrasonic Level Transmitter:

We will refer to Fig-3 Functional Block Diagram to clarifying the physical structures of an Ultrasonic Level Transmitter.

A micro-controller based Control Circuit monitors all the activities of the ultrasonic level transmitter. There are two Pulse Transmission Circuits, one for transmitter pulse and the other for the receiver pulse. The pulse generated by the transmitter pulse is converted to Ultrasound pulses by the Ultrasonic Sensor (Transmitter) and targeted towards the object.

This ultrasound pulse is reflected back as an echo pulse to the Ultrasonic Sensor (Receiver). The receiver converts this Ultrasonic pulse to an electrical signal pulse through the pulse generator. The time elapsed, or the reflection time is measured by the counter. This elapsed time has a relation to the level to be measured. This elapsed time is converted to level by the Control Circuit. There is a Timing Generator Circuit which is used to synchronize all functions in the ultrasonic level measurement system.

The level is finally converted to the 4-20mA signal. 4mA is 0% level, and 20mA is the 100% level (see Fig-1). This 4-20mA output signal carrying the level data can be transmitted to long distance to Process Control Instruments.

5. Advantages of Ultrasonic Level Transmitter:

The ultrasonic level transmitter has no moving parts, and it can measure the level without making physical contact with the object. This typical characteristic of the transmitter is useful for measuring levels in tanks with corrosive, boiling and hazardous chemicals. The accuracy of the reading remains unaffected even after changes in the chemical composition or the dielectric constant of the materials in the process fluids.

6. Limitations of Ultrasonic Level Transmitter:

Ultrasonic level transmitters are the best level measuring devices where the received echo of the ultrasound is of acceptable quality. It is not so convenient if the tank depth is high or the echo is absorbed or dispersed. The object should not be a sound absorbing type. It is also unsuitable for tanks with too much smoke or high-density moisture.