Question

Draw and explain the working principle of ultrasonic level measurement system. Also give advantages, disadvantages and applications of it.

Answer 1

Working principle: 1. It operates by generating an ultrasonic wave or pulse and measuring a time it takes for the echo to return. 2. There are two way of measurement of liquid level: Doppler Type Time difference type 3. The ultrasonic waves generated by transmitter and directed towards the liquid surface in the tank which is to be measure. 4. These waves get reflected from the surface of the liquid and are received by the receiver. 5. The time take by the wave is a measure of the distance travelled by the wave. Therefore the time‘t’ between transmitting and receiving a wave is proportional to the distance ‘d’ between ultrasonic set and surface of the liquid in the tank. 6. As the distance ‘H’ between ultrasonic set and the bottom of the tank is fixed time ‘t’ is measure of level ‘l’ 

Advantages: 1. Non-contact type level measurement technique. 2. There is no moving parts. 3. The output is unaffected by changes in composition, density, electrical conductivity of process fluid. 

Disadvantages:  1) Temperature compensation is essential. 2) Accuracy may get affected by the dirt, contamination, solid suspensions in process fluids. 3) Ultrasonic Transmitter is subject to many interferences so as strength of the signal may dilute. 4) Ultrasonic level measurement devices do not work satisfactorily in areas involving vacuum or high pressure conditions 

Application: 1. It is used in a nuclear power factory to ensure enough water to be turned into steam. 2. It can be used to control the flow rate by means of detecting level of process fluid in tank. 3. It is used to measure level of tank of height more than 50 feet.

Answer 2

Ultrasonic level detectors operate either by the absorption of acoustic energy as it travels from source to receiver or by the attenuation (frequency change) of a vibrating diaphragm face, oscillating at 35 to 40 KHz. It operates by generating an ultrasonic pulse and measuring the time it takes for the echo to return. When an ultrasonic transmitter is mounted at the top of the tank, the pulse travels in air at a speed of 331 meter/second at 0ºC. The time of travel is an indication of the depth of the vapour space above the liquid in the tank. If an ultrasonic transmitter is mounted on the bottom of the tank, the time of travel reflects the depth of liquid in the tank and the speed of travel is a function of what that liquid is. In order to measure the time of travel of the echo of an ultrasonic pulse, it is essential that some of the sonic energy by reflected. Liquids and solids with large and hard particles are good reflectors. Loose dirt have poor reflecting characteristic as they tend to absorb the sonic pulse. Since the angle of reflection is equal to the angle of incidence, it is important that the reflecting surface be flat. If the sonic pulse is reflected from a sloping surface, its echo will not be directed back to the source and the round-trip travel time will not accurately reflect the vertical distance. Irregular surfaces result in diffuse reflection where only small portions of the total echo travels vertically back to the source.

Advantages:-

1. Ultrasonic level detectors are non-contact type measurement techniques. They have the ability to measure level without making physical contact with the process material. 2. They have no moving parts.