sound waves with frequency greater than human hearing range. ie frequency greater than 20kHz is termed as ultrasonic waves (sonic means hearing range and ultrasonic means above hearing range.) . ultra sonic level transmitter uses sound waves for level detection .
How it measures level ?
Pulses of ultrasonic waves sends from the source circuit of device. They get reflected back from liquid surface or solid surface.After reflection they reaches the device itself.The receiver circuit receive this reflected wave . The time taken for the ultrasonic wave in this travel is calculated using proper electronics.This time of flight of ultrasonic pulse is directly proportional to the distance to the surface from the source.A dedicated ultrasonic level transmitter is a source of ultrasonic waves and receiver and proper signal conditioning circuits . .The transmitter circuit which sends ultrasonic pulses,receives the reflected pulses through proper circuits . After that they transmit a electrical signal to monitoring systems (like 4-20 ma signal,FF signal ,etc) proportional to the level in the tank .
Time of flight and level relation.
See figure above
Take C is the velocity of sound wave . And t is the time taken to travel from source to surface and back to source, then
speed of sound wave = Distance travelled/time of travel
Distance travelled = speed of sound wave × Time of flight
Distance between ultrasonic level
Transmitter and liquid surface = D
Distance travelled by the ultrasonic wave during the time t ( time of flight)= 2D
2D =Ct
D = Ct/2
Empty distance = E
Actual level L = E – D
L = E – (Ct/2)
So by measuring time of flight of ultrasonic waves,we can calculate empty distance and also the level in the tank.
In Modern ultrasonic level transmitters we need to program various parameters inorder to properly calculating the level and also to transmitting the output which corresponds to the measurement range .some of them are described below .
Empty distance
E Empty distance
It is the distance from the sensor face to the tank bottom or it is the distance from the sensor to the bottom surface when tank is empty.
Blocking distance
BD -Blocking distance
Ultra sonic level Sensors transmission and receiving of ultrasonic waves takes place .So the device act as both Transmitter and receiver. But not at the same time.They send ultrasonic pulses and act as Transmitter .And the device waits for the reflected wave The wave get reflected on the surface and heading to the device. .And it receives the reflected wave .And the time of flight for the wave is calculated .This way level is measured .
So the cycle of process involves
1.sending ultrasonic pulses.
2.Reflection takes place .
3.receiving the pulses .
4 .calculating the time needed .
5.sending the next pulses …..and this continues .
If the reflecting surface is too close to the ultrasonic level Transmitter ,the device couldn’t properly analyze these ultrasonic pulses .And measurement get affected .This is because the time of flight of the wave become too small than the time required for the device to complete its functioning as transmitter and receiver .So a minimum safety distance must be ensured always for enough time of flight of the wave .Beyond that level measurement will not be accurate .We should be careful for not to rise the level beyond this point .This minimum safety distance from the sensor face to the maximum allowable level of the product is called the blocking distance .
Measurement span
F .Measurement span
It is the range of measurement whose upper and lower values corresponding to minimum and maximum value of measurement .
L .Actual level
D .Distance of sensor from fluid level .
