Working Principle of Ultrasonic Level Transmitter:
- Ultrasonic level transmitters are essential to control the fluid level in many process industries.
- The ultrasonic level transmitter uses dynamic Gain, Amplitude, and Power (GAP) technology to detect the correct echo signal.
- This uses the least amount of GAP at every possible distance within its measurement range.
- The ultrasonic level transmitter works on the principle of “Time of Flight” using the speed of sound.
- Generally, this level transmitter emits a pulse of high frequency in the range of 20 kHz to 200 kHz.
- This pulse is transmitted in a cone, usually about 6° at the apex.
- Ultrasonic level sensors transmit sound waves towards a target to determine the distance by calculating the time taken for the reflected waves to return back to the receiver.
- Usually, in all applications these ultrasonic level transmitters are installed on the top of the tank or storage bin facing downward to measure the material by transmitting an ultrasonic pulse down into the tank or storage bin.
- This UL transmitter consists of a single 4-20 mA DC analog output with HART 5 and 7 relay outputs.
- The traveling speed of this pulse is equivalent to the speed of sound the pulse from the liquid surface is reflected back to the transmitter.
- The level transmitter calculates the actual time delay between the transmitted pulse signal and the received echo signal.
- This electronic pulse is converted into an acoustic pulse through the transducer.
- The converted acoustic pulse is controlled towards the surface of the measuring material.
- When this acoustic pulse reaches the surface of the material, the same energy is reflected back to the transducer as an echo signal.
- This electronic pulse is sent back to the microprocessor and the timer is stopped to determine the signal “Time of Flight”.
- This level transmitter consists of a microprocessor that launches an electronic pulse to the transducer and starts a timer immediately.
- The microprocessor calculates the level of the product by combining both the speed of sound through air and the “Time of Flight” of the pulse signal.
- Ambient noise and false echoes from the signal are removed by electronic filters.
- The microprocessor calculates the liquid level present in the tank when the level transmitter is programmed with a bottom reference called the bottom surface of the tank.
- The microprocessor computes the actual distance to the liquid surface using the formula.
Distance (S) = [Speed of Sound in Air(C) * Time Delay (T)]/ 2
S: Actual Distance to be measured,
C: Speed or velocity of sound in air,
T: Time Delay
- The tank level is calculated using the relation
Level = Tank Height – Distance
Minimum Measuring Distance (Xm):
- This is known as the “Dead Band”
- This is a common feature of all ultrasonic-level transmitters.
- This is a minimum range in front of the sensor in the ultrasonic device that cannot measure.
Maximum Measuring Distance (XM):
- Maximum Measuring Distance is the highest range under the ideal working environment where the device can easily measure the distance.
- But beyond this distance, measurement is not possible.
- The ultrasonic level transmitter converts the actual distance of wave travel into a measure of the level in the tank.
- The time interval between the firing of the sound burst and receiving the echo signal in return is in direct proportion to the length between the transducer and the material present inside the vessel.
- Generally, the medium encountered is air above the material’s surface but it may be a blanket of some other gases or vapors.
- The frequency range for ultrasonic is in the range of 15 to 200 kHz.
- The lower frequency instruments are used for more difficult applications such as long distances and solid level measurement.
- Higher frequency instruments are used for shorter liquid-level measurements.
- The speed of the ultrasonic pulse depends upon a change in process temperature.
Installation guidelines for Ultrasonic Level Transmitter:
- The Ultrasonic level transmitter is weatherproof and it can be installed outside the wall or chassis plate using the holes provided.
- It has a protection class of IP65
- It can also be installed inside the tank using another suitable enclosure
- Avoid facing of the LCD display to direct sunlight can cause the display to fail.
- Do not install this level transmitter in a high-vibration area, and close to the vicinity of an electrical cable.
- In the case of wall mounting ensure that the wall supports greater than fourtimes the weight of the level transmitter.
Further, the installation of Ultrasonic Level Transmitter is divided into 6 sub-sections:
- The level transmitter must be installed at the normal blanking distance above the highest-level point by confirming the specification for the blanking distance.
- Use of the plastic isolation kit is recommended, note that this kit must be fixed to a rigid support and must not be allowed to swing. The use of stainless steel is not recommended, because it can cause ringing and may increase the blanking distance but mild steel or a suitable plastic can be used instead.
- The transducer must be installed perpendicular to the material surface but not above beams or filling points since it measures with a clear line of sight.
- If the level transmitter must be positioned in the middle of the cone in case of the coned vessel or coned tank receives the true echo.
- If we are using a plastic standpipe, the width of that standpipe must be more. But the base must have a 45-degree chamber to reduce the echo size from the bottom of the standpipe.
- If any large electrical equipment is installed in the vicinity, then an earthed steel conduit must be used.
General Features of Ultrasonic Level Transmitter:
- Range to 50 ft. / 15 m
- The isolated output of 4 – 20 mA with HART 7
- Graphic LCD Display
- Integrated Analytical Software
- 5 Configurable Relays / 8 Amp
- Configurable as Open Channel Flow Meter
- Pump Cycling and Pump Monitoring
- Remote Totalizer Count Outputs
- Automatic changeable Gain & Power for grueling Applications
Describe various types of Ultrasonic sensors:
There are two types of ultrasonic sensors
- Proximity Detection: Here, a moving object is detected within its preset range and generates the proportional output signal. This detected point doesn’t depend upon the size, reflectivity, and material.
- Ranging Measurement: Precise length of a moving object to the sensor, and from the sensor is calculated through time intervals between transmitted and reflected ultrasonic pulses. change in length is computed continuously and displayed.
What types of sensors are suitable for liquid-level measurement?
Depending upon the industrial application sensors suitable for liquid-level measurement
- Point-Level Sensors: These are known as Digital Sensors these sensors sense the height of a single liquid at its preset level. These sensors are well suited for use of high or low alarms and flags either overfill conditions or at the low level, indicating liquid levels below the desired threshold.
- Continuous-Level Sensors: These are known as Analog Sensors, these sensors provide monitoring of the level of an entire system. They measure liquid levels in a range, rather than at a single point, outputting an analog signal that correlates to the liquid level inside your tank or vessel.
Applications of Ultrasonic Level Transmitter:
- Various Liquids Including Water Storage Tanks
- Medium solid Range like Loading & Bagging Hoppers
- Mineral Oil
- Open Channel Flow Measurement in Weirs and Flumes.
Advantages of Ultrasonic Level Transmitter:
- Ultrasonic-level transmitters are easy to install on empty tanks or on tanks containing liquid.
- Set up is simple
- Programming can be configured in minutes.
- Measurement is not affected by changes in process pressure.
- These devices are maintenance free
- Highly resistant to corrosion from condensing vapors
- The level measurement is not affected by changes in the density of liquid, dielectric, or viscosity because the device is a non-contact type.
- Measurement performance is well on aqueous liquids and many chemicals.
Disadvantages of Ultrasonic Level Transmitter:
- The ultrasonic level measurement is comparatively expensive.
- Materials of construction generally limit the process temperature to around 158 °F (70 °C) and pressure to 43 psi (3 bar).
- The presence of foam on the surface causes unreliable measurement because foaming will often dampen out the return echo.
- It’s not very useful if the tank is deep or if the echo is absorbed or spread out. The object shouldn’t be one that absorbs sound.
- It is also not good for tanks with too much smoke or high-density moisture.