Ultrasonic level measurement sensors basically works with sound waves for detection of liquid level. They usually work over the frequency range between 20 kHz to 200 kHz. Read the following Ultrasonic Level Measurement Questions and Answers.
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An ultrasonic sound pulse is pulsed 4 times per second from the base of the transducer.
The pulse travels through air, reflects against the process medium below and returns to the transducer.
The sensor measures the pulse time of flight between sound generation and receipt, and translated this into distance or liquid level height within the vessel.
Many application liquids have challenging characteristics that negatively impact contact type sensor performance, maintenance and/or replacement costs.
Examples include corrosive, coating, scaling, viscous and dirty liquids. Ultrasonic sensors are non-contact, and therefore impervious to these characteristics.
The application variables to avoid with ultrasonic are extreme foam, vapor, turbulence or installation in tall fittings with narrow risers.
Foam, vapor and turbulence can absorb and/or deflect away a substantial portion of the return signal.
Tall riser fittings can disrupt the acoustic signal path. Appropriate product selection and installation is important.
The 4-20 mA represents the sensor’s measurement span.
The 4 mA set point is normally placed near the bottom of the empty tank, or the greatest measurement distance from the sensor. The 20 mA set point is normally placed near the top of the full tank, or the shortest measurement distance from the sensor.
The sensor will proportionately generate a 4 mA signal when the tank is empty and a 20 mA signal when the tank is full.
Analog 4-20 mA signals have the following advantages:
The sensor can be installed up to feet from its point of termination using a shielded, 18-20 gauge twisted pair cable and 24 VDC power supply.
The total loop resistance should not exceed 900 ohms.(Changes from vendor to vendor).
While the 4 mA and 20 mA set point configuration is not fundamentally required, it’s recommended because it optimizes the sensor’s installed performance.
Yes, the sensor can be configured such that the 20 mA set point is empty and the 4 mA set point is full.
Yes, as long as the total loop resistance is less than 900 Ohms (Changes from vendor to vendor).
NPT (National Pipe Threads) are the American standard tapered threads which are applied throughout North America, South America and parts of Asia. G (metric) ISO threads are the European standard straight threads applied throughout Europe and parts of Asia.
Select a sensor range that covers the tank height with at least 25% margin.
In challenging applications with light foam, vapor and/or turbulence, select a sensor range that is double the tank height. The additional sensor will improve performance in such conditions.
Electronic enclosures are fire retardant polycarbonate, cables are polypropylene and transducers are PVDF. The mounting gasket is Viton.
The sensor should be located so that the acoustic signal path is free of obstructions from side-walls, ladders, pipes, mixers and other instruments.
The sensor should be mounted level and/or perpendicular to the liquid surface. If substantial foam, vapor and/or turbulence are present, install the sensor in a stand-pipe.
The sensor can be broadly installed in tank adapters, couplings, flanges, brackets and stand pipes of appropriate size relative to the transducer.
Avoid fitting types that substantially recess the transducer into the fitting or compress the transducer against an inner pipe stop. Use plastic fittings whenever possible.
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In most installations, the sensor is hand tightened only. If you use a wrench or tool, be careful not to over-tighten the sensor within the fitting.
The sensor can be elevated up to a maximum of 12″ high in a 6″ diameter riser, 8″ high in a 4″ diameter riser, 6″ high in a 3″ diameter riser and 3″ high in a 2″ diameter riser.
Sensors with a 1″ transducer should be installed in 2″ stand-pipes. Sensors with a 2″ transducer should be installed in 3″ stand-pipes.
On plastic tanks, use a plastic self-aligning bulk head fitting that enables the sensor to be rotated perpendicular relative to the liquid surface. On metal or fiberglass tanks, use a plastic flange fitting with a short riser that accommodates for the slope.
The side mount bracket is a good choice. Users can also fabricate equivalent brackets based on custom size or material requirements.
Typically only one sensor is installed per tank. If you wish to install two sensors, the signals must be isolated to avoid cross-talk interference. This can be done by installing one or both sensors in a stand-pipe.
NEMA 6 enclosure sensors are designed to resist infrequent submersion for short periods of time.
NEMA 4 enclosures are designed to resist frequent splashing the equivalent of direct hose spray and should never be submerged. Generally speaking, the sensor should be located to avoid submersion and/or direct spray under normal conditions.
No, the sensors have non-volatile memory which can only be erased by performing a factory reset.
Factory reset erases any previous user configuration and returns the sensor to its original factory set points.
Check the wiring for an open circuit and verify the power supply.
Check the installation fitting to ensure that its consistent with our guidelines. If the sensor is installed in a metal fitting, switch it to a plastic fitting.
Check the fill height configuration set point. Make sure that the fill height setting corresponds to the tank full level (as measured from the empty tank bottom up), and not the distance from the installed sensor to the liquid level (as measured from the sensor down). what is range, tank height, fill height and dead band?
Check to make sure that the sensor is installed correctly. Check to make sure that there is no surface foam or obstructions within the beam path. If so, install the sensor in a stand-pipe to separate its point of measurement from the surface foam, eliminate the obstruction or relocate the sensor.
Ultrasonic sensors are devices that generate ultra-high-frequency sound waves greater than 20 kHz, beyond the range of human hearing. They use electrical energy and a ceramic transducer, which both transmits and receives sound waves. Sound waves are essentially pressure waves that travel through solids, liquids and gases and can be used in industrial applications to measure distance or detect the presence or absence of targets.
The sensor has a ceramic transducer that vibrates when electrical energy is applied to it. The vibrations compress and expand air molecules in waves that travel from the sensor face to a target object. The ultrasonic sensor measures distance by emitting a sound wave and then "listening" for a set period of time, allowing for the return echo of the sound wave bouncing off the target, before retransmitting.
Because ultrasonic sensors use sound rather than light for detection, they work in applications where photoelectric sensors may not. Target color and/or reflectivity don't affect ultrasonic sensors, allowing them to operate reliably in high-glare environments. Ultrasonic sensors are also a great solution for clear object detection and for liquid level measurement.
Ultrasonic sensors offer advantages when sensing clear objects, liquid level or highly reflective or metallic surfaces. Ultrasonic sensors also function well in wet environments whereas an optical beam may refract off the water droplets. However, ultrasonic sensors can be susceptible to temperature fluctuations or wind, things that don’t affect radar sensors. With optical sensors, you can also have a small spot size, fast response, and in some instances, you can project a visible spot on a target to help with sensor alignment.
Because ultrasonic sensors use sound waves, any acoustic noise at the same frequency as the ultrasonic sensor uses may interfere with the sensor's output. This includes high-pitched noises such as those created by a whistle, and the hissing of relief valves, compressed air, or pneumatic devices. You may also experience acoustical crosstalk by placing two ultrasonic sensors of the same frequency close together. Electromagnetic devices create an alternating flow of electrons which makes vibrations. These vibrations can be picked up by electrical devices such as transformers, which create a hum that can interfere with ultrasonic sensors.
Temperature fluctuation affects the speed of an ultrasonic sensor's sound waves. As temperature increases, the sound waves travel faster to and from the target. While the target may not have shifted, the sensor may pinpoint the target as being closer. To solve this temperature problem, many of Banner’s ultrasonic sensors, including the T30UX Series and QS18U Series, compensate for temperature fluctuations to deliver high-accuracy performance across a wide range of ambient temperatures. Air currents caused by pneumatic equipment or fans may also deflect or disturb the path of the ultrasonic wave. This could lead a sensor to fail to recognize the correct location of a target.
When a sensor is first powered up, the individual components heat up and also heat the surrounding space and components. This fluctuation in temperature from a cold start to the operating temperature is called "warm-up drift." Until all components have reached the correct operating temperature, the accuracy of measurements may be affected.
A dead zone refers to the area directly in front of the transducer face where the sensor cannot reliably make measurements. This is due to a phenomenon called ringing. Ringing is the continued vibration of the transducer after the sound wave, or excitation pulse, is sent. The energy must dissipate before the transducer can listen for a return echo. Because of this, targets should be located beyond the specified dead zone of an ultrasonic sensor. Some ultrasonic sensors, including Banner’s S18U Series, have a minimal dead zone and/or have the ability to eliminate the dead zone when used in retrosonic mode.
The best targets for use with an ultrasonic sensor are large, flat, solid surfaces of materials such as metal, ceramic, glass, or wood. They should always be placed perpendicular to the sensor. Soft or irregular surface targets, such as pellets, sawdust, or foam are not detected effectively, and are better served by other types of sensors.
When setting up an ultrasonic sensor, teach the sensor the "background" as your good condition. By teaching an ultrasonically reflective background surface as the good condition, any object that is positioned between the sensor and the background will be detected, therefore causing the output to switch.
For more information about how precision measurement with ultrasonic sensors from Banner Engineering can improve your operations, contact a Banner engineer today.
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