Selecting the right sensor for your bin-level monitoring application can seem like ordering off of a menu when you don’t speak the language. There are volumes of information about different types of continuous-level measurement technologies and technical data about how they work.
Although helpful, it’s likely you don’t have time to digest so much information, and bin-level sensors are only a fraction of the equipment you need to worry about. This article will give you high-level knowledge about your options, and let you know what you need to know before calling someone to discuss your needs.
Your investment in being prepared with a few facts will pay off in ruling out technologies that won’t work in your application or won’t fit your budget. When you contact your vendor, you’ll save time if you’re prepared with 10 key pieces of information about your application:
A few other things you might want to consider for continuous inventory management are:
When the level of material needs to be monitored on an ongoing basis and the information needs to be accurate, continuous level measurement sensors can output data to a console, using specialised software, send the information to a PLC or the Internet for anywhere, anytime access.
Advanced systems can report the data from all of the bins on site or multiple sites, making it easy to monitor inventory status for an entire operation.
Radar, ultrasonic, and 3D Level Scanners have a default blanking distance commonly referred to as a dead zone. The sensor does not account for or measure that distance. So, you need to account for this distance when setting up the device to trigger alerts when the full level is reached.
For example, if using a 3DLevelScanner, the area from the process connection to the bottom of the device (19 inches) is a dead zone. A measurement to the bottom of the dead zone would be considered a full tank. Dead zones can be increased if a lower full point is desired. Most manufacturers have the dead zone preset in the controller, based on the unit selected.
A weight-and-cable sensor, or plumb bob sensor, works like an automatic measuring tape, without the danger and hassle of climbing bins to take measurements. The sensor is mounted on the top of the bin, generally 1/6 of the way in from the outer perimeter for the best accuracy.
The sensors are programmed to take measurements at predetermined intervals, such as every 30 minutes, once an hour, every 6 or 8 hours, or once a day. Plumb bob measurements are highly accurate, taking the measurement in the same location with reliable repeatability. Depending on the system selected and operational needs, data is sent to a PLC, console, PC, or to the Internet.
Measuring range is from the tip of the bob (also referred to as a probe or weight) when the cable is fully retracted to where the bob contacts material at the bottom of the vessel. The dead zone is minimal, just 4 to 8 inches measured from the process connection to the tip of the sensor probe hanging from the cable when the unit is fully retracted.
Weight-and-cable sensors measure headroom from a single point on the material surface directly below the sensor’s mounting location.
The acoustic technology used in sensors like BinMaster’s 3DLevelScanners, generically called scanners, differs greatly from other types of sensors. As the name implies, these devices scan the material surface to take multiple measurements, taking into account the high and low spots in the silo.
The data from multiple measuring points is processed using advanced firmware and algorithms, and when combined with the silo’s parameters loaded into the software, provides highly accurate level and volume information.
The measuring range starts at 19” below the threads on the process connection (upper dead zone).
Unlike any other technology, the 3DLevelScanner takes measurements from multiple points within the silo. These points consider irregular material topography to determine the volume of material in the bin. Measurement points are not simply averaged to calculate bin volume – instead, an advanced algorithm assigns each point a “weight” to determine the true volume of material in the bin.
Guided wave radar utilises time domain reflectometry (TDR) to measure the distance to the material by sending a low-power microwave signal along a cable and calculating the level based on the time of flight.
Guided wave radar is used to measure powders, bulk solids, and liquids. Depending on the material's characteristics, different diameters and lengths of cables are used. Measurement data is output to a PLC, a graphical display on the device, or a local display unit.
The measuring range generally starts from 14” to 36” below the threads on the process connection (upper dead zone), although some of the newer models available state smaller dead zones. Guided wave radar also has a lower dead zone, generally about 4” above the top of the counterbalance weight. It measures the level of headroom at a single point where the cable is located in the vessel to the top of the lower dead zone.
Open-air radar transmits radio-frequency (RF) energy to the material surface, and the energy is reflected back, much like sound waves. A small portion of the reflected energy returns to the radar. This returned energy, which is called an echo, is processed to determine the distance to the material in the bin.
There are many different models of open-air radar devices, using different types of antennas and operating frequencies, primarily ranging from 6 GHz to 76 GHz. The model of open-air radar device that will perform successfully in operation will depend on the material and container parameters.
The measuring range varies, with the upper dead zone generally ranging from 14” to 36”, depending on the type of antenna and horn installed on the device. Open-air radar measures the level of headroom at a single point on the material surface directly below where the unit is aimed.
For liquids, it is generally pointed straight down (vertical), and for bulk solids, it is aimed at the discharge to prevent the signal from bouncing off an angled hopper bottom, as this can cause false reflections.
Ultrasonic sensors are used for continuous, non-contact level measurement in tanks, bins, silos, and conveyors. They work by transmitting an ultrasonic pulse of pressurised air to the material's surface in a vessel.
The pulse reflects off the material and returns to the sensor in the form of an echo that is received by a microphone. The sensor then sends the measurement data directly to a control system or display module, with some systems allowing data to be sent to a PC running utility and diagnostic software.
The measuring range generally starts from 4” to 14” below the threads on the process connection (upper dead zone). Ultrasonic measures the level of headroom from a single point on the material surface directly below where the unit is aimed.
For liquids it is generally pointed straight down (vertical), and for bulk solids it is aimed at the discharge to prevent the signal from bouncing off an angled hopper bottom causing false reflections.
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