Selecting Continuous Level Sensors: A Practical Approach
Although helpful, it’s likely you don’t have time to digest full-line brochures and long-winded product articles to determine which bin level sensors your operations need.
BinMaster is here to arm you with high-level knowledge about your options and what to consider before calling someone—or us—to discuss your needs. Being prepared will pay off, easily helping you rule 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 ten key pieces of information about your application.
- What material(s) are you measuring?
- What`s the bulk density of the material in lb./ft.³?
- Does the material tend to be sticky or create buildup?
- Is the material corrosive?
- What is the amount of moisture present?
- What are the temperature and pressure in the silo?
- Are excessive noise or vibration present?
- Is dust, foam, steam, or vapor expected to be within the vessel?
- What`s the size and shape of the silo?
- Are there limitations to where the sensor can be mounted on the silo?
A few other things you might want to consider for continuous inventory management are:
- How often do you need to measure or access the data?
- How many people need access to the data? How will the information be shared?
- Is viewing one bin at a time okay, or do you need to monitor multiple bins simultaneously?
- Do you need notifications or alerts if levels reach a certain high or low point?
- What are your budgetary constraints for equipment?
When the material level 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 specialized software, then 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.
What is the "Dead Zone?"
Radar, ultrasonic, and 3DLevelScanners have a default blanking distance, which is commonly referred to as a dead zone. That distance is not accounted for—or measured by—the sensor. When setting up the device, you`ll need to account for this distance 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. 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.
Types of Continuous Sensors
Weight & Cable
A SmartBob® weight-and-cable sensor, or plumb bob, works like an automatic measuring tape—minus the danger and hassle of climbing bins to take measurements. The sensor is mounted on the top of the bin, generally 1/6 inward 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 six or eight hours, or once a day. SmartBob measurements are highly accurate, taking a measurement in the same location with reliable repeatability. Depending on the system selected and operational needs, data can be sent to a PLC, console, PC, or to the Internet.
The measuring range is from the tip of the bob 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 the level of headroom from a single point on the material surface directly below the sensor’s mounting location.
Please see PDF for the detailed pros & cons list.
3D Acoustic Sensor
The technology used in sensors like BinMaster’s exclusive 3DLevelScanner is very different from other types of sensors. As the name implies, these devices scan the material surface and take multiple measurements, accounting for the uneven high and low spots within the silo.
The data from multiple points is processed using advanced firmware and algorithms. When combined with the silo’s parameters—pre-loaded into the software—the data provides high-accuracy level and volume information. Measurement points are not simply averaged to calculate bin volume; instead, each point is assigned a “weight” to determine the true volume of material in the bin.
The measuring range starts at 19” below the threads on the process connection—known as the upper dead zone.
Please see PDF for the detailed pros & cons list.
Guided Wave Radar Sensors
Guided wave radars utilize time domain reflectometry (TDR) to measure the distance to the material by sending a low-power microwave signal along a cable, then calculating the level based on time of flight.
These sensors are used to measure powders, bulk solids, and liquids. A variety of different diameters and lengths of cables can be used, fully dependent on the characteristics of the bulk material being measured. Measurements are 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 upper dead zone, though some newer models on the market 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.
Please see PDF for the detailed pros & cons list.
Open-Air Radar
Open-air radar sensors transmit radio-frequency (RF) energy to the material surface. A small portion of the reflected energy returns to the radar. This returned energy—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 80 GHz. Measuring range varies, with the upper dead zone generally ranging from 14” to 36”; it is dependent 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, 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.
Please see PDF for the detailed pros & cons list.
Ultrasonic Transmitters
Ultrasonic sensors are used for continuous, non-contact level measurement in tanks, bins, silos, and conveyors. They transmit an ultrasonic pulse of pressurized air to the material surface within a vessel.
This pulse reflects off the material and returns to the sensor in the form of an echo. 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 upper dead zone.