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Selecting Continuous Level Sensors: A practical approach

Selecting Continuous Level Sensors: A practical approach
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Selecting Continuous Level Sensors: A practical approach

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ABOUT CONTINUOUS LEVEL SENSORS

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 arm you with some 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:


  • Material being measured
  • Bulk density of the material in lb/ft³
  • Material’s propensity to be sticky or create buildup
  • Corrosiveness of the material
  • Amount of moisture present
  • Temperature and pressure in the silo
  • Excessive noise or vibration
  • Presence of dust, foam, steam, or vapor
  • Size and shape of the silo
  • Limitations 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 I 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 OK, or do I need to monitor multiple bins simultaneously?
  • Do I need notifications or alerts if levels reach a certain high or low point?
  • What are my 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, and send the information to a PLC or the Internet 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 a dead zone?

Radar, ultrasonic, and 3DLevelScanners have a default blanking distance commonly referred to as a dead zone. That distance is not accounted for, or measured by the sensor. 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. 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.

Weight & Cable

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. SmartBob measurements are highly accurate, taking the measure 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.


The 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 the level of headroom from a single point on the material surface directly below the sensor’s mounting location.

Table 1. Pros and cons of weight-and-cable level sensors

Pros

Cons
Not affected by dust or other adverse process conditionsThe on-demand system does not provide an instantaneous response to changes in the material level
Not affected by material buildup on the sensorSeasonal maintenance may be required to clean out a mechanical cavity in very dusty conditions if air- purge is not used
Performs in extremely light, signal-absorbing materialsNot recommended for use in high-pressure bins
Can be used to measure tall bins with cable lengths up to 180 feet
Not affected by material characteristics, such as low dielectric constant or angle of repose
Sensor requires no calibration
High-temperature models available up to 1,000°F
Low purchase cost relative to most other continuous-level sensors
Very simple setup and installation
Consistent, repeatable, and accurate measurements
Minimal contact with stored material
Networkable PC software can be used for monitoring levels
A variety of digital and analog outputs accommodate different types of operations
Wireless interfaces are available to reduce cabling costs
Hazardous location approvals are available for high-dust environments

3DLevelScanner

The acoustic technology used in sensors like BinMaster’s 3DLevelScanners, generically called scanners, is very different 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 are processed using advanced firmware and algorithms, and when combined with the silo’s parameters loaded into the software, provide a 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 take into account 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.


Table 2. Pros and cons of acoustic (3DLevelScanner) sensors

ProsCoins
Multiple point measurementElevated background noise can impact the performance of acoustics technology
Continuous level measurementSetup requires care in mounting the sensor in the proper location and Accurately mapping the vessel dimensions
Non-intrusive, non-contact designThe time required to process multiple pulse echoes limits the sample rate
Measures uneven powder or solid material surfacesNot recommended for liquid applications
Detects cone up, cone down, and sidewall buildupThe corrugation on small vessel walls can cause false echoes
Provides minimum, maximum, and average distancesNot recommended for materials with a bulk density under 11 lb./cu. ft. due to absorbing the acoustic pulse
Performs in extreme levels of dust
Calculates highly accurate bin volume due to mapping the surface of the material with multiple measuring points
Measuring range up to 200 feet
Self-cleaning with minimal maintenance
Models for high-temperature applications available
Automatic compensation for temperature changes
Analog and digital communication options
Networkable PC software available for multiple vessel monitoring
Can generate a 3D image of material surface
Wireless interfaces are available to reduce the need for cabling
Approved for hazardous locations
Not affected by material characteristics or low dielectric constants

Guided Wave Radar

Guided wave radar utilizes 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. A variety of different diameters and lengths of cables are used dependent on the characteristics of the material. 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.

Table 3. Pros and cons of guided wave radar level sensors

ProsCons
Continuous level measurement in powders, granules, bulk solids, and liquidsThe sensing probe is in constant contact with the material
Performs in vessels prone to dust, humidity, temperature, pressure, and bulk density changesDoes not perform in materials with a very low dielectric constant
Microwave energy is focused and travels along a waveguide, concentrating the radar beam within a small diameterMeasurement range is limited by the maximum cable length stated by the manufacturer
Suitable for vessels of most any shape or diameter, including narrow tanksUse in heavier materials may be limited due to the tensile load on the cable
Can be used on high-pressure vesselsMaterials such as large rocks may damage the probe and be difficult to sense
Models available for high-temperature applications
Highly accurate distance measurement

Open Air Radar
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 an open-air radar device that will perform successfully in an operation will be dependent on the parameters of the material and container. Measuring range varies, with the upper dead zone generally ranging from 14” to 36” 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 (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.


Table 4. Pros and cons of open-air wave radar level sensors

ProsCons
Continuous level measurementLow dielectric materials are difficult to measure as there is not enough radar energy to be reflected from the product surface
Non-intrusive, non-contact designFrequent maintenance for air purging of the horn, requiring air to be run to the top of the bin, plus the cost of air
Ranges generally up to 100 feet for 26 GHz or less, over 300 feet for 76 GHzMay not perform reliably in environments with excessive dust
Models for high-temperature applications are availableSusceptible to condensation and product buildup on the antenna, which may cause signal attenuation that adversely affects performance. The higher the frequency, the more signal attenuation. Higher frequency units have smaller antennas, thus the same level of coating or condensation on a smaller antenna naturally has a greater effect on the performance
In liquid applications, radar is not adversely affected by steam or foamMeasuring distance may be impeded in units with wide beam angle
Measurement is virtually unaffected by changes in process temperature, pressure, density or gas/vapor composition within the vesselCone-bottom vessels can sometimes be problematic when nearing empty. The cone acts as an excellent reflector, throwing energy around the vessel sometimes confusing the transmitter

Ultrasonic transmitters
Ultrasonic sensors are used for continuous, non-contact level measurement in tanks, bins, silos, and conveyors. They work by transmitting an ultrasonic pulse of pressurized air to the surface of the material 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.

ProsCons
Continuous level measurementPerformance affected by dusty conditions, pressure fluctuations, turbulence in a vessel, and large particulate size
Non-intrusive, non-contact designCan be problematic when used for measuring solids
Performs very well in liquids with ranges from a few feet up to 90 feetNot recommended if the steam will be present in the vessel
High-temperature models are offered by many manufacturersWill measure the surface of the foam, if present
Sanitary models are available with tri-clover fittings for use in food and pharmaceutical applicationsPerformance affected by high pressure, follow manufacturer specifications
Tend to be lower purchase costs than radar and acoustics
Self-cleaning transducer face for minimal maintenance
Automatic compensation for temperature changes
Analog and digital communication options
PC software available to diagnose and calibrate sensor is generally available
Very easy to install and calibrate
Most manufacturers offer multiple voltages and units for varying distances

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