How to Select a Continuous Level Sensor

This whitepaper is intended to help operations choose a continuous level sensor without searching through lots of technical data. To maintain efficiency, prevent costly process interruptions and keep you up-to-date on your inventory level, a continuous level sensor can be used to monitor the material level in your storage vessels. This whitepaper explains the types of sensor available, the pros and cons of each type, and explain what information you need to collect before working with a sensor supplier.

A continuous level sensor is ideal for monitoring material inventory in your vessels to prevent downtime. A continuous level sensor constantly calculates the quantity of material in a storage vessel, instead of just indicating if the material is above or below a specific point, like a point level sensor.

The continuous level sensor can output data to a console or panel, depending on the sensor type and supplier. It can send information to a HMI, PLC, or PC, or via SMS text or to the internet for any time, anywhere access.

Data can be reported from multiple vessels at your site or from vessels at multiple sites when utilizing an advanced system using multiple sensors, making it simple to monitor your entire operation’s inventory status. However, choosing the right continuous level sensor for your application can be difficult without prior knowledge.

A vast amount of information is available concerning the different types of sensors and their technologies. A level sensor is one of the numerous pieces of equipment you should be knowledgeable about. To determine which sensor you need, it is important to know some basics about how different sensor types work and their pros and cons.

Prior to learning these basics it’s crucial to understand that a continuous level sensor (which is typically mounted on the top of a vessel) has a default dead zone or ‘blanking distance’ that it cannot calculate which is the area between the vessel’s top and the highest point the sensor can measure. The sensor will indicate that the vessel is full when the material extends to the bottom of the dead zone.

The dead-zone height differs by sensor type but can shift between 4 and 36 inches. Most suppliers preset the dead-zone height in the sensor controller depending on the sensor type, but a dead zone’s height can be made larger if the application needs a lower full point.

Types of Sensor

Types of continuous level sensors include 3D scanners, guided-wave radar, weight-and-cable, ultrasonic, open-air radar, and laser. Each type works differently and has pros and cons.

3D Scanners

The acoustics-based 3D scanner sensor takes measurements at multiple points within the vessel which is unlike other continuous level sensors. The multiple measuring points take into account the material’s irregular surface topography.

The scanner utilizes three independent transducers, which create acoustic sound pulses by converting electric energy. The sound pulses bounce off the material in numerous positions on the material surface, and the sensor records the time it takes for the pulses to return (or echo) back to the transducer.

The 3D scanner has a measuring range of around 200 feet and a 19 inch dead zone. The sensor uses an advanced algorithm that gives each measuring point a weight to concisely establish the material volume and create a 3D image of the material level.

Pros

• Creates a 3D map of the material’s surface and accurately calculate the material volume by measuring multiple vessel points
• Offers a 3D visualization of the vessel topography
• Provides continuous level measurement
• Doesn’t contact the material and is nonintrusive
• Requires minimal maintenance and self-cleans
• Is available in high-temperature models up to about 350 °F
• Measures uneven material surfaces, including sidewall buildup and cone-up or cone-down formation
• Can report maximum, minimum, and average material levels
• Can be used in vessels up to about 200 feet tall
• Is approved for use in hazardous locations
• Isn’t affected by material characteristics
• Works in very dusty conditions

Cons

• Must be carefully located and mounted to accurately map the material surface
• Limited its sample rate, requires time to process multiple sound pulse echoes
• Has a high purchase cost compared to other sensors
• May not perform well in an environment with a lot of background noise
• Isn’t recommended for measuring a material with a bulk density less than 12 lb/ft3 because such a material will absorb the sound pulse
• May not perform well in small vessels with corrugated walls, which can create false echoes

Guided-Wave Radar

A guided-wave radar sensor utilizes time-domain reflectometry in order to calculate the distance between the material and the sensor. A low-power microwave signal is dispatched along a sensing probe (a cable with a counterbalance weight at its end) to function as a wave-guide, concentrating the radar signal within a concentrated diameter around the probe. The sensor calculates the material level based on the signal’s flight time.

Depending on the vessel size and the material’s characteristics, the sensor’s cable diameter and length will vary. The sensor measures the material level at a single, specific location in the vessel (along the cable) between the top of the lower dead zone and the bottom of the upper dead zone.

The guided-wave radar sensor can be used in granules, powders, pellets and other bulk solids, and is usually for vessels up to 100 feet tall. It can be utilized in materials with a dielectric as low as 1.3 dependent on the model. Guided wave radar works in high dust or humidity and is immune to condensation.

Pros

• Provides highly accurate measurements
• Low maintenance
• Can be used in high-pressure vessels
• Supplies continuous level measurement
• Performs well in vessels prone to changes in condensation, dust level, pressure, humidity, material bulk density, and temperature
• Is suitable for almost any vessel shape or diameter including narrow silos
• Is available in high-temperature models up to about 800 °F
• Is quite easy to set-up and install

Cons

• Has a relatively high purchase cost (but usually cheaper than 3D or open-air radar scanners)
• Not all models perform well in materials with a very low dielectric constant
• May not be suitable for use with heavy or abrasive materials, such as large rocks, which are hard to measure and can impose a high tensile load on the cable and damage it
• Has a sensing probe that is always in contact with the material
• Measures a sole point along the cable
• Measuring range is limited, it typically has a maximum cable length of less than 100 feet
• Upper and lower dead zones vary by manufacturer and model

Weight-and-Cable

A weight-and-cable (or plumb bob) sensor acts like an automatic measuring tape: The sensor lowers a cable with a weight (also called a bob or probe) connected to its end into the vessel. It measures the amount of cable that has been released when the weight arrives at the material’s surface to calculate the material’s level. Next, the sensor returns the weight to the vessel’s top by retracting the cable.

The weight-and-cable sensor is programmed to record measurements at predetermined intervals, such as every 30 minutes, once an hour, every 6 or 8 hours, or once a day, so it is not strictly a continuous sensor. The weight-and-cable sensor is precise and reliable. It measures a single point or vessel distance directly below its mounting location and its measuring range can be up to about 150 feet. The dead zone is low, between 4 to 8 inches measured from the weight’s tip when the cable is fully retracted to the sensor’s mounting location.

Pros

• Unaffected by dust or other adverse process conditions
• Can keep the mechanical components clean in very dusty conditions when equipped with an air purge
• Has a low purchase cost in comparison to the other sensors
• Can be used in vessels up to about 150 feet tall
• Is not affected by material buildup
• Is not affected by material characteristics, such as low dielectric constant or angle of repose
• Can measure extremely light, signal-absorbing materials
• Is approved for use in hazardous, high-dust locations
• Is available in models that can handle temperatures up to 1,000 °F
• Is simple to install and set up
• Has little contact with the stored material
• No calibration needed
• Provides accurate, consistent, and repeatable measurements

Cons

• Measures a single point or vessel location distance
• Doesn’t respond to material-level changes instantaneously
• Can require periodic maintenance
• Shouldn’t be used in high-pressure vessels

Ultrasonic

An ultrasonic sensor releases an ultrasonic pulse of pressurized air onto the surface of the material. The pulse bounces off of the material as an echo which is then received by the sensor's microphone. The sensor is normally pointed at the vessel discharge to prevent false measurements caused by the signal reflecting off a nearly empty vessel’s angled hopper bottom.

The ultrasonic sensor’s dead zone is usually between 4 to 14 inches, and its measuring range is generally restricted to around 40 feet. It can be higher in some low frequency models. The sensor calculates the material level at a specific point on the material’s surface. Ultrasonic is perfect for use in liquids and is frequently used for continuous level measurement in tank inventory management.

Pros

• Requires hardly any maintenance
• Provides continuous level measurement
• Is available in high-temperature models up to about 300°F
• Has a relatively low purchase cost compared to the other sensors
• It doesn’t contact the material and is nonintrusive
• Excellent for liquid applications
• Is easy to calibrate and install

Cons

• Is not generally recommended for powders or solids
• Isn’t recommended for vessels containing steam
• Measures a single vessel location
• May not perform well in high-pressure vessels
• Typically restricted to measurements below 40 feet
• Doesn’t perform well in dusty conditions or with large particle sizes, pressure fluctuations, or vessel turbulence

Open-Air Radar

An open-air radar sensor transmits a radio-frequency signal to the material surface. This reflects a small part of the signal back to the sensor’s antenna. The sensor uses the signal that is returned to calculate the material’s level. The sensor’s antenna is normally aimed at the vessel’s discharge. This prevents the signal from reflecting off the angled bottom when the vessel is almost empty, which could result in false measurements.

Various operating frequencies (typically ranging from 6 GHz to 80 GHz) and antenna types are available. The model to be employed in an application depends on the material being measured, the vessel height, the sensor’s operating frequency, and the presence or absence of dust.

The sensor’s measuring range differs based on the operating frequency. A 26 GHz radar measures in a 10° beam angle, while a 80 GHz radar measures in a very focused 4° beam angle that is ideal for precise targeting. Sensors with frequencies of 26 GHz or less can measure up to about 100 feet, while a 80 GHz sensor can measure almost 400 feet. All open-air radar sensors measure the material level at a single point depending on where the sensor is aimed.

Pros

• Is versatile for use in liquids, solids, and slurries
• Is virtually unaffected by changes in process temperature, pressure, or material bulk density
• High frequency (80 GHz) models can be precisely targeted to avoid structures
• Provides continuous level measurement
• Is nonintrusive and doesn’t contact the material
• High frequency (80GHz) models can be used in vessels up to 393 feet tall
• Is not affected by corrugation
• Updates quickly for accurate tracking of filling or emptying activity

Cons

• The high frequency 80 GHz radar has a flush lens antenna highly resistant to product buildup, eliminating the need for air purge and making it suitable for use in dust. May not perform reliably in very dusty environments with lower frequency models.
• Measures a single location
• May need air purging in lower frequency models, which could mean a supply of compressed air needs to be run to the sensor
• Must be carefully located and pointed to the chosen measurement location

Laser

A laser sensor is mounted on top of the silo using an adjustable 10° mounting flange for aiming the laser beam to the chosen output location. Maximum and minimum measuring distances are configured on the sensor using 4 and 20 inputs.

The sensor dispatches timed laser pulses to the surface of the material and the distance is then measured by utilizing complex algorithms which convert the laser pulses to a data output. Based upon the angle of the beam to ensure accurate level measurement, a compensation for “slant range” is made.

Laser does not perform reliably in high dust, is for low or no dust environments. It is perfect for level control in narrow vessels which contain solids or plugged chute detection. Laser can also be utilized in hoppers and restrictive chutes where exact targeting is needed. When installed above or pointed to the sidewall, it can also be used to monitor buildup. Laser sensors can operate in opaque liquids in vessels where the beam must be precisely targeted to avoid walls or structure.

Pros

• Is simple to configure in the field using a USB port
• Possesses a fast update rate of 8 times per second
• Has a narrow beam that can be directed to avoid obstructions
• Can also be applied in opaque liquids
• Can be configured without emptying or filling the vessel
• Has built-in dust protection for minimal maintenance

Cons

• May require an air purge option to keep lenses free of dust for reliable performance
• Is not recommended for liquids with excessive vapor that is too opaque for the laser to “see through”
• Only measures a single point on the material surface
• Does not measure accurately in dusty environments
• Is subject to interference from falling materials

Accuracy of a Single Point Inventory Measuring System

1. Printed Measured Accuracy

The measured distance from the sensor to the material surface is the “printed accuracy stated” of any single point level device. This is not the accuracy of the conversion from a measure of distance to volume or mass.

2. Volume

The calculation is based on the internal vessel dimensions and the measured level of material at a point on the material surface when converting measured distance from the sensor to the material surface to volume. Incorrect vessel geometry will add to the overall error in the volume calculation. The sensors placement and the location of the discharge outlet and fill inlet will also have an effect on the overall accuracy of volume.

3. Mass

The bulk density will have a big impact on the accuracy when converting volume to mass (weight). There are a number of factors involved when obtaining an accurate evaluation of the bulk density.

• A material’s general name (e.g. polyethylene or cement) gives little information about its bulk density. An estimated number corresponding with a name may be inaccurate by 50% or more depending on the specific factors.
• The specific grade (composition), the particle size distribution, and moisture or volatile content affect the material’s bulk density. Tendencies towards segregation when handling should also be examined.
• A material’s density will fluctuate depending how much it is compacted or fluidized.

It is crucial to use an average – not the stated amount designated to the materials general name to calculate bulk density. This is possible by taking a measurement before and after a known weight load is put in the vessel and adjusting the bulk density to match this weight.

When using a single point level measuring system there will always be error related to the conversion of mass/distance/volume. Usually, the measured distance of most single point technologies will be about ± 0.25% of the distance measured.

The calculated volume accuracy will depend on the accuracy of sensor placement, the vessel dimensions, and location and number of fill/discharge points. When using a single point measuring device, a vessel with center fill/center discharge with material that flows symmetrically will give the best results.

It is difficult for any provider of single point level measurement devices to accurately pinpoint the exact calculated value of mass. With strategic placement of the sensor, accurate vessel geometry, and a good average bulk density, the accuracy of mass may be figured around ±8-15%.

Accuracy of a Multi-point Inventory Measuring System

Unlike standard devices that establish a single distance by measuring one point, the 3DLevelScanner takes measurements from multiple points within the silo. These points are used to measure the volume of material in the bin. Each point is allocated a “weight” or strength of accuracy rating assigned by an algorithm to determine the true volume of material within the bin, instead of being averaged to calculate bin volume.

This technology considers variations that can happen on material surfaces by mapping the high and low points. The 3DLevelScanner gives an accurate profile of the top surface within a storage vessel. This is useful with materials that do not fill/discharge symmetrically, or when differences occur in the material surface due to multiple fill and discharge points.

The volume accuracy is still dependent upon the accuracy of the vessel dimensions, and sensor placement with the 3DLevelScanner. Due to bulk density variables, when converting the volume to mass there will still be inherent inaccuracies. However, the improved accuracy of the volume calculation will mean determining the correct mass calculation is more accurate.

A volume accuracy of ±3-5% can be expected given correct vessel geometry and proper sensor placement. With a good average bulk density, the accuracy of the mass may be around ±5-10%.

Evaluating Your Level Measurement Needs

It is useful to know some basic information about your vessel, your material, and your operation before you contact a supplier to decide which level sensor is best for your application. This preliminary information will help you rule out sensor types that won’t work for your budget or application quickly.

Your Vessel:

• What’s the temperature in the vessel?
• What are the vessel’s dimensions?
• Are there limitations to where the sensor can be mounted?
• Is there excessive noise or vibration in the vessel?
• What’s the pressure in the vessel?

Your Material:

• What is its bulk density in pounds per cubic foot?
• What is the material?
• Is it sticky?
• Does the material create dust, steam, or vapor?
• Does it tend to create buildup?
• What’s the material’s moisture content?
• Is it corrosive?

Your Operation:

• How many people need access to the data, and how will it be shared?
• What is your budget?
• Is monitoring the material level in one vessel at a time okay, or do you need to monitor levels in multiple vessels simultaneously?
• How often do you need to measure the material level or access the data?
• How accurate do you need the measurements to be?
• Do you need a notification or alert if the level reaches a certain high or low point?

This information has been sourced, reviewed and adapted from materials provided by BinMaster.

For more information on this source, please visit BinMaster.