Open-air Radar
An open-air radar sensor transmits a radio-frequency signal to the material surface, which reflects a small portion of the signal back to the sensor’s antenna. The sensor processes this returned signal to determine the material’s level. The sensor’s antenna is typically aimed at the vessel’s discharge to prevent the signal from reflecting off the angled bottom when the vessel is nearly empty, which could cause false measurements.
Sensor models are available with different antenna types and operating frequencies (typically ranging from 6 GHz to 80 GHz). Which model will perform successfully in an application depends on the vessel height, the material being measured, the presence or absence of dust, and the sensor’s operating frequency.
The sensor’s measuring range varies depending on the operating frequency. Sensors with frequencies of 26 GHz or less can measure up to about 100 feet (approx. 30.5 meter) , while a 80 GHz sensor can measure almost 400 feet (approx. 122 meter). 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. All open-air radar sensors measure the material level at a single point where the sensor is aimed.
Pros
The open-air radar sensor:
- Provides continuous level measurement
- Is nonintrusive and doesn’t contact the material
- Can – with high frequency (80GHz) models – be used in vessels up to 393 feet (approx. 120 meter) tall
- Can – with high frequency (80 GHz) models – be precisely targeted to avoid structures
- Updates quickly for accurate tracking of filling or emptying activity
- Is versatile for use in solids, liquids, and slurries
- Is not affected by corrugation
- Is virtually unaffected by changes in process temperature, pressure, or material bulk density
Cons
The open-air radar sensor:
Measures a single location
Must be carefully located and pointed to desired measurement location
May require air purging in lower frequency models potentially requiring a supply of compressed air be run to the sensor
May not perform reliably in very dusty environments with lower frequency models. 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.
| Accuracy of a Single Point Inventory Measuring System |
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1. Printed Measured Accuracy: The “printed accuracy stated” of any single point level device is the measured distance from the sensor to the material surface. This is not the accuracy of the conversion from a measure of distance to volume or mass. 2. Volume: When converting measured distance from the sensor to the material surface to volume, the calculation is based on the internal vessel dimensions and the measured level of material at a point on the material surface. Inaccurate vessel geometry will increase the overall error in the volume calculation. The placement of the sensor and the location of the fill inlet and discharge outlet will also have an impact on the overall accuracy of volume. 3. Mass: When converting volume to mass (weight), the bulk density will have a large impact on the accuracy when converting volume to mass. There are several considerations involved to obtain an accurate assessment of the bulk density.
There will always be error associated with the conversion of distance/volume/mass when using a single point level measuring system. The measured distance of most single point technologies will be around ± 0.25% of the distance measured. The calculated volume accuracy will be dependent upon the accuracy of the vessel dimensions, sensor placement, and location and number of fill/discharge points. A vessel with center fill/center discharge with material that flows symmetrically will give the best results when using a single point measuring device. It is very challenging for any manufacture of single point level measurement devices to accurately pinpoint the exact calculated value of mass. With accurate vessel geometry, strategic placement of the sensor, and a good average bulk density, the accuracy of mass may be figured around ±8-15%. |
Ultrasonic
An ultrasonic sensor transmits an ultrasonic pulse of pressurized air to the material’s surface. The pulse reflects off the material as an echo and is received by a microphone in the sensor. The sensor is generally aimed at the vessel discharge to prevent the signal from bouncing off a nearly empty vessel’s angled hopper bottom and causing false measurements.
The ultrasonic sensor’s dead zone is typically 4 to 14 inches (approx 102 to 355 mm), and its measuring range is typically limited to about 40 feet (approx. 12.2 meter), but can be higher in some low frequency models. The sensor measures the material level at a single point on the material’s surface. Ultrasonic performs best in liquids and is commonly used for continuous level measurement in tank inventory management.
Pros
The ultrasonic sensor:
- Provides continuous level measurement
- Is nonintrusive and doesn’t contact the material
- Performs well in liquid applications
- Is available in high-temperature models up to about 300°F (approx 150°C)
- Requires minimal maintenance
- Is easy to install and calibrate
- Has a relatively low purchase cost compared to the other sensors
Cons
The ultrasonic sensor:
- Measures a single vessel location
- Typically measures only up to about 40 feet (approx. 12.2 meter)
- Is not generally recommended for powders or solids
- Doesn’t perform well in dusty conditions or with pressure fluctuations, vessel turbulence, or large particle sizes
- Isn’t recommended for vessels containing steam
- May not perform well in high-pressure vessels
