- Level sensors overview
- Technology overview
Technology overview - level sensors
Impedance spectroscopy
Deposits and foam often make reliable level detection difficult. The impedance spectroscopy technology measures the electric and magnetic field strength at multiple frequencies between 50…200 MHz range. Each medium creates a unique signature profile across this high frequency spectrum sweep. At each point, three measurements are made:
- Attenuation (dampening) of the electromagnetic field
- Conductance of the electric field (ability to conduct an electric current)
- Permittivity (ability to polarize particles) of the magnetic field
When the medium is present, these measurements match the profile. With no medium present or only residue present, the measurements do not match. When the measured profile falls in the green switching zone, the output of the sensor changes state.
No media present: The image above shows the situation with no media covering the sensor tip. There is low attenuation, low conductivity and low permittivity. The signature profile is outside of the switching zone.
Media present: This next image shows the profile when media is present on the tip. Attenuation, conductivity and permittivity are all high and the measured profile is within the switching zone. The output changes state.
Residue present: When only residue is covering the tip, conductivity and permittivity are high because there are traces of the medium present. But attenuation is low because the amount of medium is small. The profile is outside of the switching zone and the output does not change state.
Other media have different profiles. Using IO-Link, the medium process values can be evaluated and used to distinguish one material from another, i.e. oil vs. water, whole milk vs. 2% milk, etc.
Features:
- Suppression of residue build-up and foam.
- Flush sealing PEEK sensor tip meets 3A requirements.
- Stainless steel body for robustness.
All versions are programmable, but factory default settings are available for water-based media, oil-based / powder media, and high sugar-content media.
Guided wave radar (gwr)
The gwr operating principle uses electromagnetic pulses in the nanosecond (microwave) range. The sensor head transmits the pulses and the pulses travel down the metal probe (guide). When the wave hits the medium, it is reflected back, collected by the metal probe, and guided to the sensor head. The time difference between the transmitting and receiving pulse (time-of-flight) is directly proportional to the distance measurement.
For proper decoupling of the radar pulse, a metal launching plate at least 150 mm² or 150 mm diameter is required. If the tank has a metal lid, that can act as the launching plate.
The image above shows a tank with a metal lid. No launching plate is required because the lid acts as the launching plate.
In a tank with a plastic lid, a metal launching plate is required. Shown is a flange that is at least 150 mm in diameter.
In an open tank, a launching plate is also required. An easy way to accomplish this is to bolt a flange onto a metal angle.
For oil-based media, the fluid surface does not reflect the radar pulse as good as water. To intensify and contain the signal, we must use a coaxial tube accessory.
When using the coaxial tube, the launching plate as described above is not necessary. This makes mounting easier. However, bridging between the probe and coaxial tube due to solids, emulsions, etc. can cause false level indication. The coaxial tube can be used with water-based media as well and the tube can be cut to length to match the probe.
Features:
- 3A authorized Clean-Out-of-Place (COP) design for some models
- Pressure rating up to 40 bar for some models
- Stainless steelmaterials of construction
- Immune to dust, fog and steam
Hydrostatic pressure
Hydrostatic pressure is the force per area exerted by a column of liquid and it is a function of the height of the container, not the container’s overall shape or volume. The equation for hydrostatic pressure is:
If the density and specific gravity of the fluid is known, the height (or level) of the fluid can be determined from the hydrostatic pressure measurement.
A common hydrostatic pressure application is measuring the level of liquid in a closed tank. A blanket of inert gas may be used to prevent the liquid from oxidizing, such as CO2 on top of a tank of beer. In this case, differential pressure can be calculated using two pressure sensors. The one at the top measures the gas pressure and the one at the bottom measures the gas pressure plus the pressure due to the liquid. The pressure of the liquid only (and therefor the level of the liquid) is the difference in the two measurements.
Capacitive point level (Kxxxxx article numbers)
Capacitive sensors detect any material with or without contact. With ifm’s capacitive proximity sensors, the user can adjust the sensor’s sensitivity to detect liquid or solids even through non-metal tanks.
For successful level detection using capacitive sensors, make sure:
- The vessel wall is non-metal
- The vessel wall is less than 6 – 12 mm thick
- There is no metal in the immediate vicinity of the sensor
- The sensing face is placed directly on the vessel wall
- Both the sensor and the vessel are grounded at the same potential
Capacitive continuous level (Lxxxxx article numbers)
ifm’s LK and LT continuous level sensors consist of 16 individual capacitive sensors that are stacked and multiplexed.
Each cell evaluates its surroundings to determine if it is covered by the media. The microprocessor evaluates all 16 cells to determine the media level.
The LK and LT families have built-in overflow protection. The algorithm that monitors overflow is independent from the general level measurement. This way, if the outputs fail to switch as desired and the level continues to rise, the overflow protection forces the outputs to switch.
Additionally, the LT series provides a separate output for medium temperature.
Ultrasonic
Ultrasonic sensors rely on the detection of sound waves reflecting off the surface to measure level. The medium surface reflects sound waves and the distance is determined via time-of-flight measurement.
Unlike photoelectric sensors, the medium’s colour, transparency and reflectivity do not affect ultrasonic technology.
Ultrasonic sensors exhibit a high degree of immunity to moisture and dust. The sensing face vibrates at a very high frequency and sheds excess moisture and dust before these substances can negatively affect the performance. However, temperature extremes may affect the accuracy since the speed of sound varies with temperature.
Photoelectric
The O1D laser distance sensor and O3D vision sensor use pmd time-of-flight technology to measure the distance to the medium surface. The time-of-flight principle monitors the time it takes for a photon of light to travel to the surface and back. The signal is then processed by a receiver element.
This technology is not appropriate for measuring the level of clear liquids. It can be used only for opaque liquids and solids.
Radar
The device works according to the FMCW method (FMCW = Frequency Modulated Continuous Wave). Electromagnetic pulses in the GHz range are sent to the medium at a constantly changing frequency between 77 and 81 GHz. Since the transmitter continuously changes the frequency of the transmitted signal, there is a frequency difference between the transmitted and the reflected signal. The frequency of the reflected signal is subtracted from the frequency of the signal transmitted at that time, resulting in a low frequency signal proportional to the distance to the level. This signal is further processed in order to obtain fast, reliable and highly accurate level measurements.
What is the advantage of 80 GHz?
Antenna size and frequency are the two main factors, which are decisive for the range resolution and accuracy of a radar sensor. Basically:
- The smaller the antenna, the bigger the radar opening angle
- The higher the frequency, the lower the wavelength
The figure shows: The high frequency 80 GHz technology enables a comparatively small opening angle using a small antenna size.
More signal, less interference
Higher focusing of the strong signal through the small opening angle enables the detection of low dielectric media, as the high focus increases the reflection to the sensor. The high focusing also prevents the detection of agitators and jet cleaners which would lead to signal interference.
High resolution and accurate level measurement of the whole tank height
For applications such as industrial level sensing, range accuracy (down to millimeter) is a key priority. The accuracy of measurements and range resolution (i.e. how precise changes to the level are detected) depends on the emitted frequencies. The wide bandwidth available in the 77 to 81 GHz band makes range measurements very accurate. 80 GHz radar sensor can achieve 20x better performance in range resolution and accuracy compared to a 24 GHz radar. Also, the high resolution helps separate the fluid level from any unwanted reflections at the bottom of the tank. This enables the sensor to accurately measure the fluid level over the whole tank height, minimizing the blind zone at the bottom of the tank. And since high resolution improves the minimum measurable distance, it helps measure the fluid level until the very top of the tank when the tank is full.