Capacitance level measurement in tanks
Level sensors that work according to the capacitive measuring principle are used in different liquids, pastes, foams and slurries and can be implemented in all types and shapes of tanks, metallic or non-metallic. They have a very high level of precision in both conductive and non-conductive media. If you look at the whole thing in practice,
it becomes clear how versatile the capacitive measuring principle is in very different process conditions.
For a steel-producing plant, a continuous level measurement, which works unaffected by steam or pressure fluctuations within a very small measuring range, was implemented with the NivoCapa® NC 8000 in water for gas cooling.
The proven technology with active buildup compensation and a tunable oscillator was a flexible solution for optimizing the manufacturing process.
One of the top 10 steel producers worldwide was looking for an insensitive and at the same time precise continuous level measurement of cooling water within very small, scattered tanks for gas cooling.
As part of the modernization of the production facilities in India, several plants were to be equipped with smart level monitoring. In addition to filling and emptying by means of pumps, which cause pressure fluctuations within the tanks, the presence of H2 gas was to be viewed as a critical factor for the selection of suitable measurement technology.
Several on-site tests carried out by Sumit Majumder, Managing Director of UWT India, and his team with various sensors quickly made it clear that the tanks can be safely monitored with the capacitive measuring principle of the robust NivoCapa® NC 8000 probe. For the small measuring range of the application of 1150 mm (3.77 ft.), the rod version with a total measuring range of up to 5 meters (16.41 ft.) (in the rope version up to 25 meters (82 ft.)) offered a well-matched solution.
The NivoCapa® NC 8000 is a simple and cost-effective level monitor for water-based liquids or oils as well as flowable powders. The electrical capacitance is measured in the process, which is proportional to the level in the container. The mechanical probe consists of the measuring area with flexible length and the active build-up compensation with fixed length. It supplies the electrical capacitance value of the measuring range in relation to the environment (container wall, measuring/ ground pipe or conductive material) and is connected to the transmitter.
The level sensor can be used in large silos or tanks as well as small process vessels, buffer storage tanks, hoppers, tubs, bypasses or downpipes. The stable, wear-resistant and abrasion-proof stainless steel probe made of 1.4404 (316L) is suitable for use in aggressive chemical media with an optionally available PFA coating.
The different device versions enable installations from above, below and, thanks to the detached housing, even with major vibrations or very little space. The housings are powder-coated aluminum and the high sensitivity from a DK value of 1.5 ensures a high level of functional reliability and allows the level sensor to be used flexibly in many media. Different dielectricity is easily evaluated and detected. The NC 8000 is a 2-wire instrument with an analog 4… 20 mA output signal according to NAMUR NE43. It combines easy-to-adjust electronics with field-tested probe arms.
Active Shield technology
The additionally built-in “Active Shield” technology protects with active build-up compensation against measurement errors, which arise from bridging, foams, material deposits and caking. The flexible measuring range combined with a very high resolution ensures precise measuring results, even at extreme process temperatures within a range of -40 °C to +200 °C (-40 °F to +392 °F) and within a process overpressure of up to 35 bar (507.63 psi).
The rod sensor NivoCapa® NC 8100 implements reliable level detection within the cooling tank with a process temperature between 80 °C and 150 °C (176 °F to +302 °F) without any problems at the Indian production sides. The capacitance measurement technology realizes a very fast measurement (<1s) and is also not influenced by the prevailing steam or pressure fluctuations. The installation on a nozzle also allows a measuring range to be realized up to the tank cap and to work with maximum accuracy over the entire length of the sensor.
The sensors implement a user-friendly user interface including an LCD display with control buttons and diagnostic function.
In addition to a WHG certification, the sensors of the complete NivoCapa® series also have the international approvals required for use within the cooling system for areas at risk of gas and dust explosions.
By implementing the practical solution for functional and safe level measurement in accordance with the high requirements and standards of the steel producer, smooth plant operation is optimally supported.
Capacitive measurement technology
To explain capacitive level measurement technology, it is important to understand the basic principle of a capacitor.
A capacitor consists of two conductive electrodes insulated from each other. It has the ability to store energy between its electrodes within an electric field. If non-conductive material (= the dielectric Ɛr) gets between the electrodes, the amount of storable energy increases and thus the capacitance of the capacitor. This is additionally determined by the dimensions of the electrodes (area A) and their distance (d) from each other.
The capacitance of a plate capacitor is generally described by the formula C = Ɛr * A/d.
With this understanding, the operation of a capacitive level sensor can now be explained. This consists of a medium-contacting metallic probe (rod or cable) which, together with a conductive container wall, can be understood as the two electrodes of a capacitor. A non-conductive medium, such as oil (non-conductive media < 1 µS/ cm), forms the dielectric between these two electrodes. If the level now rises, the electrode area increases, which in turn leads to an increase in capacitance.
This change in capacitance is detected by the sensor and converted into a signal proportional to the level.
Due to their function, capacitive level sensors often have to be adjusted and calibrated to the medium (empty and full capacitance). The medium adjustment is not necessary if the medium to be measured is conductive, such as water (conductive media > 100 µS), since in this case the medium itself represents the second electrode. The insulating PFA rod sheathing serves as dielectric, whereby the basic principle of a capacitor is given again.
„Inverse Frequency Shift“ Technology
- A small change in capacitance results in a relatively large change in frequency
- What also distinguishes this capacitive measurement technology is the unique ”Inverse Frequency Shift” technology used in the sensor.
- The capacitance is not measured directly, but rather the change in a generated frequency is evaluated.
Very high measurement accuracy
Imagine you have an empty container, the probe is not covered, the frequency is high and the capacitance is low. Now the container fills, the probe is covered and thus the frequency drops and the capacitance increases. What can be clearly seen is that a small change in capacitance results in a relatively large change in frequency. Because the frequency can be evaluated, even small changes in the level can be detected. This ensures a very high measurement accuracy and the sensor can be set very sensitively.