Level measurement is the one of the best ways of keeping track of inventory based on volume and weight. It is a key component for obtaining accurate and reliable measurements of the contents of a tank or silo.
Level measurement is important not just for inventory control, but also for custody transfer, which occurs in situations where material that is bought and sold is transferred based on level measurement. The number is converted and read as a volume or weight. Accuracy is a major factor in this process—an error even as small as 1/8 inch or 3 mm of measured level is a significant error that can greatly complicate the accuracy.
Level measurement is carried out by sensors. Sensors are available in a number of varieties, each following a different principle. They are available in capacitive, vibronic, and paddle technologies.
Capacitive Switch Technology
Capacitive switch technology is based on the principle of detecting signals thorough capacitance or resistive coupling of identified generated input pulses. In other words, capacitive measurement depends on the principle of capacity changes. This method takes advantage of the fact that the contents of a tank or silo have a constant difference in their electrical charge.
Capacitive measurement works via a capacitive probe inserted into a tank. As the fluid level rises, it covers the probe, in the process changing the capacity level.
Vibronic Switch Technology
The vibronic principle was invented by Endress+Hauser in 1967, using the direct correlation between oscillation and damping in media. Vibronic instruments monitor point levels in tanks, silos, and pipes. This is done primarily through a tuning-fork shaped sensor attached to the instrument, which is excited to its resonance frequency.
In liquids measurement, the tuning fork sensor is placed horizontally and oscillates based on the height level of the liquid. When the sensor is not in contact with the liquid, it oscillates at a rapid pace. When it is fully submerged, it oscillates at a slow pace. This oscillation is converted into a switching signal.
In solids measurement, the tuning fork faces downward. As it is covered by solids, the oscillation is dampened, and the change in amplitude is converted into a switching signal.
Paddle Switch Technology
Paddle switch sensors typically involve a transmitter with a sensor shaped like a golf club or paddle. The paddle is inserted into a silo. As the material inside pushes onto the paddle, it sends a signal to the transmitter via a slip-slide clutch.
If the material is stopped by the material, the motor moves from the “rest” to the “switch” position, which activates two switches. The first signals the level condition, while the second switches the motor off.
Industries for Capacitive, Vibronic, and Paddle Switches
Endress+Hauser capacitive switches, such as the Liquicap FMI51, are especially reliable for continuous level monitoring in liquids and bulk solids, particularly in any material that forms build-up and in extremely high temperatures. These are typically used in the food and chemical/petrochemical industries.
Vibronic sensors from Endress+Hauser are also used with bulk solids or liquids. These can be modified to measure a range of physical properties, including conductivity, density changes, pressure and temperature.
Paddle switches, such as the Soliswitch FTE20, are mostly used for level measurement in bulk solids. By installing these in silos holding such products as rice and grains, manufacturers in the food industry gain tremendous advantages as only Endress+Hauser can provide.