Level Measurements

With level detection sensors, there are few parameter restrictions. Basically, if you can imagine it—it is being sensed; from food and beverage products to rocks and gravel, hot wax and glue. Nearly all manufactured products require some type of level detection in the production cycle—whether it involves the end product or is part of a reciprocal process. The challenge is to accurately sense the levels of substances despite the obstacles commonly found in manufacturing facilities.

Specifically designed for the plastic industry, this capacitive sensor features excellent EMC and ESD immunity, allowing it to reliably detect the level of plastic pellets in a hopper while withstanding environmental interference.

For example, it can be crucial to accurately gauge level in a container rather than merely report whether a container is empty or full. Medias that “run dry” can damage other parts and processes in the line. Over-fills are not just a difficult and time-consuming process to clean, they may pose safety risks from the media being sensed as well as to the environment location—as is the case in many hazardous materials and locations.

When selecting a sensor, the material used for the container that stores the media is also important, so is the application environmental along with other factors. For many such applications—such as detecting liquids in plastic or glass containers as well as solids, including powders and rocks—consider capacitive sensors.

“Seeing through” to sense
By adjusting a potentiometer, capacitive sensors can “see through” lower dielectric materials such as plastic or glass to detect higher dielectric materials.

Capacitance is a function of the surface area of two electrodes, the distance between the electrodes and the dielectric constant of the material between the electrodes.

The sensors function when two metallic electrodes (A and B) are placed in a feedback loop of a high frequency oscillator. When no target is present, the sensor’s capacitance—the function of the surface area of two electrodes, the distance between the electrodes, and the dielectric constant of the material between the electrodes—is low and the oscillation amplitude is small.

When a conductive target enters the sensor’s field, it forms a counter electrode to the active face of the sensor, decreasing the distance between the electrodes and increasing their average surface area—resulting in an increase in the capacitance and the amplitude of oscillation. Conversely, when a non-conductive target enters the sensors field, it acts as an electrical insulator between the electrodes. An evaluating circuit then measures this amplitude of oscillation and generates a signal to turn the output ON or OFF.

The dielectric constant of the sensed material is a measure of its ability to conduct electricity. Air has a dielectric of 1, with metals having dielectric constants in the thousands. Since all liquids and solids have a greater dielectric constant than air, the capacitance of metallic and non-metallic targets is always greater than the capacitance of a sensor’s circuit in the absence of the target. In most situations, the greater the dielectric constant of the material, the greater the sensor’s achievable operating distance. However, as operating distance increases the required free zone also increases.

Through this method of sensing, capacitive sensors can handle a variety of challenges that may be presented by the container, the media, and the surrounding environment.

Containers and capacitive sensors
Containers—which hold the medium for level detection—are constructed from plastic, glass, metal, steel as well as a variety of other mediums and range in size, shape and special properties. While there is no official standard for the materials containers are made from, they affect what mediums can be contained and what type of sensor can be used per container.

The size and shape of a container depends on the application and greatly influences the types of sensor used as well as the reaction time of the sensor and the rate of fluid change. For instance, very tall containers with small diameters prove challenging for many sensors. Probes will not fit inside the container, and ultrasonic “see-through” sensors will not mount flush to the container walls. Capacitive sensors, however, feature wide sensitivity bands that enable them to sense very small metal parts through a plastic tube.

Material challenges
The variety of containers is perhaps only surpassed by the gamut of materials sensed, which include fine powder like flour and some types of makeup to large broken rocks and boulders in a quarry. The type of media to be sensed will influence your choice of sensor.

For instance, the rapid transfer of plastic pellets in hoppers can cause high static electricity. When transfer occurs for extended periods of time—a common occurrence in many applications—a high amount of static electricity causes electromagnetic interference with the internal components of some sensors. High static electricity in these applications can also cause the plastic pellets to stick or “clump” together, generating a false reading by the sensor. For these applications, capacitive sensors can be mounted on a plastic sight glass or directly in the hopper containing plastic pellets for point level detection.

When a conductive target enters the sensor’s field, it forms a counter electrode to the active face of the sensor, decreasing the distance between the electrodes and increasing their average surface area.

When a non-conductive target enters the sensor’s field, it acts as an electrical insulator between the electrodes.

While standard capacitive sensors are prone to failure due to the electrostatic discharge (ESD) frequently found in pellet silos and systems, some newer sensors are specially designed to combat these issues and resist ESD. Additionally, capacitive sensors cannot sense through metal. However, applications that require liquid level detection through a metal container wall can use special sight glass and tank well fittings.

Capacitive sensors can be used to detect liquids of varying viscosities. Liquids that may cling would not necessarily prevent a capacitive sensor from giving an accurate reading. However, it is a good idea to ensure against any liquid touching the actual sensing portion of a capacitive sensor, as it may cause the sensor to fault and lock on to a constant ON or OFF readout. Also consider the potential for product build-up for liquid level detection applications, as wax, glue and other adhesives can build-up on the sides of the containers and interfere with the flow as well as the sensor’s reading. To prevent this interference, some sensors are designed to ignore product build-up.

Environments and housing
Finally, consider the environment where the capacitive sensor will be located prior to selecting a specific sensor for level detection. Rugged and sanitary environments, temperature variables and hazardous areas all require uniquely designed devices to withstand the rigors associated with each area. Intrinsically safe NAMUR capacitive sensors, for example, are designed for use in explosive areas, such as grain elevators, to detect materials ranging from rice and barley malt to corn and soybeans. Additionally, the materials used in capacitive sensors allow them to be used for applications, such as pharmaceutical, where metal is not allowed.

Capacitive sensors are available in a wide variety of housing options, including barrel, rectangular and probe and are applicable in many environments and applications, providing a versatile sensing solution—even for challenging applications.

TURCK USA
www.turck.us