Are all capacitive sensors for liquid level detection created equal?

When standard capacitive sensors are used for liquid level detection in an indirect level detection application, the sensor must be adjusted to the point where it ignores the container wall but reliably detects the capacitance change caused by the changing liquid level. Typically, a standard capacitive sensor can be adjusted to disregard a wall thickness of approximately 4mm. In addition, the dielectric strength of the liquid must be higher than the container wall for reliable level detection.

Capacitive sensors detect any changes in their electrostatic sensing field. This includes not only the liquid itself, but also application-induced influences such as condensation, foaming, temporary or permanent material build-up. High viscosity fluids can cause extensive delays in the accurate point-level detection or cause complete failure due to the inability of a standard capacitive sensor to compensate for material adhering to the container walls.

A perfect capacitive sensor for non-invasive level detection applications would not require any user adjustment after the initial setup process. It would detect the true liquid level of any type of water-based liquid through any non-metallic type of tank wall while automatically compensating for material build-up, condensation, and foam. While ignoring these interferences, the sensors would still detect the relative change in capacitance caused by the liquid but use additional factors to evaluate the validity of the measurement taken before changing state.

These sensors would be fundamentally insensitive to any non-conductive material like plastic or glass up to 10mm thick, which would allow them to be utilized in non-invasive level applications. The enhanced capacitive sensors only limitation is it would require electrically conductive fluid materials with a dipole characteristic such as water to operate properly. This is a great concept but does a sensor like this exist today? The short answer is yes, and it is called SMARTLEVEL!

SMARTLEVEL sensors work with a high-frequency oscillator whose amplitude is directly correlated with the capacitance change between the two independently acting sensing electrodes with each electrode trying to force itself into a balanced state. That is the reason why the sensor measures the capacitance of the container wall without ground reference and the capacitance of the conductivity of the liquid with ground reference (contrary to standard capacitive sensors).

Without having target material introduced into the sensor’s field, the amplitude of the oscillator is at a nominal state. An empty container wall creates a capacitance change: the amplitude rises above the nominal level; however, it will never lead to a valid switching condition unlike a standard capacitive sensor. As a conductive liquid rises in the container, a second, much higher capacitance with respect to ground will be created. This second capacitance, being significantly higher than that of the container wall, affects the oscillation inversely to the capacitive effect of the wall and pulls the amplitude down to 0V, triggering the sensors output.

Enhanced capacitive sensor detection schematics – no target present

Enhanced capacitive sensor detection schematics – no target present

Enhanced capacitive sensor detection schematics – automatic tank wall compensation

Enhanced capacitive sensor detection schematics – automatic tank wall compensation

Enhanced capacitive sensor detection schematics – liquid level presence detection

Enhanced capacitive sensor detection schematics – liquid level presence detection

 

The key advancement is that SMARTLEVEL sensors compensate for differing conditions by operating at a field frequency higher than that of an ordinary capacitive sensor. This high AC frequency reduces the reactance segment of the impedance between the active sensor surface and the material build-up, allowing the sensor to detect true levels without interference.

SMARTLEVEL sensors provide cost-effective, reliable point-level monitoring for a wide array of medical, biotechnology, life sciences, semiconductor processes, and other manufacturing processes. Balluff offers this technology exclusively in a wide array of sensor configurations all aimed at accommodating the full breadth of science and industry’s most stringent application-specific level monitoring requirements. SMARTLEVEL capacitive sensing technology brings considerable advantages to the area of liquid level detection, not only offering alternative machine designs, but also reduced assembly time for the machine builder as well as drastically reduced field service.

Discrete indication tasks like fluid presence detection in reagent supply lines, reagent bottle level feedback, chemical levels and waste container overfill prevention are now a distinct competence for capacitive sensors. Reagents and waste liquids are composed of different formulas depending from customer to customer or and even sometimes the end-user. The sensing technology has to be versatile enough to compensate automatically for changing environmental or media conditions within high tolerance limits.

Applications that require precision and an extraordinary amount of reliability, such as blood presence detection in cardiovascular instruments or hemodialysis instruments, medical, pharmaceutical machine builders, equipment builders for semiconductor processes can rely now on SMARTLEVEL capacitive sensing technology.

 

This entry was posted in All posts, Capacitive Sensors, Object Detection Sensors and tagged , , , , , , , . Bookmark the permalink.

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