Capacitive Sensor Foam Liquid Detection
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Solution Overview
Problem
Existing methods for detecting the liquid surface in laboratory and medical devices are unreliable, especially when dealing with foaming liquids, as they fail to accurately distinguish between foam and liquid contact, which is crucial for precise pipetting operations and automated measurement procedures.
Innovation Solution
A capacitive measuring device is used to differentiate between foam and liquid contact by applying input signals at different frequencies and evaluating the resulting output signals for distinct jumps, with a quotient comparison to determine the presence of a phase boundary, allowing for reliable detection of foam or liquid contact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional liquid level detection methods are used, then the detection process is simple, but the reliability of detection is insufficient when dealing with foaming liquids
Solution Approach 1:
The detection process is divided into multiple sequential steps: first detecting a phase boundary, then determining whether it is foam or liquid through additional measurements. This segmentation allows reliable foam detection while maintaining a structured, manageable approach to the increased complexity
Solution Approach 2:
The method uses changes in capacitive parameters at different frequencies to distinguish between foam and liquid. By measuring capacitive reactance at multiple frequencies and analyzing the relationship between these parameters, the system achieves reliable foam detection through parameter analysis rather than through complex hardware
2Object-affected harmful factors
If the pipette tip is positioned just below the liquid surface, then contamination is minimized, but the risk of sucking in air increases if the liquid level varies
Solution Approach 1:
The system continuously monitors the liquid level using capacitive sensing and provides feedback to adjust the pipette tip positioning. By detecting phase boundaries and determining whether they represent liquid or foam, the system dynamically adjusts the aspiration depth to maintain optimal positioning just below the liquid surface while avoiding air intake
Solution Approach 2:
Before performing the actual pipetting operation, the system preliminarily detects the liquid level and foam presence to determine the correct aspiration depth. This preliminary detection ensures that when the pipette tip is positioned, it is correctly located to minimize contamination while preventing air suction
3Measurement precision
If the fill level is precisely determined, then pipetting accuracy is improved, but the detection must handle great variation in fill levels across different containers
Solution Approach 1:
The capacitive sensing system serves multiple functions: it detects phase boundaries, determines liquid versus foam, measures liquid level, and adapts to different container types and fill levels. By using the same sensor for multiple detection purposes and analyzing different capacitive parameters, the system achieves precise measurement across varying conditions without requiring separate specialized sensors
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method ensures reliable distinction between foam and liquid contact, ensuring accurate positioning of pipette tips and preventing contamination during pipetting, thereby enhancing the precision and reliability of liquid level detection in various applications.
Implementation Method 1
the gas-liquid phase boundary can also be determined via a change in capacitance
Implementation Method 2
Since a gas and a liquid have significantly different dielectric constants
Data Source
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AI summary
Method and apparatus for distinguishing between contact with foam (2) or liquid (1) in a liquid container (5) of a device (100) by means of a capacitive measuring device (M) with a sensor (3) that can be moved up and down in the liquid container (5), wherein at least one output signal (sout(t)) is processed by the measuring device (M), the method comprising the following steps: a. Performing an up or down movement (B) of the sensor (3) in the liquid container (5); b. Applying an input signal (sin(t)) with a first frequency to the sensor (3); c. Applying the input signal (sin(t)) with a second frequency to the sensor (3), wherein the second frequency and the first frequency are different; d. As part of a first process - evaluating the output signal (sout(t)) to detect a first step of the output signal (sout(t)); e.In a second process: - evaluating the output signal (sout(t)) to detect a second jump in the output signal (sout(t)); f. comparing the first jump with the second jump; g. if the comparison exceeds a specified value, outputting an identifier (Ks) that signals contact between the sensor (3) and foam (2) in the liquid container (5).