This article describes how Capacitance Liquid Level Detection (cLLD) works and what changes were introduced with the new Multisense™ AirFCA.
The liquid level detection function is activated at Z-start, as the tip moves down towards the liquid. If no liquid is detected when the tip reaches Z-max, an exception message is reported. The capacitive liquid level sensor of the pipetting channel detects the liquid level in a container by measuring the capacitance change between the conductive tip and the grounded deck of the instrument while the tip moves downwards from the reference environment (air) to the liquid.
A threshold value for the change in measured capacitance is defined to trigger positive feedback when the tip comes in contact with the liquid surface. cLLD works with conductive disposable tips, which can be recognized by their black color. Transparent tips do not work with cLLD.
The Fluent uses cLLD technology with real-time signal evaluation. In case of troubled liquid level detection conditions, the system will trigger an immediate stop and will automatically perform a liquid level detection retry. All liquid FCAs and older AirFCAs use Adaptive Signal Technology™ (AST), with the “slow” and “fast” signals where cLLD requires the liquid to be assigned to one of the three sensitivity groups according to its conductivity and relative static permittivity. Each group has a specific sensitivity profile, which is a function of the tip distance to Z-max.
For new (2023) AirFCAs with MultiSense technology, there is only one signal that is processed. Therefore, setting the sensitivity groups 1, 2, or 3 is not needed, as the absolute capacitance is measured. Hence, a MultiSense setting should be used. The FluentControl software from version 3.3 onwards will automatically detect the arm type and offer either the sensitivity groups or MultiSense (Figure 1).
The sensitivity profile reflects the signal variation expected in case of contact with the conductive liquid, as the tip moves downwards:
- The closer the liquid level is to Z-max, the smaller the liquid volume and, as a result, the weaker the expected signal.
- This dynamic threshold setting allows the system to detect even the smallest volumes while simultaneously minimizing susceptibility to interference.
Figure 1. cLLD sensitivity groups
Existing liquid classes do not require any adaptation when they are used on an instrument with a MultiSense FCA. FluentControl automatically detects the arm and uses the “MultiSense” setting for the cLLD sensitivity group in the liquid class.
The new MultiSense arm also provides the possibility to use Pressure Liquid Level Detection (pLLD) in combination with our new technology of PMP™-AI and PhaseSeparator™.
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