A tip that plunges to the bottom of every well drags liquid up its outside and carries contamination from well to well. A tip that never reaches the surface aspirates air and short-fills. Getting the depth right, well after well, in plates that are never filled to exactly the same height, is what liquid level detection is for. It lets the instrument find the surface, submerge the tip just enough, and follow the meniscus down as the volume drops.
Capacitive detection
Capacitive detection, often abbreviated cLLD, senses the change in electrical capacitance when a conductive tip touches a conductive liquid. It is fast, gentle, and works down to small volumes, which makes it the default for aqueous buffers and salt solutions. The catch is in the word conductive: it needs a conductive tip and a liquid that carries charge. Pure organic solvents, oils, and low-ionic liquids can be effectively invisible to it, and heavy foam can trigger a false surface.
Pressure-based detection
Pressure-based detection, pLLD, watches the pressure inside the channel as the tip descends. Contact with the surface produces a small, detectable pressure change regardless of whether the liquid conducts. That makes it the answer for solvents and other liquids capacitive sensing cannot see. It is generally less sensitive at very small volumes, so the two methods are complementary rather than interchangeable.
Monitored pipetting during the transfer
Some instruments keep watching pressure throughout the aspiration and dispense, not just at first contact. This monitored-pipetting mode turns the channel into a sensor for things that should not be happening: a clot that blocks flow, a tip that hit the bottom, a well that was empty, foam that broke the column. Instead of silently delivering the wrong volume, the run can flag or skip the offending transfer, which is the difference between catching a problem on the deck and finding it in your data a week later.
What this means for your liquid class
Detection is not a separate setting bolted on beside the liquid class; it shapes several of its parameters.
- Submersion depth: how far the tip sits below the detected surface. Too shallow and it sucks air as the level drops, too deep and it wets the outside and worsens carryover.
- Surface tracking: whether the tip follows the meniscus downward during aspiration, which keeps submersion constant instead of drifting.
- Detection method per liquid: aqueous classes lean on capacitive sensing, solvent classes often must use pressure sensing.
- Error handling: what the class does when detection fails, from retrying to skipping to aborting.
Level detection is only as good as its match to the liquid. Confirm that your chosen method actually sees the liquid before you trust a class that depends on it.