The best option ultimately depends on the liquids being processed and the requirements of the application. Fixed Probes, based on stainless steel needles, work with either conductivity or capacitance level detectors.
Although conductivity requires two needles or a needle and a separate conductor wire, the signal generated is generally extremely stable in all situations and can provide more information with problematic samples where clots or bubbles are present. Conductivity can also be the most reliable option when processing samples where the volume available in the tube may be less than 50µl such as with some paediatric samples.
Capacitance detectors have the advantage of working with a single needle and ultimately allow for probes with a smaller profile. This is almost essential for aspirating from 1536 well plates and for processing tubes fitted with septa. The Capacitance level detector can also be used with carbon filled tips which perform better during reagent addition steps where the probe needs to detect liquid on repeated visits to the same container. Furthermore, capacitance detection will sense low polarity fluids such as organic solvents incuding DMSO, Hexane, Methanol etc. The capacitance sensor can also be used to provide liquid level detection with conductive (carbon filled) disposable tips.
A highly sensitive pressure sensor is used to detect the minute pressure changes which occur when a disposable tip touches the meniscus of a liquid. This technology allows the detection of liquids with non-carbon tips. It also enables the small changes that arise within the tip during dispensing to be monitored, recorded and checked. As well as the detection of the liquid surface, errors caused by clots on aspiration and blockages on the dispense can be identified and appropriate error recovery and user intervention routines initiated as specified by the user in the instrument configuration or test definition software parameters.