Fail-safe method and apparatus for aspirating and/or dispensing liquids in automated laboratory instruments

A method for preventing either the introduction of air into a liquid aspirated from a container through an aspiration line passing through a container cap, or the over-filling or non-filling of a container by a liquid dispensed through a dispensing line passing through a container cap, said method comprising the steps of simultaneously detecting, using a first detector, that (a) the container cap is properly positioned on and secured to a liquid container from which liquid is to be aspirated, or into which a liquid is to be dispensed, and (b) detecting, using a second detector, that the level of liquid in the container is either above or below a predetermined level,

• wherein said first detector comprises a proximity switch mounted in said container cap for sensing the proximity between said cap and a mouth of a container to which said cap is to be attached, and said second detector comprises a float switch, and
• wherein said proximity switch comprises a switch-actuating member that is slidably-mounted on either of said aspiration or dispensing tubes for movement between first and second axially-spaced positions, and a spring for urging said switch-actuating member towards a switch-opening position.