Pipettes and pipettors are widely used to transfer liquids in a range of applications. Pasteur pipettes are used for cell culture and often in conjunction with a handheld pipette controller. Pipettors used with pipette tips are used for small volume, single and multi-channel, applications. Air-displacement pipettors are best suited for liquid transfer, where the air cushion protects sample integrity. Positive-displacement units are best for viscous or volatile samples. Additional considerations include: accuracy and repeatability or measurement, chemical resistance, and robustness to changes in environmental conditions such as temperature changes and dispensing angle.