While in the air cushion principle, an air cushion separates the liquid within the idea from the pis- ton inside the pipette. $begingroup$ In olden times, (I used to be taught to both of those suck up and blow out a pipet by MOUTH), TD and TC about included it. https://danteionli.tribunablog.com/getting-my-different-pipette-types-to-work-43735165