Reusable Pyrex® serological pipets are designed for cotton plugging. The uniform top-end openings permit performing of plugs and allow precise control of liquid column. Color-coded by size for easy identification and sorting. Colored graduations are enameled onto the glass. The two rings indicate the pipet is calibrated "to deliver" (TD) its total capacity when the last drop is blown out. Reference: ASTM E-1044.