Volumetric flasks, which have calibrated volumetric markings, come in a variety of sizes — from 1 mL cylindrical microflasks to 4 L larger flasks. Manufactured from plastics, such as polyethylene, polypropylene, or polymethylpentene, volumetric flasks are produced to meet ASTM and ISO standards of quality and dependability. Many volumetric flasks are tested and calibrated in ISOIEC 17025 certified laboratories. Certain flasks protect contents from harmful UV light, have PVC coating for break resistance, or are rated for meeting Class A or Class B tolerances. Examples include Thermo Scientific™ Nalgene™ Class B Polymethylpentene (PMP) Volumetric Flasks with Screw Caps and Thermo Scientific™ Nalgene™ Class B Polypropylene Copolymer Volumetric Flasks with Closure.