Karter Scientific Class A Volumetric Flasks are manufactured from high quality inert 3.3 borosilicate glass and meet the tolerances requirements as established by ASTM E-694 - ASTM E-542 - and ASTM E-288. Flat bottom. Volumetric flasks provide improved lab convenience, broad selection and precise volume measurement. Accuracy limits conforms to ISO 1042 Laboratory Glassware-One mark volumetric flasks. With one permanent circular mark. Printed with fused-on enamel color, resistant to most acid and alkaline solutions. Calibration is based on the poured in volume ("In") at a +20°C reference temperature.