Class A vs Class B: what the letter on your glassware actually buys you
Standards · 6 min read · Vitreon technical desk
The class letter is a tolerance contract. Here is what it means numerically, when Class B is genuinely good enough, and when it quietly ruins your results.
01The tolerance contract
Class A and Class B are defined in the ISO standards for each instrument type (ISO 1042 for flasks, ISO 385 for burettes, ISO 648 for pipettes). Class B tolerance is exactly twice Class A. A 100 mL Class A flask is guaranteed within ±0.10 mL; its Class B twin within ±0.20 mL. Neither is 'more accurate glass' — they are different guarantees from the same production line, separated by calibration effort and verification.
02When Class B is the right call
Preparing rinse solutions, rough dilutions, teaching labs, and any step whose error is swamped by a later gravimetric or instrumental measurement. Buying Class A for these is spending money on decimal places you will never read.
03When Class A is non-negotiable
Preparing standards for titration or calibration curves, pharmacopoeial assays (USP <31> explicitly requires Class A), and any measurement feeding an uncertainty budget. The tolerance term propagates into every result downstream — a Class B flask can double your combined uncertainty before you have pipetted anything.
04The Vitreon difference
Our Class A instruments do not just meet the tolerance band — each carries its measured value on a serialised certificate. Knowing your flask contains 100.021 mL, not just '100 ± 0.10', lets careful labs correct systematically and tighten budgets further.