Buyer's Guides
Class A costs more and is not always the right answer. The tolerance only matters if it is larger than the error you are already introducing elsewhere.
OMS Clinical Team
Product Specialists
Class A glassware is manufactured to a tighter tolerance than Class B, typically about half the permitted error. A Class A 250ml cylinder might be certified to plus or minus 1.0ml where the Class B equivalent is plus or minus 2.0ml. Whether that difference matters depends entirely on what you are doing with it.
The class describes manufacturing tolerance, not the quality of the glass. Both are usually borosilicate, both take the same thermal shock, both are graduated the same way. What differs is how far the true volume may sit from the marked volume, and whether the item is individually certified.
Class A items are often supplied with a certificate or a batch certificate traceable to a standard. Class B items are made to a specification but not individually verified. If your quality system requires traceable evidence that a measurement was accurate, that certificate is the thing you are actually paying for, more than the extra precision.
Reach for Class A where the measurement is the result, or where it feeds directly into one. Preparing a volumetric standard, making up a calibration solution, or any analysis where the volume enters the calculation are the clear cases. So is anything that has to satisfy an auditor who will ask how you know the volume was right.
Class B is entirely appropriate for the large majority of routine work: making up buffers that will be pH-adjusted anyway, rinsing, transferring, rough dilutions, teaching, and anywhere a subsequent step corrects for volume. Using Class A here is not wrong, it is simply money spent on precision that the rest of the method throws away.
Glassware is calibrated either to contain or to deliver, marked TC and TD, or sometimes In and Ex. A TC item holds the marked volume when filled to the line. A TD item delivers the marked volume when emptied, which accounts for the film of liquid that stays behind on the glass wall.
The two are not interchangeable. Using a TD pipette as though it were TC, or blowing out the last drop of a TD pipette that was not designed for it, introduces an error larger than the difference between Class A and Class B. Check the marking before assuming.
Volumetric flasks are almost always TC. Pipettes and burettes are usually TD. Measuring cylinders come both ways, so the marking on a cylinder is worth reading rather than assuming.
Read at eye level, with the bottom of the meniscus resting on the graduation line. Reading from above or below introduces parallax error, which on a narrow cylinder can easily exceed the Class A tolerance you paid for. Set the cylinder on a level surface rather than holding it up, because a tilted cylinder reads high.
For coloured or opaque liquids where the bottom of the meniscus is not visible, read the top of the meniscus consistently and note that you have done so. Consistency matters more than which edge you chose.
Glassware is calibrated at a stated temperature, usually 20 degrees Celsius, and the figure is printed on the item. Glass expands and liquids expand more, so measuring a hot solution in a flask calibrated at 20 degrees gives a volume that is not what the marking says. For routine work the error is small. For accurate work, let the solution come to room temperature first.
This is also why you should not use volumetric glassware straight from a drying oven. Let it cool.
A practical approach is to hold Class A only in the sizes where you actually make up standards, and Class B across the general range. Most laboratories find they need Class A in two or three volumes and Class B in everything else. Buying Class A throughout is a common way to spend three times as much for no measurable improvement in results.
If you are replacing a set, look at what has actually broken over the last year. Breakage patterns tell you which sizes are in daily use and which are sitting in a cupboard, and that is a better guide to what to reorder than the original purchase list.
