Glassware, Calibration Of Volumetric
By Phillip Carter.
While most prudent laboratories realise the importance of testing all Class A volumetric glassware on receipt and before use, it is surprising how few know the correct procedures for testing volumetric calibrations. An incorrect calibration, while only playing a small part in a study, can jeopardise the integrity of the studies finding. This is why major manufacturers of quality volumetric glassware recommend that all Class A glassware be checked before use.
Unfortunately though, so few laboratories or text books contain the correct procedures for calibration of volumetrics. Everyone should know to take into account temperature variations on the water and glassware, but how many people take into account the different shrinkage rates between Borosilicate glass and Soda Lime glass. With Borosilicate's tighter molecular structure than Soda Lime glass it is less responsive to changes in temperature resulting in minimal thermal expansion which needs to be taken into consideration when testing calibrations. So how do I tell them apart I hear you ask. As a general rule Soda Lime glass is labelled in amber stain, while Borosilicate glass not being able to be stained successfully with conventional amber staining techniques, will be labelled in white or blue ink. Some brands such as Duran are a trade name for Borosilicate glass 3.3, so these will all be in Borosilicate.
Calibration procedures are also influenced by air pressure and temperature at the time of reading, regardless of what type of glass you are testing. By measuring a barometer reading and calculating the air temperature the results may be used in an "Air Pressure And Temperature Correction Table" to obtain a correction figure which is taken into account to obtain a final gravity result to ensure a consistently accurate result is obtained.
We strongly recommend that to comply with good laboratory practice, all Class A volumetric glassware should be check calibrated on receipt. If you consistently find that the glassware falls outside the Class A tolerances for accuracy or delivery time, it should be sent back to your supplier.
For your convenience we at Bacto Laboratories Pty Ltd have collated all of the relevant procedures and tables from ISO Standards and British Standards with thanks to Volac Volumetric Glassware.
Also check our technical tip on the Properties Of Glass.