Extracted from Volumetric Measurement in the Laboratory, Brand GmnH
Two types of glass are used for the production of blanks:
These glass types meet the stringent laboratory requirements for chemical and physical resilience.
High-quality blanks and strict statistical testing of the required quality characteristics are the basis for producing high-quality volumetric instruments. For example, thermal stress in the glass blanks must be eliminated by a controlled heating and cooling process.
This results in optimum mechanical stability, a requirement for keeping the volume constant despite any subsequent temperature fluctuations. Therefore, BLAUBRAND® and SILBERBRAND volumetric instruments can be heated up to 250 °C in a drying cabinet or sterilizer without any resulting volume changes. As with all glass instruments, however it should be noted, that uneven heating or sudden temperature changes produce thermal stresses, which may result in breakage.
Therefore: Always place glass instruments into a cold drying cabinet or sterilizer; then heat slowly. At the end of the drying or sterilizing period, allow instruments to cool off slowly inside the switched-off drying cabinet or sterilizer. Never heat volumetric instruments on a hotplate!
Every glass volumetric instrument is individually calibrated at BRAND®, i.e. the instrument is accurately filled with a defined quantity of water, and a calibration mark is applied at the lowest point of the meniscus. In the case of graduated instruments, two calibration marks are applied. Computer-controlled systems ensure maximum precision in a fully automated production line. 'Statistical Process Control' (SPC) guarantees production of volumetric instruments with the smallest deviation from nominal capacity (accuracy) and narrow scatter of individual values (coefficient of variation).
Volumetric instruments are either calibrated 'to contain' ('In') or 'to deliver' ('Ex').
The contained quantity of liquid corresponds to the capacity printed on the instrument. These instruments include e.g., graduated cylinders, volumetric flasks, and capillary pipettes up to 200 μL.
The delivered quantity of liquid corresponds to the capacity printed on the instrument. The wetting residue remaining in the instrument has already been taken into account in the calibration. These instruments include e.g., graduated and bulb pipettes, and burettes.
The standard reference temperature, i.e. the temperature at which volumetric instruments will contain or dispense their volumes, is 20 °C. If the adjustment or calibration is performed at a temperature that deviates from this standard, the corresponding measurement values must be corrected.
Due to the small coefficient of expansion of the glass material, the reference temperature is of minor significance in practical use since the measurement deviations resulting from volume expansion of the measuring instrument are generally smaller than the error limit.
Figure.Calibration of graduated pipettes
Calibration is followed by silk-screen printing of marks and inscriptions. BRAND® uses stretchable screen stencils for all graduated pipettes, burettes, graduated cylinders, and mixing cylinders. These stencils can be stretched to match the calibration marks accurately, so that the measuring precision is maintained for all intermediate volumes.
Pipettes are additionally marked with 'color-code' rings at their upper end, which make it easier to identify similar pipette sizes clearly. The ISO 1769 industry standard defines color-coding for different nominal volumes.
BRAND® uses quality inks manufactured especially for glass volumetric instruments:
Figure.Automatic silk-screening of volumetric flasks
Firing or annealing the screened ink is the last step on the way from the blank to the finished volumetric instrument. A carefully controlled annealing process, along with the specially produced quality inks, is a prerequisite for durable graduations. This involves a gradual heating and cooling of the volumetric instruments. Depending on the type of glass, temperatures of 400 to 550 °C are reached in this process.
Quality assurance is implemented at BRAND® through ongoing testing during production and statistical testing in the final inspection. (For detailed information, see page 39.)
BLAUBRAND® bulb pipette
on every volumetric instrument:
The following information may also be added:
Volumetric instruments are generally available in two accuracy classes:
Volumetric instruments of class A and AS have identical error limits as established by DIN EN ISO. These are generally implemented only in glass volumetric instruments. Exceptions are BRAND® plastic volumetric flasks made from PFA and PMP and plastic graduated cylinders made from PMP, which are designed to meet the highest requirements and likewise correspond to class A. Measuring flasks with wide neck are marked with class AW and have a higher error limit compared with class A measuring flasks. For class AS volumetric instruments, calibrated to deliver (TD, Ex), the additional 'S' means swift delivery.
Class AS volumetric instruments have become quite well established. The risk of clogging is low in pipettes and burettes with a larger tip opening. The delivery behavior of various liquids is compensated by observing the defined waiting time (see 'Delivery and waiting times' on page 11).
The long graduation marks extend over at least 90% of the tube perimeter or are present as ring markings.
Volumetric instruments of class B are available in glass or plastic. Class B instruments generally have twice the error limits of class A/AS. Examples of BRAND® class B plastic volumetric instruments are volumetric flasks and graduated cylinders made of PMP or PP, as well as bulb and graduated pipettes made of PP. For class B measuring instruments calibrated to deliver (TD, Ex), no waiting time is specified.
The long graduation marks extend about 20-40% of the tube perimeter.
There is no all-round material to meet every single requirement in the laboratory. The decision to use glass or plastic is guided by the application, product design and after considering the specific properties of the materials and economic aspects. Volumetric instruments of plastic excel by their high resistance to breakage and low weight. PP, PMP and PFA are proven materials.
The accuracy of bulb pipettes, graduated pipettes, volumetric flasks, and graduated cylinders made from PP corresponds to that of class B error limits. PMP and PFA are also used for measuring instruments that correspond to class A error limits, e.g., volumetric flasks (PMP/ PFA) and graduated cylinders (PMP). Due to its higher purity, PFA is preferably used in trace analysis applications.
Graduated cylinder made of PMP, class A