Pipettes are tools that laboratory analysts use to accurately measure small amounts of liquid. Accuracy is important in scientific analysis as it is integral to a test that materials used are used in known quantities. Calibration of a piece of equipment simply means that the equipment has been checked to ensure that it works properly and accurately in the way that it is supposed to. Common methods of pipette calibration involved setting pipette volumes and weighing the resultant liquid picked up by the pipette against the expected weight. Once this is within an acceptable margin of error for the laboratory, then the pipette passes calibration.
Precision tool-making expertise goes into making pipettes for the scientific and industrial markets. Basically, a pipette is a way to separate out a specific volume of liquid, which may be several milliliters, down to tiny volumes like ten microliters. Each pipette is designed to be accurate, and the pipette user needs to know that the pipette is definitely accurate, and that it hasn't become damaged or worn.
Although the manufacturers of the pipette produce the tools to be accurate within a chosen range, sometimes tools break or lose function. Analysts therefore perform a pipette calibration on their pipettes often. The time scale between pipette calibration times depends on the rules of the particular laboratory or industry, and the thoroughness of the test can also differ. Often, an analyst performs a short calibration daily before a certain pipette is used, but the pipette has to undergo a much more complicated calibration every few months.
Simple calibrations, such as those performed daily, typically involve a weighing balance, some water, and the pipette. This involves weighing out specific volumes of water and comparing their weights on the balance to their expected weights according to the settings on the pipette. Typically, the necessary settings are chosen by the laboratory, and commonly these represent a low and a high volume that the pipette can be set to. For example, a pipette that is designed to be accurate between 1ml and 5ml may need calibration volumes of 1.5 ml and 4ml.
The analyst extracts the necessary volumes with the pipette and places the water onto a container on the weighing balance, which must have been calibrated previously so its accuracy is indisputable. Then he or she waits for the balance to settle and register the weight of the liquid dispensed from the pipette. This process is repeated with one or more volumes of water. Sometimes a calibration procedure calls for the pipette tip to be wet several times with the water first to ensure that the correct volume is taken in. After the weights are recorded, the analyst can check that the actual weight of water is within the acceptable limits of error for that pipette.
Professional calibration services can also perform more complicated checks on pipettes. As these types of pipette calibration tend to take longer, and be more thorough, they can be more expensive to do. They do, however, ensure that the pipette is still working within its initial tolerances and is suitable for further use.