Types of pipettes

There are two main types of pipettes used for accurately dispensing volumes of solutions: volumetric and graduated. Pipettes are manufactured in a range of styles, and it is important to understand the differences.

Graduated vs Volumetric

Volumetric pipettes are designed to accurately measure a single fixed volume and thus only have a single graduation mark and usually feature a bulb half way up the pipette. Graduated pipettes are slightly less accurate than a volumetric pipette, however they are designed to measure multiple volumes and thus have multiple graduations on.

Mohr (drain out) vs Serological (blow out) graduated pipettes

Pipettes may be labelled ‘BLOWOUT’ which indicates that every drop of liquid in the pipette is counted as part of the volume, meaning that the last drop needs to be ‘blown out’ of the pipette to ensure the correct volume is measured. Non-blowout pipettes differ in that the final drop of liquid that remains in a pipette is accounted for with the graduations and should not be forced out of the pipette when measuring volumes TAPING SURFACE . Drain out pipettes are often marked with a zero at the start of their conical end where as blow out do not display zero marks. 

Volume graduations

Pipettes vary with the graduations as to whether they run from 0 to the pipette volume from the end of the pipette to the top or vice-versa. To take a 5 mL pipette as an example, on pipettes that start with 0 mL at the tip (top image), if you wanted to measure 3 mL of liquid, you would draw the liquid to the 3 mL mark which would then dispense the correct volume. Whereas for a 5 mL pipette where the tip starts at 5 mL (bottom image) you would calculate the difference, therefore you would fill the pipette to the 2 mL mark in order to dispense 3 mL.

Pipette Fillers

Fillers come in three main designs (for each image the pipette is inserted into the bottom of the filler):

1. Three valved bulb fillers (left). There are valves which control the operation of the pipette. One valve allows the air to be emptied from the bulb. A second valve uses the reduced pressure of the bulb to draw up liquid in the pipette. The third valve opens the pipette to atmosphere which will cause the column of liquid to fall in the pipette.

2. Wheeled fillers (centre). These allow liquid to be drawn up into a pipette by rolling the wheel which pushes up the piece of plastic from the filler and draws liquid up the pipette. There is usually a valve to allow the filler to be reset to enable larger volumes to be drawn up.

3. Electronic fillers (right) usually provide a switch to draw up and release liquid. 

Regardless of the pipette filler used, the same principles apply for accurately measuring and dispensing volumes of liquids using pipettes. The instructions below are for using the valved bulb fillers and will need minor modifications if using a different type of filler.

It is important to make sure that the pipette and pipette filler are always held vertically when there is any liquid in the pipette. Inverting the pipette can cause the liquid to flow into the pipette filler, which can damage it and prevent it from working correctly.