Eastern Connecticut State University

Department of

Biology

Measurement

One of the most basic techniques in laboratory work is accurate measurement. This may involve weight, volume, time, or some other variable. In all cases it is important that you are familiar with the appropriate equipment, and know how to use it.

How to measure:

Volumes

Units of volume

The basic unit of volume is the litre (L: note: in the US this is usually spelled liter). The litre is subdivided into 1000 milliliters (often abbreviated as mls) and each milliliter is divided into 1,000 microlitres (often shown as µL or the Greek letter lambda l). You should become familiar with converting from one unit to the other as a matter of routine.

Measuring devices

It is important to use the correct type of measuring device, which will depend on the volume you are working with. The devices listed below are those typically used, however you may find some variation or overlap (i.e. for 10 mls you could use either a graduated cylinder or a 10 ml pipette)

For volumes greater than 25 mls
A graduated cylinder or a volumetric flask are appropriate. Do not use beakers or conical flasks for measurement purposes, they are not accurate enough.

Graduated cylinders. Note: they come in various sizes, from 10 mls to 2000 mls. They may be made from glass or plastic and vary in style. Each cylinder can be use to measure any volume, up to its maximum, however you should always use the cylinder that is closest to the volume you want. So, if you want to measure 40 mls, use a 50 mls cylinder, not the 100 or 1000 ml.

The error on measurement is typically 1-2%. For most experiments this is sufficiently accurate.

Volumetric flasks. Again, they come in various sizes, but each one can be used to measure just one volume, however it is more accurate than using a measuring cylinder.

For volumes between 1 and 25 mls

It is usual to use a pipette. These come in sizes from 0.1 ml to 25 mls but most volumes of less than 1 ml are now made using a micro pipette. Each pipette can be use to measure any volume, up to its maximum, however you should always use the pipette that is closest to the volume you want. So, if you want to measure 4 mls, use a 5 ml pipette, not the 10 or 25 ml. Glass pipettes are usually washed and reused, while plastic ones are typically discarded after use (check with the instructor)

For volumes less than 1 ml

Almost all measurements of 1 ml (1000 µL ) or less are done using a micro pipette. These are precision instruments costing more than $250 a piece, so be careful when using one. there are several brands and each works slightly differently. Watch the instructional movie below for general information, but check with the instructor for details on how to use each type of micro pipette before using one for the first time.