Measuring Variables

Every practical comes down to the same question: is this number trustworthy? That depends on picking the right instrument for the job, knowing how finely it can read, and being honest about how far out it might be.

Accurate is not the same as precise

These two words get used interchangeably in conversation, and separating them is one of the quickest ways to sound like you know what you are doing in an evaluation.

Measuring mass

• Mass is measured with a digital balance, which normally reads to two decimal places.
• Balances must be tared (set to zero) before use, so the reading shows the mass of the sample and not the container.
• The standard unit is the kilogram (kg), but grams (g) are used far more often in biology. 1 kg = 1000 g.

Taring is worth understanding rather than just doing. If you forget, every single reading is wrong by the same amount — the mass of the weighing boat. That is a systematic error, and repeating the measurement will not reveal it.

Measuring the volume of liquids

Common units are cm³, dm³, ml and l. A millilitre is the same as a cm³, so labels using either can be read the same way.

The three you need to distinguish

• Measuring (graduated) cylinders are used for approximate volumes where high accuracy is not important. They come in sizes from about 10 cm³ up to 1 dm³.
• Volumetric pipettes are the most accurate way of measuring a fixed volume, usually 10 cm³ or 25 cm³. They have a scratch mark on the neck, which you line up with the bottom of the meniscus.
• Burettes are the most accurate way of measuring a variable volume, typically between 0 and 50 cm³. The awkward part is that the scale runs from top to bottom — 0.00 cm³ is at the top of the column.

Measuring the volume of gases

• A gas syringe is the usual apparatus: gas produced by a reaction pushes the plunger out along a graduated barrel.
• A graduated measuring cylinder or burette inverted in water also works, but only if the gas is not water-soluble.
• If the gas is heavier than air and coloured, an upright cylinder can be used.

In biology the classic case is oxygen from a catalase or photosynthesis practical. Oxygen has low solubility in water, so collecting it over water is fine. Carbon dioxide is far more soluble, so a gas syringe is the safer choice.

Measuring time

• A stopwatch or stop-clock is usually accurate to one or two decimal places.
• Units are normally seconds or minutes (1 minute = 60 s), with longer units for very slow processes.
• The limiting factor is human reaction time. It has a significant effect when the intervals being measured are short — under a second or so.

This is a useful evaluation point. If a reaction finishes in 3 s, a reaction time of about 0.2 s is roughly 7 % of the measurement. If it takes 300 s, the same 0.2 s is negligible. The fix is to slow the reaction down so the time being measured is longer.

Measuring temperature

• Laboratory thermometers usually have a precision of half a degree or one degree.
• Digital temperature probes are more precise, often reading to 0.1 °C.
• Traditional thermometers rely on the uniform expansion and contraction of a liquid; digital probes can be just as accurate, if not more so.
• Units are degrees Celsius (°C).

Measuring length

• Rulers measure to the nearest millimetre; the standard unit of length is the metre (m).
• Larger distances — a transect on a rocky shore, for instance — need a tape measure.
• Many biological structures are far too small for either, which is why microscopy uses micrometres and nanometres.
• Watch your units: 1 cm = 10 mm and 100 cm = 1 m.

Making counts

Counting cells

Counting the cells in a known volume of a culture lets you work out the concentration of the whole sample.

• Direct counting uses a haemocytometer or counting chamber — a slide with a volumetric grid etched into it, divided into squares of known volume. You count the particles in a known volume and scale up.
• Plate counts involve plating a known volume of culture onto a petri dish with a growth medium and counting the colonies that grow.
• Direct methods do not need highly specialised equipment and are easy to perform, but they are time consuming.

Counting organisms

A great deal of ecology involves counting organisms, and it is far harder than counting cells:

• Many organisms move, or are inconspicuous and easily missed.
• Accurate total counts are rare, so population sampling is used instead and scaled up.
• Counts carry errors from the size of the population, from failing to detect individuals, and from direct counting mistakes such as miscounting large groups or misidentifying individuals.

Recording what you see

Biological drawings

To record observations made under the microscope, or from a photomicrograph, a labelled biological drawing is made. These are line pictures showing the specific features that were actually observed, and they follow a set of conventions: clean continuous lines, no shading or sketchy strokes, label lines that do not cross, and a stated magnification.

Qualitative observations

Not all data is numerical. Classifying organisms is a good example of a qualitative observation — data classified by attributes such as sex, colour of fur or number of limbs. The attribute being observed cannot be measured on a scale, so it counts as qualitative data.

Worked examples