1.6.3 Litres and kilograms
The two physical units of measurement that you will probably come across most often in your workplace concern volumes of liquids and weight measurements. It's important to get a feeling for what various factors of ten look like, so that you can spot when there seems to be a mistake in a value that you've calculated or have been given by someone else.
The litre is the main unit of measurement for liquid volumes (written as liter in America), but what does a litre of fluid look like? What about a millilitre (ml; one thousandth of a litre) or a microlitre (μl; one millionth of a litre)?
A litre is the volume of liquid or gas that would fit into a cube measuring 10 cm on each side (10 cm × 10 cm × 10 cm = 1000 cubic centimetres (cm3) = 1 litre). A millilitre is the volume of a cube measuring 1 cm on each side (1 cm × 1 cm × 1 cm = 1 cm3 = 1 ml). A microlitre is the volume of a cube measuring 1 mm on each side (1 mm × 1 mm × 1 mm = 1mm3 = 1μl)
A typical carton of fruit juice has a capacity of a litre.
A teaspoon holds about 5 millilitres of liquid.
One raindrop is about thirty microlitres.
Look around your workplace to find out the typical volumes used for various applications: e.g. what's the capacity of a blood bag or a drip bag? How about the various syringes or pipettes you might use? If your work is lab-based, then you will probably be measuring volumes down to a smaller scale of microlitres. Again, get used to the volumes of typical containers (e.g. 7 ml and 20 ml specimen tubes; 2 ml, 1.5 ml and 0.5 ml micro-centrifuge tubes) and the equipment appropriate to measure out each of these various volumes (measuring cylinders 1-1000 ml; pipettes 0.1-30 ml; micropipettes 0.1-1000 μl).
The kilogram is the basic unit of measurement for weight. Again, what does a kilogram feel like? What about a gram (g; one thousandth of a kilogram)? What about a milligram (mg; one thousandth of a gram)?
A litre of water weighs one kilogram.
A £20 note weighs about one gram, whilst a pound coin is almost 10 grams. Originally, a 'pound' was the monetary value given to a pound weight of sterling silver (an alloy of 92.5% silver and 7.5% copper) - hence 'pound sterling'.
About 12 grains of salt weigh one milligram.
Once more, look around your workplace and find the typical weights that you might deal with. Get used to the different sizes of various tablets and medications and the typical doses that patients are given. This will help you spot when a calculation might be wrong and you need to double-check with someone.