Concepts of Metrology and Quality Testing
Senior 2 · Entrepreneurship
Concepts of Metrology and Quality Testing
Lesson 2 of 4
22 min
A metre of fabric in Kigali must be the same length as a metre in Kampala or Paris, or trade between them breaks down. Before shared units, traders used local measures such as arm-lengths and baskets, which varied from place to place and invited cheating. Today almost every country, including Rwanda, uses the International System of Units (SI), adopted internationally in 1960.
A base unit is one of the small set of units from which every other unit is built.
| Quantity | Base unit | Symbol | Everyday business use |
|---|---|---|---|
| Length | metre | m | fabric, timber, plots of land |
| Mass | kilogram | kg | sugar, beans, coffee, cement |
| Time | second | s | working hours, delivery times |
| Electric current | ampere | A | electrical appliances and wiring |
| Thermodynamic temperature | kelvin | K | cold storage of milk and medicines (usually stated in °C) |
| Amount of substance | mole | mol | chemistry laboratories, fertiliser and medicine makers |
| Luminous intensity | candela | cd | lamps and lighting |
Since 2019 all seven have been defined by fixed values of constants of nature, so any well-equipped laboratory in the world can reproduce them exactly.
Derived units are built from base units: area in square metres (m²), volume in cubic metres (m³), speed in metres per second. Some non-SI units are accepted for everyday use alongside SI: the litre (1 L = 1 dm³ = 1,000 cm³), the tonne (1 t = 1,000 kg), minutes and hours, and the degree Celsius (°C).
| Prefix | Symbol | Multiplies the unit by | Example |
|---|---|---|---|
| mega | M | 1,000,000 | 1 MW = 1,000,000 W |
| kilo | k | 1,000 | 1 km = 1,000 m |
| centi | c | 0.01 (one hundredth) | 1 cm = 0.01 m |
| milli | m | 0.001 (one thousandth) | 1 mL = 0.001 L |
Mass (in kilograms) is the amount of matter in an object. Weight is the force of gravity on it, measured in newtons. In everyday trade people say “weight” but mean mass: a bag of beans “weighing 50 kg” has a mass of 50 kg.
Written by Ganzaa.
Tip
Memory hook. To go to a smaller unit, multiply (km → m: × 1,000). To go to a bigger unit, divide (g → kg: ÷ 1,000). The answer should have more units when the unit is smaller. Learn the seven base units as a list: metre, kilogram, second, ampere, kelvin, mole, candela.
Solution: (a) kilogram, kg; (b) second, s; (c) ampere, A.
Solution: 3.2 × 1,000 = 3,200 g.
Solution: 450 ÷ 100 = 4.5 m.
Solution: No. It is an accepted unit of volume equal to one cubic decimetre, derived from the metre, a base unit.
Solution: “Net mass 500 g” (the symbol is g, with no plural and no capital).
Summary
Sources and credits
The conversion examples and the practice items are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
