Concepts of Metrology and Quality Testing
Senior 2 · Entrepreneurship
Concepts of Metrology and Quality Testing
Lesson 3 of 4
22 min
Entrepreneurs buy in one unit and sell in another: sugar by the 25 kg sack and by the 500 g packet, fabric by the roll and by the metre, juice by the jerrycan and by the bottle. Every conversion is a chance to make or lose money. Using units correctly lets you price fairly, compare suppliers and label products honestly.
To compare products sold in different sizes, convert each price to the price per unit (per kg, per L, per m).
Worked example (written by Ganzaa). A shop sells rice in a 2 kg bag for 3,000 Frw and in a 500 g packet for 800 Frw.
Worked example (written by Ganzaa; prices are illustrative). Uwase buys a 25 kg sack of sugar at 1,400 Frw per kg and repacks it into 500 g packets that she sells for 800 Frw each.
If her scale over-fills each packet by just 20 g, she can fill only 25,000 ÷ 520 ≈ 48 packets, and her profit falls. Accurate measurement protects the seller as well as the buyer.
The mass of a container is its tare. Gross mass = product + container; net mass = gross mass − tare. A label must declare the net quantity. A jerrycan of honey with gross mass 21.5 kg and tare 1.5 kg holds 21.5 − 1.5 = 20 kg of honey.
Milk and many medicines must be kept cold. Temperatures are usually stated in degrees Celsius, but the SI base unit is the kelvin: K = °C + 273.15. Milk chilled to 4 °C is at 4 + 273.15 = 277.15 K. A thermometer that reads 2 °C too low could let milk spoil without anyone noticing, so cold-chain thermometers must also be calibrated.
No filling machine is perfect. A tolerance is the allowed difference between the declared and the actual quantity. A juice factory might set a tolerance of ±5 mL on a 500 mL bottle (an illustrative figure): any bottle holding between 495 mL and 505 mL passes.
Tip
Exam technique. Put every quantity into the same unit before you divide or compare (kg with kg, mL with mL). Write the unit on every line of working, and check that the answer is sensible: 40 bottles from 20 litres is reasonable; 0.04 bottles is not.
Solution: Jerrycan: 12,000 ÷ 5 = 2,400 Frw per L. The jerrycan is cheaper by 2,600 − 2,400 = 200 Frw per litre.
Solution: 10 kg = 10,000 g; 10,000 ÷ 250 = 40 packets.
Solution: 50.8 − 0.8 = 50 kg.
Solution: 25 + 273.15 = 298.15 K.
Summary
Sources and credits
All worked examples and practice items are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
