Metrology, Quality Testing and Accurate Measurement

1Why this matters

A sack of coffee from a cooperative in Nyamasheke is weighed at the washing station, again by the exporter, and again by the buyer overseas. If the three weights disagree, someone is paid too little or too much. Fuel at a petrol station, sugar at a kiosk and medicine at a pharmacy all depend on measurements that both sides can trust. For an entrepreneur, accurate measurement is fairness, profit and reputation all at once.

2Measurement

To measure is to compare a quantity with an agreed unit. A result always has a number and a unit: “2.5 kg”, not just “2.5”.

3Metrology

Metrology is the science of measurement. It does three linked jobs:

• defining units that everyone agrees on (the metre, the kilogram, the second);
• realising those units as physical reference standards, such as reference weights kept in a national laboratory;
• traceability: linking every everyday measurement back to the reference standards through an unbroken chain of calibrations.

It has three branches:

• Scientific (fundamental) metrology: establishing units and the most accurate reference standards.
• Industrial (applied) metrology: making sure the instruments used in factories and workshops measure correctly.
• Legal metrology: the laws and official checks on instruments used in trade, health and safety, such as fuel pumps, shop scales and medical thermometers.

4Accuracy and precision

• Accuracy is how close a measurement is to the true value.
• Precision is how close repeated measurements are to each other.

A kiosk scale that shows 950 g, 951 g and 950 g each time a certified 1 kg reference weight is placed on it is precise (its readings agree) but not accurate (they are about 50 g below the true value). Adjusting it against the reference weight is called calibration.

5Quality and quality testing

Quality is the degree to which a product or service meets its requirements, whether set out in a standard or expected by the customer. Quality testing checks products, by measuring and inspecting them, against those requirements.

6How the three are linked

1. A standard states requirements in measurable terms: “net mass 500 g”, “fat content at least a given percentage”.
2. Quality testing measures the product to see whether it meets them.
3. Those measurements are only trustworthy if the instruments are accurate: calibrated and traceable to reference standards. That is metrology's job.

Without accurate measurement, quality testing gives false results; without quality testing, a standard stays on paper.

In Rwanda, the Rwanda Standards Board (RSB) provides metrology services. Through legal metrology it verifies instruments used in trade and health, including fuel dispensers, non-automatic weighing scales with a capacity below 2,000 kg, volume measures from 1 L to 5,000 L, and medical instruments such as clinical thermometers.

Worked example: the cost of an inaccurate scale

Written by Ganzaa. A kiosk scale shows 1 kg when only 950 g of sugar is on it. The shopkeeper sells 200 “kilograms” of sugar in a month at 1,500 Frw per kg.

1. Sugar missing from each “kilogram” = 1,000 g − 950 g = 50 g.
2. Total missing = 200 × 50 g = 10,000 g = 10 kg.
3. Value lost by customers = 10 × 1,500 = 15,000 Frw in one month.
4. If instead the scale read low, the shopkeeper would give away sugar and lose money. Either way an inaccurate instrument is unfair and, when discovered, destroys trust.

Practice