When you go to a wet market to buy some fruit, you can get charged by the catty or by the weight of the fruit. Let’s say you buy 450g apples. How do you know that it weighs 450g and not 500g or 350g? The Hong Kong government has a copy of the international prototype kilogram, against which it can be measured.
Metrology is the formal name for the study of measurement. It covers everything from the unit of temperature kelvin, to the time’s second and the unit of brightness, the candela.
In Hong Kong, the final authority on weights and measures is the Standards and Calibration Laboratory (SCL) of the Innovation and Technology Commission. At their main lab in the Immigration Tower in Wan Chai, they do their part to keep the gears of society more or less the same size and mass.
The SCL is the benchmark for calibration, for everything from an electricity or gas meter to a stopwatch. All of these measurements must be compared to their standards, which can then be compared further to international standards. This ensures that measurements will be accurate, and any errors can be traced.
This traceability is most evident with how we measure mass, using the humble kilogram. Other units are defined by certain absolute physical values. For example, the definition of a metre is: the distance light travels in a vacuum in around one three millionth of a second.
But the kilogram is defined by the weight of “the kilogram”. Specifically, the International Prototype Kilogram (IPK), currently held in France under the care of the International Bureau of Weights and Measures (BPIM).
Hong Kong has its own copy of the IPK, copy number 75, which we received in 1993. As befitting its importance, it was delivered by the Queen’s Messenger through the British embassy in France.
This small artefact is made of an alloy of 90% platinum and 10% iridium to a purity of one part per billion. It is also very dense – a one kilogram weight in stainless steel is over twice the size of the artefact, which itself is the size of a ping pong ball and is kept in a safe with double locks; one person holds the key for the lock, while another person has the combination. Inside the safe, it’s kept under two glass bell jars, keeping it airtight and clear of dust.
On top of that, it is used very rarely to avoid wear and tear and is sent to France for recalibration only once every ten years – and it will be making the trip again in September this year. Human hands do not come into contact with the weight when it is cleaned and washed in two very meticulous processes.
Beyond that, the lab itself is climate controlled, with the temperature and humidity kept constant by its own air-conditioning that is separate from the rest of the building.
All staff and visitors must even wear masks and either special lab shoes or shoe coverings when entering the mass calibration room. And once the setup for a calibration is done, staff usually leave the room and observe the process from a nearby computer station.
The reason for all this pomp and ceremony is the sheer fineness of detail the lab works with: the microgram, or one millionth of a gramme. At such a small scale, even the presence of a person in the room and slight differences in the environment can cause errors. For each calibration, the staff even has to adjust for the magnetization of the metal weights.
But for all the fine detail and security and obsession with cleanliness, the science of metrology has very real-world applications. “We design procedures to maximise accuracy and precision. It takes passion,” says Dennis Lee Wah-kwan, head of the SCL. “And it also takes vision, to know why we do all this work. To aid in Hong Kong’s research and development, and underpin commercial trade.”