The scales at the airport told me my baby weighed 8.2kg. But the set at the medical clinic told me he was 7.5kg. Could the national carrier be charging for excess baggage based on faulty scales?

This story was first published on Stuff.

Think of the last time you used a set of scales. Or purchased something that had been weighed or measured before being packaged.

Perhaps you’ve taken some medication today. Or bought chia seeds at the supermarket. Put petrol in your car. Posted a parcel. Poured cement.

You trust those numbers, don’t you? Of course you do. The Weights and Measures Act sets out rules for selling goods by quantity.

It’s often only when measurement tools tell us something odd we start to notice them.

Like when you use the scales at the airport to weigh your baby. I mean, your bag. Or your baby. And then you weigh him – your baby – again at the doctor’s office and the difference in weight is 700g.

It doesn’t sound like a lot but for an 11-month-old, it is.

Let me explain.

The Air New Zealand scales at Auckland Airport’s gate 28 said my son’s weight was 8.2kg. But the following afternoon, the scales at the medical clinic said his weight was 7.5kg.

Could the airline be charging passengers for excess baggage based on faulty scales? It’s not beyond the realm of possibility. In 2008, an investigation into scales at major UK airports found widespread weighing errors. Ticket counterweights at US airports have also been found to be inaccurate.

I contacted Air NZ’s media team: “Is someone able to explain how the scales are calibrated? Is there consistency across the country?”

A spokeswoman told me the scales are “standard calibration”.

But not all scales, even digital ones, are created equal. To be used for trade they need to be approved and verified.

This is where Mike Adamson, a Trading Standards-accredited weighing scale service technician – or metrologist – comes in.

Adamson is managing director at ScaleLogic Limited, a company specialising in supplying, maintaining and repairing weighing, filling and dosing systems, as well as calibrating measuring instruments across industries. He calibrates and certifies scales from small lab balances in food factories through to 200-tonne silos for weighing grains.

This involves using legally-verified “test masses” of an approved type with an unbroken measurement chain back to the International Prototype of the Kilogram, or IPK.

The IPK, a very special lump of metal stored in France, is part of an often unnoticed international framework, called the International System of Units (SI). It underpins the 20,000 weighing scales in supermarkets around Aotearoa, for example. It ensures you pay for the right measure of chia seeds.

As my colleague Keith Lynch wrote: “The concept of a kilogram is unmoved by politics, disinterested in social media chatter – a dispassionate, unbending foundational truth.”

For those in the know: Yes, the kilogram was redefined in 2019. But I’m not going to go into that here because the IPK is still used for calibrating mass standards around the world.

Every year, Adamson sends his test masses to Trading Standards in Wellington, the agency that administers and enforces the nation’s system of trade measurement. The masses are tested against one of a higher classification that in turn has been calibrated by an even higher classification. And so on, right up to the IPK.

“That’s how you get measurement traceability,” Adamson says.

“Our 20kg weights are verified to an accuracy of ±0.6g which is ±0.003% of maximum permissible error.”

I went back to Air NZ’s media team: “Can you tell me anything further about the process [of calibration]?”

They could – or would – not: “Thanks for your follow up questions but we don’t have anything further to add here, sorry.”

A week later, my son and I boarded another flight. I weighed him on a different set of Air NZ scales. They said he weighed 8.3kg.

This time, I noticed several stickers on the scales. A “mark of verification” and a “certificate of accuracy” provided by Wedderburn, an Australian scales business since 1896.

National service manager Brent Taylor tells me certificates are awarded by an accredited person and show compliance with the requirements of regulation 20 of the Weights and Measures Regulations 1999. Wedderburn issues in excess of 10,000 a year.

“Every 12 months, we’ll roll up and put a heap of hand weights on those machines and issue them with certificates of accreditation.”

Wedderburn also works with Plunket and other health agencies to ensure their scales are accurate.

I ask whether my son is 7.5kg, 8.2kg or 8.3kg.

He explains the scales at the airport are designed to weigh items from 100g to 100kg. Most paediatric scales have a weight capacity of 20kg and a graduation interval of 10g. Machines in neonatal intensive care units are accurate down to 2g.

“So the airport scales count by a bigger number,” Taylor says. “The paediatric scales will be more accurate for smaller weights.”

That’s assuming they’ve been calibrated correctly. But general practitioners aren’t using their scales to trade by weight. Meaning they don’t need to be trade certified.

“I’d suspect the baby scale your doctor is using mightn’t be 100% correct.”

Hospital scales get tested for accuracy because they’re used to inform medication doses. In my son’s case, we’re simply tracking growth over time.

But to satisfy my curiosity, Taylor arranges for a colleague in Christchurch to weigh my son on a set of new and precise paediatric scales. “They’ve just been tested,” we’re told.

I take off my son’s boots and am assured – with a demonstration involving hand weights – his clothing weighs no more than 50g. We watch the numbers tick over before they stop at 8.03kg.

Finally, we have an answer.

The GP is underestimating my son’s weight while Air NZ may be overestimating it, by a fraction. Presumably, no one is fined for having baggage 200-300g above the limit.

So the next time I want an accurate weight, for my son or myself, I know where I’ll be going.