We buy lots of goods by either weight or volume: petrol or diesel by the litre at the pump, and many items by weight at the supermarket. A system administered by the Ministry of Consumer Affairs' Measurement and Product Safety Service (MAPSS) keeps everything honest.

The MAPSS weights and measures system

The system covers any measurement used for trade purposes - weight, volume, or length. And quite a system it is too. Every instrument and machine used to weigh or measure goods must meet the following:

Type approval

Every measuring device must be type-approved. This involves the device being laboratory tested to prove its accuracy. Type approval can only be granted by approved laboratories, either here or overseas. A type-approval certificate number must be attached to the device.

Verification by accredited persons

Every measuring device must be verified before it can be used. This means that it must be individually checked for accuracy.

A sticker is attached to the device to confirm that it has been verified. If a piece of equipment is repaired, then it must go through the verification process before it can be used again.

Certificates of accuracy

Once the device has been type approved and verified, it can be used for trade - and from then on its accuracy is checked through a voluntary system of certificates.

These certificates of accuracy last for 1 year, although a small number of traders replace their certificates on a 2-yearly basis. While the certificate system is voluntary, it's used extensively by traders because it gives them confidence that their measuring devices are working as intended.

It also provides traders with some legal protection, if a problem arises.

Proactive inspections

As well as the above checks, MAPSS runs a system of market surveillance - around 1000 different weighing instruments and 400 measuring instruments (such as fuel dispensers) are spot-checked for accuracy every year.

A team also purchases and weighs pre-packaged goods to make sure they stay within internationally accepted tolerances.

Investigation and enforcement of complaints

All complaints from the public are investigated and followed up. Any necessary action is taken.

Does the system work?

Just to see if the system seemed to be working, we spot-checked 5 supermarkets in the Wellington area.

In their self-weighing fruit and veg section, 4 of the 5 supermarkets had scales without certificates of accuracy. That's OK up to a point - neither certificates, nor verification are required here because the scales are not used for "trade" (that is, a selling price is not set using these scales).

All the supermarket checkout scales had current certificates of accuracy.

Our advice

• The system is reasonably rigorous. You can have confidence that the goods you purchase have been weighed accurately and the fuel you buy dispensed accurately. But cracks may develop in any system - and failures can occur.
• We think supermarket product scales should have current certificates of accuracy.
• If you think you've been short-measured, you can contact the MAPSS service directly (see More information, below). It will investigate.