Someone has to do it. There are laws in NZ pertaining to how the stated volume of bottled liquids corresponds to their actual volume. If, for example, you are selling beer in 375 ml capacity bottles, you need to make sure that your bottling plant is working to the NZ definition of what 375 ml actually means. In a bottling plant, the volume of liquid supplied to a bottle is often controlled by back-pressure. This is the same mechanism that causes a petrol-pump to cut-out when your tank is full. Generally speaking, it gives an adequate measure of when your bottle is filled with the appropriate amout of liquid.
There will always be variations in the amount supplied. One bottle will never contain exactly the same amount as the next. So for trading purposes, 375 ml must have an appropriately practical definition. One of the talks at the Measurement conference last week, by Chris Sutton, looked at some of the issues behind this. Chris talked about the current law – I didn't write this down – but it includes such things as the average volume per bottle not being less than 375 ml when sampled over a certain number of bottles, and restrictions on just how much below the stated volume of any individual bottle can be. However, there's no point having any laws or industry standards if it's not possible to measure it.
And there is the problem, really. Measuring volume isn't an easy thing to do. One could sample lots of beer bottles and tip out the contents into calibrated measuring containers. Such things exist. The problems with that, however, are that the process is slow and your small craft brewery doesn't enjoy having a significant fraction of its output being destroyed in the process of checking it's obeying the law. Consequently, it's actually better to measure volume by using mass and density. Here, one first would measure the density of a sample of the beer being fed into the bottling line. Then a number of empty bottles are chosen, and accurately weighed. The bottles go back into the production line, and after they are filled they are weighed again. That gives the weight (and therefore mass) of the beer that's been added. Knowing the density of the fluid inside, one can then do a simple calculation of volume = mass/density to find the volume in each bottle. That way, the volume is measured without significant loss of the end product. That keeps the small breweries happy.
Except, there is a problem with this. That's the carbon dioxide content. The density of the beer changes with the concentration of CO2 dissolved. So when we talk about volume of beer, do we mean with or without the CO2? Currently, the most robust way of defining a measurable standard is for de-gassed beer. Get rid of the CO2 and then measure. But doing this is a destructive process – you don't get your beer back afterwards. So, how do we come up with a practical standard for the case of carbonated drinks that keeps both the maker and the consumer happy? It's still an open problem. Answers to Chris Sutton, please.