By Marcus Wilson 22/10/2015

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.

0 Responses to “How do you measure the volume of beer in your bottle?”

  • Another great post Marcus, I do enjoy the way you think. In medicine, of course, they’d not measure the volume at all, merely a surrogate for the volume. I could well imagine an experiment (involving lots of students) in which the ability to walk in a straight line is calibrated against volume of beer consumed. Once upon a time a nomogram would have been devised, but these days an app perhaps, in which the “straight line” ability along with confounders (% alcohol in the drink, sex, weight etc) could be entered and out would pop an estimate of the volume.

  • An interesting question but whats missing from this problem is the level of precision required, is it +/- 0.1ml or 5ml? Understanding that and then the impact of dissolved CO2 on volume will inform the quality of measurement required. I would suggest that the easiest technique is to accurately measure the height that 375ml of water (or equivalent to allow for dissolved CO2) reaches on the neck of a good sample of bottles and then use that height as a proxy for volume. This is how any pub measures the volume of beer sold.

    • The trouble is, not every bottle is exactly the same. Measuring volume well is a time-consuming process (so I’m told). Volume sold in a pub is interesting. As far as I’m aware, in NZ there is no standard. When a pub offers ‘beer’ for sale, they don’t have to specify a standard measure. Coming from a place where beer is sold in pint or half-pint measures, in glasses that are clearly marked as such, that seems a rather major oversight in our trading laws, but there we are.

  • Unless things have changed a lot, the petrol pump nozzle is triggered by the venturi effect of the flowing fluid. There is a small uptake air line near the tip of the nozzle that flows to an air chamber with a diaphragm above the liquid venturi. Once the air line has liquid instead of air in it, the pressure changes and the nozzle diaphragm immediately blocks the fuel flow off. The most well known manufacturer of fuel pump nozzles in NZ was ZVA, withe the V standing for venturi, if I recall correctly.

    Carbonated drinks are typically filled by a counter-pressure system – the bottle is top filled with carbon dioxide to desired pressure ( which also removes undesirable oxygen ), and then filled from a liquid line near the bottom of the bottle with the pressure controlled by venting the CO2 through the top line. This is all well-known technology and available from several specialist carbonated beverage bottling plant manufacturers. The same technique can be used for home-brewers, craft brewers, and large scale brewers. The commercial reality is that bottling plants will be designed to perform within national regulation controls for volume, without offering too much extra beer to clients.

    Details of the above technology is routinely available from fuel nozzle manufacturers and carbonated beverage bottling plant manufacturers, but I’d struggle to believe the fuel nozzle and beer filling are based on the same concept.

    • I stand corrected. I may have misinterpreted the notes that I took during the talk. But both petrol-pump and bottling methods for controlling volume of liquid are both pressure-based at least.

  • The small beer maker is bottling beer that has natural carbonation. So the insertion of CO2 is not carried out here. Breweries, it can be assured, take extraordinary measures to make sure they minimise wastage. ie no more than they can possibly get away with legally. The regulator problem is the major breweries have a lot of bulkbeer production. Whereas lots of boutique brewers have small batches and lots of them.

  • Counter-pressure filling is not typically used for adding CO2, it’s for ensuring CO2 is not lost during bottle filling, and also ensuring undesirable oxygen in the empty bottle is removed – although some bottling systems use vacuum instead. With counter-pressure filling, the remaining opportunity for loss is during capping, and the beer is usually chilled before bottling and capping to reduce such losses.

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