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October 2015 Archives

I take back what I said last week about amazing vehicle management systems on milk tankers. Last night a GPS took a forty-two tonne tanker onto the three-tonne-rated Cambridge High Level Bridge, in what could have been a catastrophe. The bridge, with which I am very familiar, was designed for people, horses-and-carts, and the occasional small mob of sheep or a few head of cattle. Out of necessisty (witness the traffic mayhem this morning)  it now takes cars. NO trailers, and absolutely NO trucks. It's not as if it's difficult to see the warning signs - the approach is designed to slow you right down before you get onto the bridge. But then, if the GPS tells you to go that way, what are a few large, conspicuous warnings on a narrow 117 year-old bridge other than a mere distraction. 


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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.


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I'm currently at the Metrology Society of Australasia conference in beautiful Queenstown. For those that don't know, which might be most of you, metrology is the science of measurement. How do you measure things well?

At this conference, we've got presentations on measuring temperature, pressure, liquid volume (a surprisingly tricky one this - if you want to do it accurately, quickly and non-destructively), electrical properties, so on and so forth. There's lots of industry engagement here - unsurprising since making measurements well can make real differences in a company's bottom line.

For example, Richard Suckling from Fonterra talked this morning about some of the problems that Fonterra faces in terms of collecting milk from farms, and the measurement technology that is packed into each milk tanker. Milk tanker routing is a real example of a 'travelling salesman problem' - how do you optimise the route that a tanker takes to go between all its pick-up points? There's a lot of computer power that goes into doing just that - to ensure that the minimum number of tankers are sent out, they arrive within time constraints, with enough spare capacity to take on board the milk, but with enough weight on board already in the right part of the truck  to get traction of the more tricky farm tracks, be able to turn in and out of the farm safely and so on. Coupled with large seasonal changes in milk production, optimized tanker routing means lower fuel costs, and that's a huge saving. Then there's all the technology that goes into measuring just how much milk is being taken onboard at each farm, monitoring the temperature of the milk, taking samples for testing quality, and so on. This is just collecting the milk. He didn't go into what happens after that.

But for me the most interesting comment was regarding the colouring and finish of the tankers, because it has parallels with military stealth technology , the area in which I used to work. There was a time when the tankers were just shiny metal, but a series of night-time accidents changed that. Several incidents occured where cars (with headlights on) drove into the sides of tankers. The shiny metal just wasn't visible at night, even when illuminated with a car's headlights? Why? The shape was such that the large majority of the light from the car was reflected away from the car. Only a small fraction was reflected back to where it came from, meaning that the driver wouldn't necessarily see the large object straight in front of him. The current finish, including retroreflective paint and diffuse surfaces is much easier to see at night - and can be made into a nice attractive logo to boot.

This afternoon we had an industry 'site visit' to Gibbston Valley Winery, to check out the measurement technology involved in the wine-making industry. Sugar content, pH, yeast content, etc, all need to be measured (Or so some winemakers say. Others just go on 'experience'.) And there were lots of nice samples for us to 'measure' too...

I'm sure we'll get a good lot of talks tomorrow, too.



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In our first-year physics lab we have the following horticultural experiment. 


Here we have some bulbs growing on a rotating turntable. The array of five pots is placed on the turntable so that the centre pot is at the centre of the turntable; the left- and right-hand pots are at the perimeter.The turntable is rotating at about half-a-revolution a second. What happens as the plants grow?

Actually, this is a demonstration of centrifugal force just as much as it is a horticultural experiment. First the biology bit. Plants grow, pretty-much, towards the light. I'm sure someone will tell me the mechanism by which this happens, but for now I'll just state that as true. In this case, however, what is towards the light is constantly changing. The plants get equally illuminated from all sides. So we can take light out of the equation. 

The other direction plants grow is upwards. What do we mean by 'up'. It's against gravity. Again, someone will tell me the mechanism by which they achieve this (rather than sending their shoots straight down into the ground). But what is 'up' when you're on a turntable. 

Imagine you're standing on the spinning turntable. To remain 'upright' you'd have to lean into the centre. Why? In the rotating frame of reference, the one you're in, you experience centrifugal force pushing you outwards. You need to counter-act that. The same is true for the rotating plants. They effectively experience gravity as being downwards and a little bit outwards. Consequently they grow upward and a little bit inwards. Note how the centripetal force is proportional to the radius at which the plants are growing, so the ones on the ends of the line have more of a lean than the ones in the centre. (Unfortunately the left-hand plant as we see it has been a bit slow-off-the-mark, but you can still clearly see the lean.) Indeed, the central plant, sitting on the axis of rotation, experiences no centrifugal force and it grows straight upward. 

We can get a little more mathematical. The turntable takes T=1.7 seconds to do one revolution (I've just gone and timed it) and the outer plants are about r=20 cm off the axis. This means the centrifugal acceleration is given by omega squared times r, where omega is the angular velocity (= 2 pi / T). Doing the calculation we then get a centrifugal acceleration of about 2.7 metres per second squared outward. Compare this with the acceleration due to gravity, which is about 9.8 metres per second squared downwards. It's about a quarter the strength of gravity. So, for every four centimetres the outer plant grows upwards, it should grow by one centimetre inwards. A glance at the image will tell you that seems to correspond with what actually happens. 

Finally, then, a challenge to those who say centrifugal force is just something that you think is happening when you go round a bend in your car - it's not a real thing. Plants don't have a brain. They aren't just thinking they are experiencing centrifugal force. They ARE experiencing centrifugal force. 


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