Dry gases
Question: Recently I saw a large ore-carrying truck on which was a small
plate stating that the tyres on the truck were inflated with 鈥渄ry nitrogen鈥�.
Can you tell me what dry nitrogen is and why it would be used to inflate
heavy-duty truck tyres?
Answer: Road vehicle tyres are made from both natural and synthetic
vulcanised rubber, which perishes when exposed to water and oxygen due to oxides
and bacterial decay. By inflating tyres with nitrogen, and not air, which
contains oxygen, the perishing process is slowed.
Advertisement
The atmosphere is the cheapest source of nitrogen (about 80 per cent by
volume) but this contains substantial amounts of water vapour. So nitrogen
obtained from dried air is used and the working life of tyres can be more than
doubled by this means.
The use of nitrogen for inflation is not exclusive to truck tyres. Some
trains on the Paris Metro have rubber tyres, and these are also inflated with
nitrogen. This gives them an average running life of more than 200 000
kilometres.
Lewis Lesley
Professor of Transport
School of the Built Environment
John Moores University, Liverpool
Answer: Oxygen in the air of normal tyres slowly leaks out through the rubber
walls, and the resulting underinflation leads to higher tyre wear with a
consequent decrease in safety and higher fuel costs.
Nitrogen leaks out much more slowly and keeps the tyre inflated to the
correct pressure for longer. This reduces maintenance costs and, more
importantly, also improves safety.
Because it is inert, nitrogen also reduces the degradation of the rubber that
frequently occurs in the presence of moist oxygen.
Nitrogen has been increasingly used where there is a lot of wear, or where
performance and safety are key issues, such as in heavy-duty trucks, Formula 1
and Indycar motor racing, military and civil aircraft, and also in the space
shuttle. In the cases of racing cars, aircraft and the space shuttle, the
reduced risk of tyre fires is seen as an added advantage.
John Irven
Air Products
Basingstoke, Hampshire
Mountain mystery
Question: On a recent mountain trek, I came across a lake fed by mountain
streams and melted snow. The only outlet from the lake was a high waterfall, yet
the lake was full of fish. How did they get there?
Answer: The answer may depend on the type of waterfall that leads from the
lake. If the water falls as a series of steps and each vertical section is less
than 3 metres high, the fish in the lake are probably leaping migratory fish
such as salmon, which return in autumn in large numbers to mountain streams via
mountain lakes to spawn. However, if the falls are over sheer cliffs, the fish
could be a transient group of eels, which can actually migrate overland.
It is less likely that the fish represent a permanent population because
lakes at very high altitude are usually too small and too poor in nutrients to
support such a closed aquatic ecosystem. However, if the conditions are right,
in temperate regions they could have swum up a melting glacier at the end of the
last glacial period, and ended up in the corrie lake hollowed out by the moving
ice at the head of one of the glacier鈥檚 tributaries.
When the ice completely melted, the water flowing out of the former tributary
would form 鈥渁 hanging valley鈥濃�攁 high waterfall on the side of the main
U-shaped valley.
In such a scenario, the waterfall would have formed after the fish arrived,
and consequently the fish population must have been cut off for about 10 000
years. By now that population would have evolved into its own subspecies, like
the whitefish in upland lakes in the British Isles which date from this
period.
It is also quite possible that an outside agency could have seeded the
population more recently.
Organic candidates include birds which could, in a short time, transport
large numbers of tiny fish eggs in water plants that they are carrying in mud on
their plumage or feet, from one body of water to another. In this case, the
quick journey time would prevent dehydration of the eggs. Eggs could even have
been carried on the muddy boots of mountain trekkers. This is not as unlikely as
it sounds because trekkers often wade through streams containing newly laid fish
eggs. These can remain hydrated in the mud found on the walkers鈥� boots.
The inorganic candidate for fish transportation is the weather. Tornadoes are
known to pick up items, and have been responsible for 鈥渞aining鈥� frogs and fish
when they dump their burden later in a different place. A ready-made ecosystem
could be transported in this way, although it has to be pointed out that this is
a rare occurrence.
However, once the pioneering fish were in place it is very likely that they
would survive because of the lack of competition in the new environment.
Dorothy Reich
Chilton, Oxfordshire
Answer: The appearance of fish in mountain lakes is very interesting.
However, there is usually an obvious explanation especially if the lake in
question is in Britain.
In Victorian times, when the great hunting estates were being developed in
the north of England and Scotland, the landlords often stocked their mountain
lakes and lochs with fish, usually brown trout or Arctic char. The plan was to
use these bodies of water for game fishing.
Unfortunately these lakes often lacked the necessary nutrients for large
populations of fish to survive. However, where they did there were few predators
to cull the population. The result was a lake teeming with small fish, limited
by overpopulation and the size of the lake鈥攁nd useless for game
fishing.
William Banks
New Deer, Aberdeenshire
This week鈥檚 questions
Tonic blues: Last night my husband poured me a rather wonderful large gin and
tonic but we were both rather concerned to notice that our tonic water had a
curious bluish colour to it. What was going on here, or had I had one too
many?
Mary Bird
Warlingham, Surrey
Suck it and see: I have heard that if you drink a can of beer through a straw
you will become intoxicated more quickly. Many of my friends have heard it too.
Is it an urban myth or is it true and if so, why?
Haitsu Shiroyama
Chiba, Japan