Bar
The bar is one hundred thousand pascals, almost exactly the air pressure at sea level. That is what makes it so handy: the atmosphere is about 1 bar, a tyre 2–3 bar, a scuba cylinder 200 bar. Under water the pressure grows by 1 bar every 10 metres, and at the bottom of the Mariana Trench, almost 11 km down, it is about 1 100 bar — a tonne on every square centimetre.
The bar is not part of the SI, but the laws of the European Union and Russia allow it on a par with the pascal. In meteorology it has been replaced by the hectopascal and in engineering by the megapascal, but in tyres, compressors, diving and hydraulics the bar has stayed: a number in bar tells you straight away how many times the pressure exceeds atmospheric.
| Notation | bar · mbar · names: bar, millibar |
| Definition | 1 bar = 10⁵ Pa = 10⁵ N/m² exactly |
| Neighbours | 1 atm = 1.01325 bar · 1 at = 0.981 bar · 14.5 psi |
| Under water | p = pₐ + ρgh · seawater ≈ 1.005 bar per 10 m |
| Status | non-SI, allowed by EU and Russian law |
| Typical values | air — 1 · tyre — 2.2 above atm. · cylinder — 200 · ocean floor — 400 · Mariana Trench — 1 100 |
01 · Definition
One hundred thousand pascals — and a 10-metre column of water
Pressure is force per area. A pascal is a newton per square metre, which is very little: a sheet of paper lying on a table presses with about 1 Pa. A bar is 100 000 pascals. That is how hard the column of air above us presses, about 10 tonnes per square metre, and so does a column of water about 10 metres high.
Under water the pressure is the air above the sea plus the weight of the water overhead: p = pₐ + ρgh. Every 10 m adds about a bar. It does not depend on the shape of the vessel or on how much water is around — only on depth. At the bottom of the Mariana Trench, at a depth of 10 935 m, the pressure is about 1 100 bar: on a palm it presses like a hundred and fifty cars.
How many tonnes press on a viewport
Depth is the slider, the window is the buttons. The force is the water pressure above the atmospheric pressure inside the vehicle, multiplied by the area of the window. The depth scale goes down to the bottom of the Mariana Trench.
The dive: every 10 metres, one more bar
The main action is “Start the dive”. The vehicle leaves the surface for the Challenger Deep, 10 935 m. Light fades by 200 m, total darkness sets in by 1 000 m, and below that there are only floodlights and the living light of deep-sea animals. At the side you can see at what depth each kind of dive ends: diver, submarine, whale. On the right is what the pressure does to things outside: a foam cup shrinks, an air bubble gets smaller, while steel and water hardly change. The dive takes about 24 seconds; a real descent lasts hours. Choose a vehicle below the scene — each has its own strength limit.
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02 · Conversion
| {n} | {v} | {note} |
Quick sums under water: divide the depth in metres by 10 and add 1 — you get the absolute pressure in bar. 30 m — 4 bar.
Tyres in the USA are inflated in psi: 32 psi — 2.2 bar. Multiply psi by 0.069 to get bar.
03 · Orders of magnitude
04 · Measuring instruments
05 · Writing rules
06 · Neighbouring units
07 · Historical section
The bar is defined exactly: 10⁵ Pa, with no standards at all — through the newton and the metre. Pressure standards are piston gauges: an accurately known weight presses on a piston of known area. Up to hundreds of bar their error is thousandths of a per cent.
Depth is worked out from pressure with corrections: seawater is denser than fresh water and is compressed with depth, and g depends on latitude. That is why the pressure at the bottom of the Challenger Deep is about 1 086 bar and not the 1 100 a simple formula would give — it was measured with quartz sensors, and the depth was calculated from it.
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