The thin layer of air that envelops this world is immensely important to the things that live within it. We depend on the air for life. Smaller planets than the Earth don't creat a large enough gravitational pull to stop the gases escaping and if for example, Murcury or the Moon ever had an atmosphere the great majority of it has long been lost. Even the Earth is losing some of the light gases all the time. Helium for example tends to escape the Earths pull and never builds up.

The atmopshere that remains bound to the Earth forms a blanket of gas extending roughly 50km or so above our heads. All this gas weighs down on us and although we are unaware of it there is a constant air pressure pushing down on us as a result. Of course the air is very light but there is enough of it to be significant. At high altitudes (in planes or on mountains) the air pressure is less. Also changes in weather can cause the pressure to vary by as much as 15-20% and provides a pre-warning that the weather is likely to change in a day or two.

One way of 'weighing' the air, to measure its pressure, is by using a barometer. Imagine the following thought experiment. If you could put a column of air (extending all they way to the top of the atmosphere) on one side of a measuring scale and on the other pan of the balance put a similar column of murcury in a tube, how much murcury would you need to balence the weight of all that air ? Well we can work it out :

The density of air is about 0.0012g per cubic cm at sea level but becomes less dense as we go higher up. So in other words a thimble of air, which is about 3 cubic cm, will weigh about 4mg (4 milli grams = 4/1000g). Murcury weighs 13.6g per cubic cm. So murcury weighs about 10000 times that of the same volume of air. So we need about 10000 times less murcury than air to get a balance on our scales. If the main weight of the atmosphere is about 10km above our heads then we should be able to balance a column of the air by a column of murcury of length = 10000/10000 =1m. So we can balance all the air above us using roughly 1m of murcury !

This principle (or fact) is used in the murcury barometer used to predict the weather. This is simply a glass tube filled with murcury closed at one end and which dips into a cup of murcury. If we fill the glass tube and turn it upside down into the cup the air pressure will be able to support a 1m column of murcury. If the tube is a little longer then a vacuum exists at the top. If the air pressure changes (due to weather or taking the barometer up a mountain) the height of the column of murcury within the tube will change. By clever means a dial can be attached to the top of the murcury which therefore gives us a direct reading of the air pressure. This is called a murcury barometer and is used widely for weather prediction.

Murcury is hard to get hold and rather poisonous. So to experiment a little with this facinating subject lets build a barometer but using water instead of murcury. Water has some advantages as it is roughly 10 times less dense than Murcury so a full size barometer will be about 10 times bigger, we dont need to make a full barometer to measure pressure changes but we will still get 10 times more sensitivity. A down side is that water evaporates which makes it unsuitable for measurements over a long time.

What it is
The U-tube manometer is a device to measure pressure differences and can be used as a simple barometer. It is simply a vertical U of bent plastic tubing, the bottom half of which is filled with water. When there is a pressure difference between the two ends of the tube the water is forced to move in the U-tube. This movement is the indicator of the changes in pressure.

U-tube manometer

How it works
When both ends are open to the air the forces on the water due to the atmospheric pressure are equal and so the water level on each side of the U are the same. If you blow carefully into one tube the forces will be greater on that side and this will push the water down on that side. As the water is difficult to compress the water the other side will tend to move up. The two levels of water in the U tube will therefore not be level any more. If we stop blowing the two ends will be open to air again and the levels will become equal.

If we block up one side of the U tube then that side will be seeled and fixed at the pressure at that time. If the other side is open to the air any changes in air pressure will therefore unbalance the system and register automatically by a corresponding change in the water level on the two sides.

Find some clear plastic tubing (5-10mm diameter, say 2-3m worth). Fix a portion of the tube (near to one end) to a piece of wood as shown in the picture and make a sort of U so that the bottom of the U is at the bottom of the wooden base. Coil up the tube on the side with the most tube and fit a clamp to this end - dont tighten it up yet though. Mount the device vertically. Pour some water into the other open end of the U tube so that about half the U is full of water (use food colouring (Ribena) to make the water show up more, in Rough Science Ellen made a die for me from logwood which gave a wonderful deep red colour).

When a manometer (barometer) is used to measure changes in pressure due to going up a mountain for example we call this device an alitimeter. At sea level we have the whole atmosphere of gas above us and so a great deal of weight pushing down. If we go up a hill, or mountain, as we climb we will have less air above us and so the weight will be slightly less. This will correspond to a drop in atmospheric pressure as we go up. Mountaineers and also aircraft navigators use this principle to work out their height above sea level.

Lets do a simple very rough calculation:
If 10km of air can be balanced by about 1m of murcury then about 10m of water (water is 1/10 the density of murcury) will also balance the air pressure.

Using this information and scaling down we get that:
10m of water = height of atmosphere (10km)
1m will balance 1km of air
10cm will balance 100m of air
so 1cm of water will balance about 10m of air.

So if we set up our U-tube barometer at sea level and go up a 100m high hill the water in the U-tube will move about 10cm (one side goes 5 cm down the other 5cm up, so 10cm in all) - Try it out its really works ! This is what Kate and I found in Rough Science 2 when we went from the high point at the factory down to sea level and what Ellen and I found in the altimeter challenge in RS3.

(NOTE: These calculations are a rough guide only and you may not get such a large change in water movement but you will see a change).

In order for the device to measure the pressure change we need to shut off one side of the barometer and make sure that the pressure in this side does not change. At first sight this seems very stright forward - just squeeze a clamp down onto the tube and seel it off. However when the water starts to move (as you go up in height) the volume in the closed area will change because of the water moving into this area and so the pressure wont be constant as it should be. This tends to counteract the change due to the height and makes the altimeter less sensitive / inaccurate.

The way to solve this problem is to have a lot of empty tube on the closed side of the barometer. You can just coil it up out of the way but make sure you close it at the very end of the length. Any change in volume due to the water moving will be only a small fraction of the total volume of this long tube and so will solve the problem. However, a down side to this modification is that we now have a lot more air in the closed off part of the tube and this will be sensitive (expand or contract) to changes in temperature. So we need to make sure the coiled up tube is fixed under a shade to protect from direct sunlight and minimise any heating.

Hollybury castle
my house
 'sea level'

The simple altimeter in action. In the middle picture the altimeter has both tubes open - both the levels of water on each side are therefore level. The left hand tube was then closed and we climbed up the hill 20m or so higher. The left hand picture shows the result - because the open tube is now at a lower pressure than the closed section the level of water in each side is not now equal (right tube higher than left). In the right hand picture the altimeter was set up in the same location as before but taken down hill about 20m instead. Again, the level of water is clearly not equal but because the open tube is now at a greater pressure than the closed section the high and low water levels are reversed.


Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ

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