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Make your own altimeter

Updated Tuesday, 27th February 2007

How easy is it to make your own? Our group of intrepid explorers had a go.

Supplied with a treasure map and some very basic instructions, our explorers were asked to find buried treasure on the slopes of a nearby mountain. They had to start on a mountain path and head up 80m in altitude until they found a 'Punga Island' upon which (by a wet log) they would find the place to start the search for buried treasure!

They had to build a device to measure altitude and so indicate their position along the rising track and then use the metal detector to locate the treasure.

What is altitude?
Altitude is the vertical height above a point, usually measured from sea level. Mountains are, of course, higher than sea level while some inland lakes, such as the Dead Sea, are below sea level. Devices that measure changes in altitude are called altimeters. To find the treasure they needed to measure a change of altitude accurately and so had to build an altimeter.

Air pressure
While the earth is solid, above us lies a layer of air - the atmosphere. Although we are not very conscious of it, we are constantly being weighed down by the air above us, known as air pressure. If we go up a mountain there is slightly less air above our heads than at lower altitudes. So there will be slightly less air pressure higher up a mountain than below. We can measure this quite easily. The air pressure also fluctuates with changes in the weather so the exact pressure is constantly changing.

How can we measure changes in air pressure?
One way of doing it is to make a U-shape out of plastic tube, filling it half with water and holding it up-right, so the water makes a U shape in the tube and is unable to flow out. If both ends of the tube are open to air there's equal pressures of air on top of each surface of the water and the water on each side will be at the same level.

If we cover one end of the tube, trapping the air pressure on this side, but leave the other end open, the levels will still remain the same. However if we go up in altitude, for example, up a mountain there will be less pressure on the water in the open tube but the closed side will, in principle, still have the larger pressure of the air below. Due to this difference of pressure between the two surfaces, the water will tend to move to compensate. The levels are now different and give us an indication in difference in pressure and the height we've risen up the mountain.

The device, used to measure altitude (or air pressure), is called an altimeter. A similar device used to measure air pressure changes due to the weather (but in a fixed location i.e a room in a house) is called a barometer.

We can calibrate the altimeter by taking it up a known height and measuring the change in the levels of the water. We can then use the calibration to estimate the change in height and use it to work out how far we have gone up or down in altitude. Our explorers calibrated the DIY altimeter by taking it up Sentinel Point which, according to the maps, is about 40m higher in altitude than the surroundings.

Making a water altimeter
Take about 3-4m of thin (about 6mm diameter) clear plastic tubing and attach it to a wooden board to form a U shape with sides of about 30cm in length. Drill the board and use wire or string to secure the tube. Make the U quite narrow so that the two sides are close together, about 2-3cm, making it easier to see any changes in the water level.

On one side, coil up the extra tubing (important: don't cut off the extra tube) and fix a clamp onto the end so that it can be pinched closed by tightening the clamp. Leave the other (short) end open. With the board vertical and both tube-ends open, carefully half fill the U tube with coloured water (you could soak a teabag in water for 10 minutes to colour the water) so that it is easy to see.

Other ways to calibrate an altimeter
Lifts in high-rise buildings are ideal places to calibrate an altimeter! Try going up half the total number of floors and mark off the level change and check that you get roughly double the change for the highest floor (assuming the lift starts at the bottom and ends near the top). If you can ask the building caretaker for the building plans, the height should be available and you can calibrate it properly.

Using the DIY altimeter
It was pouring down with rain when the group headed off to look for the treasure. The mountain streams were high due to the immense amount of water flowing down the hills and you could almost see them rising! They started from the bottom of the mountain track and headed off, navigating the trail using the altimeter. Before heading up they reset the altimeter to read zero at that level then walked along the track. At about 40m they stopped and one person checked a professional altimeter. He read the dial but, for the purpose of the exercise, declined to say how it compared (he revealed later on that it had been very near!).

By about 50m they arrived at a mountain river forming a gorge heading up. They followed the riverbank for sometime, having to cross the bubbling stream in order to find a dry place to walk. At about 80m there was a clearing and they could see that the river split into two at this point and this formed an island in the middle - an island covered in Punga trees!

The DIY altimeter had navigated them successfully to the hidden treasure!


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