Atmospheric pressure is the result of the weight of the atmosphere pushing from above versus the downward force of gravity. The higher the point in the atmosphere, the lower the atmospheric pressure.
An altimeter measures changes in atmospheric pressure to calculate altitude in feet or metres above mean sea level. The most common altimeters are barometric. A chamber inside contains a stack of sealed aneroid cells with vacuum inside. The cells expand and contract with changes in ambient atmospheric pressure. As a plane descends, ambient atmospheric pressure increases, compressing the cells. This is indicated as a decrease in altitude. During ascent, ambient atmospheric pressure decreases, allowing the cells to expand. This is indicated as an increase in altitude. Note, this works only in aircrafts without pressurized cabins. To compensate for changing weather conditions and to ensure the altimeter reading is correct, an altimeter must be calibrated to a known pressure value at take-off, for example, current air pressure at mean sea level.
Image: Illustration shows the altimeter instrument from an airplane
- Aneroid cells
- Kollsman Window
- Barometric scale adjustment knob
Activating the Altimeter
(Crown Position 1)
Unscrew the crown at 4 o’clock into position 1 to activate the altimeter. A red ring appears, indicating that the altimeter is in use. Important: The watch is not water resistant at this stage.
Setting the Altimeter (setting a reference value)
(Crown Position 2)
Pull the crown out to position 2 and set the altimeter by rotating the crown. There are different modes of settings, such as rotating the crown to adjust the yellow indicator to your known altitude. Another mode is rotating the crown until the reference air pressure QNH (e.g. supplied by an airport control tower) aligns with the red triangle at 6 o'clock on the central dial. The watch now displays the present altitude, yellow indicator, and the actual atmospheric pressure, red indicator.
Measuring altitude
(Crown Position 1)
Once the altimeter has been set, push the crown into position 1. Changes in valtitude are shown by the yellow indicator against the outer dial ring, on a scale from 0 – 19,700 ft, or from 0 – 6,000 metres (01 793 7775 87xx is available in both, feet and metres. The Oris X BWD Limited Edition 01 793 7775 8724 is available in feet only). The former altimeter 01 733 7705 has a scale from 0 – 15,000 ft, or from 0 – 4,500 metres (01 733 7705 is available in both, feet and metres.)
Returning to neutral
(Crown Position 0)
To deactivate the altimeter and return it to neutral, screw the crown back into position 0. This also reseals the watch so that it is once again water resistant to 100 metres.
The Oris ProPilot Altimeter is only water resistant to 10 bar/100 m when the crown at 4 o’clock is in position 0 and tightly screwed. When this crown is unscrewed in position 1 or 2, the watch case is open in order to allow circulation of air which is indispensable for the functioning of the barometric altimeter. In the crown position 1 and 2 only humidity is prevented from entering the watch case by means of a protecting PTFE membrane.
The Oris ProPilot Altimeter does not only show the accurate time, but also the altitude and atmospheric pressure, as well as altitude differences and weather trends. All measurements of the altimeter are based on the absolute barometric pressure.
Starting at 1013.25 hPa at sea level, the atmospheric pressure drops to 50 % at an altitude of 5,500 m above sea level, to 10 % at 18,400 m.
The correlation of atmospheric pressure and altitude is an approximation, as it is significantly influenced by weather dynamics, temperature and other parameters.
As a consequence, a barometric altimeter must comprise a component to adjust the altitude scale in order to compensate for changing conditions.
Still today, altimetry in aviation is commonly based on the barometric principle invented and patented in 1844 by Lucien Vidie, a French engineer and inventor.
The core of the altimeter module of the Oris ProPilot Altimeter is an aneroid cell, purely mechanical and maintenance-free. With 1½ turns of the altimeter hand, it can display a range from 0 up to 19,700 feet or 6,000 metres of altitude, depending on the dial version chosen, with an accuracy of +/– 30 m. The air-pressure scale is recessed between the altitude scale and the central dial.
Note: Pressurized aircraft cabins simulate an environment of a lower altitude than the airplane is actually flying. Every barometric altimeter, be it electronic or mechanical, as the Oris ProPilot Altimeter, indicates the current simulated air pressure when flying with air carriers. The only way to measure the correct altitude in pressurized cabins is through radar or GPS positioning. Use the Oris ProPilot Altimeter in flight operations only as a supplementary instrument.