What is the use of the leaf spring present around the partially evacuated aneroid capsule of sensitive altimeter.
The function of the spring is to pull on the evacuated capsule, such that it has a volume. Without the spring, the evacuated capsule would be entirely flat.
If the capsule were flat, a change in ambient pressure would not change the shape of the capsule; at any non-zero ambient pressure, the evacuated capsule would remain flat due to the force of the air pressure squeezing it.
Because the spring creates a force balance between the vacuum capsule and the ambient air pressure, the capsule will have a volume. Now a change in ambient pressure will change the balance between the force pulling on the capsule and the pressure of the ambient air trying to collapse it.
A new balance will be found at a different position of the spring. This change of position is translated into a rotation of the needle of the altimeter.
If the capsule was not filled with vacuum, it would expand if the temperature of the air inside it would rise, resulting in erroneous reading. With a vacuum capsule, the capsule is less sensitive to changes in temperature. Since the capsule is made of metal, temperature has an effect on the elasticity, and thus on the balance with the spring. To compensate for this, the spring is made of bimetal. This balances out the changes of elasticity of the capsule when the temperature changes.
- Aircraft Instruments & Integrated Systems, E.H.J. Pallett, Pearson Education Limited, 1992
Without the spring, the capsule would tend to collapse at or below sea-level pressure-altitude. The spring opposes the high pressure.
A variation of the principle above it to put the spring on the index mechanism.