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What is the use of the leaf spring present around the partially evacuated aneroid capsule of sensitive altimeter.

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2 Answers 2

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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.


References:

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  • $\begingroup$ So you mean to say capsule is completely vacuum from inside. Then how does it expand as we go up . $\endgroup$ Dec 17, 2021 at 4:00
  • $\begingroup$ Sir second doubt is as you are saying that spring pull up the capsule so if we go to a airfield below MSL. Then this will create huge errors. $\endgroup$ Dec 17, 2021 at 4:01
  • $\begingroup$ @SourabhJain It expands because the spring is pulling. As the pressure on the capsule decreases, the spring expands it further until a new equilibrium is reached. W.r.t. going below sea level, there is no difference to the system. The pressure increases, so the balance is found with the spring more compressed. $\endgroup$
    – DeltaLima
    Dec 17, 2021 at 19:25
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Without the spring, the capsule would tend to collapse at or below sea-level pressure-altitude. The spring opposes the high pressure.

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Source

A variation of the principle above it to put the spring on the index mechanism.

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  • $\begingroup$ so what happens at high pressure ? does spring completely block all the static pressure going towards aneroid capsule. I am not able to get that part and secondly what is there inside the capsule . Is there a gas or what $\endgroup$ Dec 17, 2021 at 3:56
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    $\begingroup$ @SourabhJain: In the capsule there is a bit of dry air, but at very low pressure, so it easy to change its shape, a small atmospheric pressure variation is enough. Without the spring the capsule would totally collapse and its shape could therefore not vary. The spring just establishes a new equilibrium shape, but the sensitivity to pressure is still high. $\endgroup$
    – mins
    Dec 17, 2021 at 11:23
  • $\begingroup$ So it's like spring is pulling the aneroid capsule right. Can you please share me the link from where did you learn about this. $\endgroup$ Dec 17, 2021 at 14:30
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    $\begingroup$ @SourabhJain: For example, or this one. Yes it pulls on the capsules. $\endgroup$
    – mins
    Dec 17, 2021 at 14:40

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