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4 Answers

Aneroid Wafer vs a Diaphragm

Asked by: 37833 views Instrument Rating

Whats the difference between an aneroid wafer and a diaphragm??

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4 Answers

  1. Kent Shook on Mar 29, 2011

    I tried to find a definitive FAA reference on this, but the only thing I could find was the Pilot’s Handbook of Aeronautical Knowledge, and based on what I see there, I think that might be the exact document causing your confusion!
    In the PHAK, Chapter 6, “Flight Instruments” they show two fairly similar-looking structures: “Aneroid Wafers” inside the Altimeter, and a “Diaphragm” inside the VSI. 
    The best clue lies in the dictionary, and points us to a very subtle difference in the instrument drawings. Aneroid literally means “without fluid” and if you look at the drawing of the altimeter, you’ll see that the static line does not connect to the aneroid wafers, but instead they have a fixed amount of air in them and they expand and contract due to the differing static pressure inside the case of the altimeter.
    On the other hand, on the drawing of the VSI, you may see that the static line connects directly to the diaphragm, and the instrument works due to the expansion and contraction of the diaphragm due to the differing pressure between the inside of the diaphragm (connected directly to static pressure) and the rest of the instrument case (connected to static through a calibrated leak).
    Thus, it would appear that the answer to your question is that the aneroid wafer is a diaphragm, “without fluid,” that is, static air pressure does not flow in and out of the aneroid wafer.

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  2. jack on Apr 20, 2012

    very good answer

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  3. Pavan Kumar on Jul 20, 2012

    Good Answer .

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  4. Hisham on Mar 19, 2017

    Excellent answer

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