What element is included within the altimeter of the Cessna 172?

The altimeter in the Cessna 172 uses aneroid wafers as its primary sensing element. Aneroid wafers are sealed metal capsules that expand and contract based on changes in atmospheric pressure. As the aircraft ascends or descends, the outside air pressure changes, causing the wafers to flex. This movement translates into an altitude reading by the altimeter.

The design relies on the principle that atmospheric pressure decreases with altitude; therefore, the aneroid wafers react to these changes precisely to provide an accurate altitude reading. This is a fundamental working principle of traditional barometric altimeters, and it's key for pilots to determine their aircraft’s altitude for navigation and safety during flight.

In contrast, hydraulic fluid is used in other aircraft systems, such as brakes, the pitot tube primarily measures airspeed, and while a pressure transducer can be an element in more advanced altimeter systems, it is not the core component of the Cessna 172's altimeter function.