What is the curved oval metal tube inside a gauge called?

The curved oval metal tube inside a gauge is called a Bourdon tube. This device is a key component in pressure measurement instruments, such as pressure gauges. When pressure is applied to the Bourdon tube, it tends to straighten out due to the internal pressure. This movement is then converted into a rotational motion that drives the needle of the gauge to indicate the pressure reading. The Bourdon tube works on the principle that a curved tube will change shape when subjected to pressure, which allows for accurate readings.

The other options refer to different devices: a thermocouple is used to measure temperature by utilizing the thermoelectric effect, a manometer is specifically used to measure the pressure of gases and liquids, often involving a liquid column, and a diaphragm is utilized in some pressure sensors but operates differently, typically responding to pressure through flexing rather than a change in shape of a tube. Understanding these distinctions highlights the unique role of the Bourdon tube in pressure measurement applications.