Which common measuring element is used in mechanical recorders?

Bourdon tubes are commonly used in mechanical recorders due to their ability to convert pressure changes into mechanical movement. The Bourdon tube is a curved, hollow metal tube that straightens out as pressure increases, which in turn moves a pointer on a dial to display the pressure measurement. This mechanical action allows for a direct visual representation of pressure levels, which is essential in various applications, including process control and monitoring.

While other options like thermocouples, strain gauges, and pressure sensors are vital in the instrumentation field, they serve different purposes or rely on electrical signals rather than mechanical movements. Thermocouples measure temperature, strain gauges measure deformation, and pressure sensors usually provide electronic output signals. In contrast, Bourdon tubes are specifically designed for mechanical recording of pressure, making them the most suitable choice for this particular application.