What is the behavior of the pen movement in a recorder that uses a spiral Bourdon tube?

The behavior of the pen movement in a recorder utilizing a spiral Bourdon tube is directly proportional to the change in tube pressure. As the internal pressure within the system increases or decreases, the Bourdon tube responds by expanding or contracting. This movement is then translated into mechanical movement, which causes the pen to move along the recording medium in a manner that accurately reflects the pressure changes being measured.

The efficiency of this relationship is due to the mechanical design of the Bourdon tube; it converts the pressure exerted on its curved shape into a linear displacement that the pen follows. As a result, the recorded data provides a direct and clear representation of fluctuating pressures in the system, allowing for precise monitoring and analysis.

In contrast, the other options do not accurately describe the operational principle of a spiral Bourdon tube. Pen movement being randomly variable would imply erratic readings that are inconsistent with the nature of a pressure-sensing device. A logarithmic scale would not typically apply to the linear response of a Bourdon tube, and a fixed circle would imply a constant movement that does not vary in response to pressure changes. Thus, the correct choice captures the essential functionality of the spiral Bourdon tube in pressure measurement applications.