The pen movement of a recorder that includes a Bourdon tube is proportional to what?

The pen movement of a recorder that incorporates a Bourdon tube is directly proportional to pressure changes. This is because a Bourdon tube is a mechanical device that measures pressure. It consists of a curved, hollow tube that straightens out as the internal pressure increases. The amount of straightening is proportional to the pressure exerted on its interior, which then translates into movement of the recorder's pen. As pressure fluctuates, the Bourdon tube responds accordingly, causing the pen to move on the graph, thus visually representing those pressure changes over time. This mechanism is widely used in pressure gauges and recorders due to its reliability and accuracy in measuring pressure.