In measuring steam flow in a pipeline with a head meter, what does the primary element sense?

In the context of measuring steam flow in a pipeline using a head meter, the primary element specifically measures differential pressure. A head meter operates on the principle of pressure differential between two points in a flow system. As steam flows through the pipeline, it creates a difference in pressure due to the varying velocity of the steam as it passes through a constriction or orifice.

The pressure measured by the primary element is directly related to the velocity of the steam flow, which can then be translated into flow rate using established fluid dynamics equations, such as the Bernoulli equation or orifice flow equations. This differential pressure is critical because it provides the necessary information to calculate the flow rate of steam in the pipeline, emphasizing the importance of interpreting this measurement correctly in various engineering applications.

The other options, while relevant to fluid measurement, do not directly apply to what the primary element of a head meter senses in this scenario. Velocity is derived from the differential pressure but not sensed directly; temperature is a separate measurement entirely and does not play a role in determining the flow rate through a head meter; and total flow rate is the outcome of calculations based on the primary measurement (differential pressure) rather than what is being sensed by the primary element itself.