The following diagram is that of a pneumatic moment-balance differential pressure transmitter, similar to the Foxboro model 13A. The term “moment” refers to the physics principle of a force acting on a lever to produce a torque.
“Moment-balance” is more appropriate than “force-balance” in this case because the device pits moment against moment, rather than force against force directly:
Describe this instrument’s response to an increasing differential pressure (increasing pressure on the “High” side, and steady pressure on the “Low” side; or decreasing pressure on the “Low” side with steady pressure on the “High” side), step by step.
Answer:
As the differential pressure increases (“high” side pressure increases relative to “low” side pressure):
End result: output pressure equals some proportion (multiple or fraction) of differential pressure across the diaphragm capsule.
1. Describe the purpose of the sealing diaphragm shown roughly mid-way along the length of the force bar.
2. Identify how this instrument will respond to obstructions (blockages) in the following locations:
→ Orifice
→ Nozzle
→ Vent (located on relay body)
Interest to add any further points? Share your answers with us through the below comments section.
Credits: Tony R. Kuphaldt
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