# Pneumatic Moment-balance Diﬀerential Pressure Transmitter

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:

## Pneumatic Diﬀerential Pressure Transmitter

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.

As the diﬀerential pressure increases (“high” side pressure increases relative to “low” side pressure):

• The diaphragm “capsule” is forced to the right.
• The Force bar rotates counter-clockwise about the sealing diaphragm, which acts as a fulcrum for the force bar.
• Flapper approaches nozzle.
• Nozzle backpressure increases.
• Increased nozzle backpressure presses up on relay diaphragm.
• Inside the relay, the ball-shaped supply valve opens and the cone-shaped vent valve closes.
• Relay output pressure increases (more than the nozzle backpressure increase, due to ampliﬁcation).
• This output pressure is sent to the bellows, which presses to the left at the range bar’s lower end.
• The range bar rotates clockwise about the “range wheel” (movable fulcrum nut).
• The top of range bar moves to the right, pulling against the force bar to move the ﬂapper away from the nozzle.
• The system reaches equilibrium, where the force exerted by the bellows against the range bar balances force exerted by the diaphragm capsule against the force bar.

End result: output pressure equals some proportion (multiple or fraction) of diﬀerential pressure across the diaphragm capsule.

#### More Questions

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)