How does a pneumatic signal indicator work?
The pneumatic indicator is now obsolete. It is only on old installations or on production facilities at remote locations with no electrical supplies.
However, the only manufacturer who still supplies this device is Foxboro. They supply the type 110 Control pneumatic indicator. This is a control room indicator which is often not used as a separate unit on its own.
The above figure shows the front view of a Foxboro 110 pneumatic indicator which displays two variables. The red mark indicates one process variable and the green one indicates the other process variable. (These are indicators which displays a maximum of three process variables). The scales are made to fi tot customers requirements.
Do you want know how does it operates?
The pneumatic signal from the transmitter (3-15 psi or 0.2-1 bar) is applied to the bellows unit (expansion type). The expanding bellows moves the moving plate. This moves the pointer drive unit. The amount of the pointer movement depends on the input signal.
The customer will decide what variable must be measured, for example temperature, pressure, flow etc. the receiver is calibrated using the span and zero adjustment. A damping adjustment is added. This stops a pulsating signal from making the pointer difficult to read. The calibration of this type of indicator will be done in the workshop using the manual.