Tuesday, April 23, 2013

Pneumatic Sensing Elements

Most pneumatic instruments use a simple but highly sensitive mechanism for converting mechanical
motion into variable air pressure: the baffle-and-nozzle assembly (sometimes referred to as a flapper-
and-nozzle assembly). A baffle is nothing more than a flat object obstructing the flow of air out of
a small nozzle by close proximity:

The physical distance between the baffle and the nozzle alters the resistance of air flow through
the nozzle. This in turn affects the air pressure built up inside the nozzle (called the nozzle backpressure). Like a voltage divider circuit formed by one fixed resistor and one variable resistor,
the baffle/nozzle mechanism “divides” the pneumatic source pressure to a lower value based on the
ratio of restrictiveness between the nozzle and the fixed orifice.
This crude assemblage is surprisingly sensitive, as shown by the graph. With a small enough
orifice, just a few thousandths of an inch of motion is enough to drive the pneumatic output between
its saturation limits. Pneumatic transmitters typically employ a small sheet-metal lever as the baffle. The slightest motion imparted to this baffle by changes in the process variable (pressure, temperature, flow, level, etc.) detected by some sensing element will cause the air pressure to change in response.

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