Baffle / Nozzle Mechanism

The principle behind the operation of a baffle/nozzle mechanism is often used directly in qualitycontrol
work, checking for proper dimensioning of machined metal parts. Take for instance this
shaft diameter checker, using air to determine whether or not a machined shaft inserted by a human
operator is of the proper diameter after being manufactured on an assembly line:

If the shaft diameter is too small, there will be excessive clearance between the shaft and the
inside diameter of the test jig, causing less air pressure to register on the gauge. Conversely, if
the shaft diameter is too large, the clearance will be less and the gauge will register a greater air
pressure because the flow of air will be obstructed by the reduced clearance. The exact pressure is
of no particular consequence to the quality-control operator reading the gauge. What does matter
is that the pressure falls within an acceptable range, reflecting proper manufacturing tolerances for
the shaft. In fact, just like the 3-15 PSI “receiver gauges” used as pneumatic instrument indicators,
the face of this pressure gauge might very well lack pressure units (such as kPa or PSI), but rather
be labeled with a colored band showing acceptable limits of mechanical fit:

This is another example of the analogue nature of pneumatic pressure signals: the pressure
registered by this gauge represents a completely different variable, in this case the mechanical fit of
the shaft to the test jig. Although it is possible to construct a pneumatic instrument consisting only of a baffle/nozzle mechanism, this is rarely done.