Pneumatic Mechanism - Continuation

A similar design of pilot valve reverses the directions of the two plugs and seats. The only
operational difference between this pilot valve and the previous design is an inverse relationship
between control rod motion and pressure:

Pneumatic Mechanism - Continuation

The sensitivity of this pneumatic mechanism may be improved by extending the control rod and
adding a second plug/seat assembly. The resulting mechanism, with dual plugs and seats, is known
as a pneumatic pilot valve. An illustration of a pilot valve is shown here, along with its electrical
analogue :

As the control rod is moved up and down, both variable restrictions change in complementary
fashion. As one restriction opens up, the other pinches shut. The combination of two restrictions
changing in opposite direction results in a much more aggressive change in output pressure as
registered by the gauge.

Pneumatic and Equivalent Electrical circuit

The following pneumatic mechanism and its electrical analogue :

As the control rod is moved up and down by an outside force, the distance between the plug
and the seat changes. This changes the amount of resistance experienced by the escaping air,
thus causing the pressure gauge to register varying amounts of pressure. There is little functional
difference between this mechanism and a baffle/nozzle mechanism. Both work on the principle of one
variable restriction and one fixed restriction (the orifice) “dividing” the pressure of the compressed
air source to some lesser value.

Pilot valves and pneumatic amplifying relays

A plain baffle/nozzle mechanism may be made extremely sensitive by reducing the size of the
orifice. However, a problem caused by decreasing orifice size is a corresponding decrease in the
nozzle’s ability to provide increasing backpressure to fill a bellows of significant volume. In other
words, a smaller orifice will result in greater sensitivity to baffle motion, but it also limits the air flow
rate available to fill the bellows, which makes the system slower to respond. Another disadvantage of
smaller orifices is that they become more susceptible to plugging due to impurities in the compressed
air.
An alternative technique to making the baffle/nozzle mechanism more sensitive is to amplify its
output pressure using some other pneumatic device. This is analogous to increasing the sensitivity
of a voltage-generating electrical detector by passing its output voltage signal through an electronic
amplifier. Small changes in detector output become bigger changes in amplifier output which then
causes our self-balancing system to be even more precise.

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.

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.

Pneumatic Application - Two- Element Boiler Steam Drum Level Control