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

Pneumatic Application - Flow Control System

Pneumatic Applications - Bio-diesel "Wash Column" Temperature Control

Pneumatic temperature, flow, and level control systems have all been manufactured to use the
same principle of 3-15 PSI air pressure signaling. In each case, the transmitter and controller are both
supplied clean compressed air at some nominal pressure (20 to 25 PSI, usually) and the instrument
signals travel via tubing.

Pneumatic Instrument - Continuation

Air pressure may be used as an alternative signaling medium to electricity. Imagine a pressure
transmitter designed to output a variable air pressure according to its calibration rather than a
variable electric current. Such a transmitter would have to be supplied with a source of constantpressure
compressed air instead of an electric voltage, and the resulting output signal would be
conveyed to the indicator via tubing instead of wires:

The indicator in this case would be a special pressure gauge, calibrated to read in units of
process pressure although actuated by the pressure of clean compressed air from the transmitter
instead of directly by process fluid. The most common range of air pressure for industrial pneumatic
instruments is 3 to 15 PSI. An output pressure of 3 PSI represents the low end of the process
measurement scale and an output pressure of 15 PSI represents the high end of the measurement
scale. Applied to the previous example of a transmitter calibrated to a range of 0 to 250 PSI,
a lack of process pressure would result in the transmitter outputting a 3 PSI air signal and full
process pressure would result in an air signal of 15 PSI. The face of this special “receiver” gauge
would be labeled from 0 to 250 PSI, while the actual mechanism would operate on the 3 to 15 PSI
range output by the transmitter. Just like the 4-20 mA loop, the end-user need not know how the
information gets transmitted from the process to the indicator. The 3-15 PSI signal medium is once
again transparent to the operator.

Pneumatic Instrumentation

While electricity is commonly used as a medium for transferring energy across long distances, it is
also used in instrumentation to transfer information. A simple 4-20 mA current “loop” uses direct
current to represent a process measurement in percentage of span, such as in this example:

The transmitter senses an applied fluid pressure from the process being measured, regulates
electric current in the series circuit according to its calibration (4 mA = no pressure ; 20 mA =
full pressure), and the indicator (ammeter) registers this measurement on a scale calibrated to read
in pressure units. If the calibrated range of the pressure transmitter is 0 to 250 PSI, then the
indicator’s scale will be labeled to read from 0 to 250 PSI as well. No human operator reading that
scale need worry about how the measurement gets from the process to the indicator – the 4-20 mA
signal medium is transparent to the end-user as it should be.

Using loop Calibrators

Special-purpose electronic test instruments called loop calibrators are manufactured for the express
purpose of 4-20 mA current loop circuit troubleshooting. These versatile instruments are generally
capable of not only measuring current, but also sourcing current to unpowered devices in a loop,
and also simulating the operation of loop-powered 4-20 mA transmitters.

Here, the loop wiring is broken at the negative terminal of the loop-powered transmitter, and
the calibrator connected in series to measure current. If this loop had a test diode installed, the
calibrator could be connected in parallel with the diode to achieve the same function. Note the
polarity of the calibrator’s test leads in relation to the circuit being tested: the calibrator is acting
as an unpowered device (a load rather than a source), with the more positive loop terminal connected
to the calibrator’s red test lead and the more negative terminal connected to the black test lead.
The same loop calibrator may be used to source (or drive) a 4-20 mA signal into an indicating
instrument to test the function of that instrument independently. Here, we see the calibrator used as a current source to send a 16.00 mA signal to the PV (process variable) input of the controller:

No transmitter need be included in this illustration, because the calibrator takes its place. Note
how the calibrator is used here as an active source of current rather than a passive load as it was in
the last example. The calibrator’s red test lead connects to the controller’s positive input terminal,
while the black test lead connects to the negative terminal.

Using shunt resistors to measure Loop Current

A similar method for non-invasively measuring current in a 4-20 mA instrumentation circuit is to
install a precision resistor in series. If the resistance value is precisely known, the technician merely
needs to measure voltage across it with a voltmeter and use Ohm’s Law to calculate current:

In electronics, such a precision resistor used for measuring current is often referred to as a shunt
resistor. Shunt resistor values are commonly very small, for their purpose is to assist in current
measurement without imposing undue voltage drop within a circuit. It is rare to find a 250 ohm
resistor used strictly as a diagnostic shunt resistor, because the extra voltage drop (1 to 5 volts,
depending on the current signal level) may “starve” loop-powered instruments of voltage necessary
to operate. Shunt resistor values as low as 1 ohm may be found installed in 4-20 mA current loops
at strategic locations.