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.