Damping Adjustment -I

The vast majority of modern process transmitters (both analog and digital) come equipped with a
feature known as damping. This feature is essentially a low-pass filter function placed in-line with
the signal, reducing the amount of process “noise” reported by the transmitter.
Imagine a pressure transmitter sensing water pressure at the outlet of a large pump. The flow of
water exiting a pump tends to be extremely turbulent, and any pressure-sensing device connected
to the immediate discharge port of a pump will interpret this turbulence as violent fluctuations in
pressure. This means the pressure signal output by the transmitter will fluctuate as well, causing any
indicator or control system connected to that transmitter to register a very “noisy” water pressure:

Zero and Span Adjustment - Continuation

Things become more complicated when the input and output axes are represented by units of
measurement other than “percent.” Take for instance a pressure transmitter, a device designed to
sense a fluid pressure and output an electronic signal corresponding to that pressure. Here is a graph
for a pressure transmitter with an input range of 0 to 100 pounds per square inch (PSI) and an
electronic output signal range of 4 to 20 milliamps (mA) electric current:

 

Although the graph is still linear, zero pressure does not equate to zero current. This is called
a live zero, because the 0% point of measurement (0 PSI fluid pressure) corresponds to a non-zero
(“live”) electronic signal. 0 PSI pressure may be the LRV (Lower Range Value) of the transmitter’s
input, but the LRV of the transmitter’s output is 4 mA, not 0 mA.
Any linear, mathematical function may be expressed in “slope-intercept” equation form:

                                                 y = mx + b

Where,
y = Vertical position on graph
x = Horizontal position on graph
m = Slope of line
b = Point of intersection between the line and the vertical (y) axis

This instrument’s calibration is no different. If we let x represent the input pressure in units
of PSI and y represent the output current in units of milliamps, we may write an equation for this
instrument as follows:

                                           y = 0.16x + 4

On the actual instrument (the pressure transmitter), there are two adjustments which let us
match the instrument’s behavior to the ideal equation. One adjustment is called the zero while the 

other is called the span. These two adjustments correspond exactly to the b and m terms of
the linear function, respectively: the “zero” adjustment shifts the instrument’s function vertically
on the graph, while the “span” adjustment changes the slope of the function on the graph. By
adjusting both zero and span, we may set the instrument for any range of measurement within the
manufacturer’s limits.
It should be noted that for most analog instruments, these two adjustments are interactive. That
is, adjusting one has an effect on the other. Specifically, changes made to the span adjustment almost
always alter the instrument’s zero point. An instrument with interactive zero and span adjustments
requires much more effort to accurately calibrate, as one must switch back and forth between the
lower- and upper-range points repeatedly to adjust for accuracy.

Zero and span adjustments

The purpose of calibration is to ensure the input and output of an instrument correspond to one
another predictably throughout the entire range of operation. We may express this expectation in
the form of a graph, showing how the input and output of an instrument should relate:

This graph shows how any given percentage of input should correspond to the same percentage
of output, all the way from 0% to 100%.

Calibration and Re-Arranging

Every instrument has at least one input and one output. For a pressure sensor, the input would be
some fluid pressure and the output would (most likely) be an electronic signal. For a loop indicator,
the input would be a 4-20 mA current signal and the output would be a human-readable display.
For a variable-speed motor drive, the input would be an electronic signal and the output would be
electric power to the motor.
To calibrate an instrument means to check and adjust (if necessary) its response so the output
accurately corresponds to its input throughout a specified range. In order to do this, one must
expose the instrument to an actual input stimulus of precisely known quantity. For a pressure
gauge, indicator, or transmitter, this would mean subjecting the pressure instrument to known fluid
pressures and comparing the instrument response against those known pressure quantities. One
cannot perform a true calibration without comparing an instrument’s response to known, physical
stimuli.
To range an instrument means to set the lower and upper range values so it responds with the
desired sensitivity to changes in input. For example, a pressure transmitter set to a range of 0 to
200 PSI (0 PSI = 4 mA output ; 200 PSI = 20 mA output) could be re-ranged to respond on a scale
of 0 to 150 PSI (0 PSI = 4 mA ; 150 PSI = 20 mA).
In analog instruments, re-ranging could (usually) only be accomplished by re-calibration, since
the same adjustments were used to achieve both purposes. In digital instruments, calibration and
ranging are typically separate adjustments (i.e. it is possible to re-range a digital transmitter without
having to perform a complete recalibration), so it is important to understand the difference.

Field Bus Standard

The general definition of a fieldbus is any digital network designed to interconnect field-located
instruments. By this definition, HART multidrop is a type of industrial fieldbus. However, HART is
too slow to function as a practical fieldbus for many applications, so other fieldbus standards exist.
Here is a list showing many popular fieldbus standards:
• FOUNDATION Fieldbus
• Profibus PA
• Profibus DP
• Profibus FMS
• Modbus
• AS-I
• CANbus
• ControlNET
• DeviceNet
• BACnet
The utility of digital “fieldbus” instruments becomes apparent through the host system these
instruments are connected to (typically a distributed control system, or DCS). Fieldbus-aware host
systems usually have means to provide instrument information (including diagnostics) in very easyto-
navigate formats.

HART multi-variable transmitters

Some “smart” instruments have the ability to report multiple process variables. A good example
of this is Coriolis-effect flowmeters, which by their very nature simultaneously measure the density,
flow rate, and temperature of the fluid passing through them. A single pair of wires can only convey
one 4-20 mA analog signal, but that same pair of wires may convey multiple digital signals encoded
in the HART protocol. Digital signal transmission is required to realize the full capability of such
“multi-variable” transmitters.
If the host system receiving the transmitter’s signal(s) is HART-ready, it may digitally poll the
transmitters for all variables. If, however, the host system does not “talk” using the HART protocol,
some other means must be found to “decode” the wealth of digital data coming from the multivariable
transmitter.

HART- Multidrop Mode

The HART standard also supports a mode of operation that is totally digital, and capable of
supporting multiple HART instruments on the same pair of wires. This is known as multidrop
mode.
Every HART instrument has an address number, which is typically set to a value of zero (0). A
network address is a number used to distinguish one device from another on a broadcast network,
so messages broadcast across the network may be directed to specific destinations. When a HART
instrument operates in digital/analog hybrid mode, where it must have its own dedicated wire pair
for communicating the 4-20 mA DC signal between it and an indicator or controller, there is no
need for a digital address. An address becomes necessary only when multiple devices are connected
to the same network wiring, and there arises a need to digitally distinguish one device from another
on the same network.
This is a functionality the designers of HART intended from the beginning, although it is
frequently unused in industry. Multiple HART instruments may be connected directly in parallel
with one another along the same wire pair, and information exchanged between those instruments
and a host system, if the HART address numbers are set to non-zero values.

Setting an instrument’s HART address to a non-zero value is all that is necessary to engage
multidrop mode. The address numbers themselves are irrelevant, as long as they fall within the
range of 1 to 15 and are unique to that network.
The major disadvantage of using HART instruments in multidrop mode is its slow speed.
Due to HART’s slow data rate (1200 bits per second), it may take several seconds to access a
particular instrument’s data on a multidropped network. For some applications such as temperature
measurement, this slow response time may be acceptable. For inherently faster processes such as
liquid flow control, it would not be nearly fast enough to provide up-to-date information for the
control system to act upon.

Hart Communicator - Superposition theorem

To apply the Superposition Theorem, we replace all the other sources with their own equivalent
internal resistances (voltage sources become “shorts,” and current sources become “opens”):

The HART communicator is “listening” for those audio tone signals sent by the transmitter’s
AC source, but it “hears” nothing because the DC power supply’s equivalent short-circuit prevents
any significant AC voltage from developing across the two wires. This is what happens when there
is no loop resistance: no HART device is able to receive data sent by any other HART device.
The solution to this dilemma is to install a resistance of at least 250 ohms but not greater than
1100 ohms between the DC power source and all other HART devices, like this:

Loop resistance must be at least 250 ohms to allow the 1 mA P-P AC signal to develop enough
voltage to be reliably detected by the HART modem in the listening device. The upper limit (1100
ohms) is not a function of HART communication so much as it is a function of the DC voltage
drop, and the need to maintain a minimum DC terminal voltage at the transmitter for its own
operation. If there is too much loop resistance, the transmitter will become “starved” of voltage and
act erratically. In fact, even 1100 ohms of loop resistance may be too much if the DC power supply
voltage is insufficient.

Understanding HART Current Lops

An important consideration in HART current loops is that the total loop resistance (precision
resistor values plus wire resistance) must fall within a certain range: 250 ohms to 1100 ohms. Most
4-20 mA loops (containing a single 250 ohm resistor for converting 4-20 mA to 1-5 V) measure in at
just over 250 ohms total resistance, and work quite well with HART. Even loops containing two 250
ohm precision resistors meet this requirement. Where technicians often encounter problems is when
they set up a loop-powered HART transmitter on the test bench with a lab-style power supply and
no 250 ohm resistor anywhere in the circuit:

The HART transmitter may be modeled as two parallel current sources: one DC and one AC. The
DC current source provides the 4-20 mA regulation necessary to represent the process measurement
as an analog current value. The AC current source turns on and off as necessary to “inject” the 1
mA P-P audio-frequency HART signal along the two wires. Inside the transmitter is also a HART
modem for interpreting AC voltage tones as HART data packets. Thus, data transmission takes
place through the AC current source, and data reception takes place through a voltage-sensitive
modem, all inside the transmitter, all “talking” along the same two wires that carry the DC 4-20
mA signal.

For ease of connection in the field, HART devices are designed to be connected in parallel with
each other. This eliminates the need to break the loop and interrupt the DC current signal every time
we wish to connect a HART communicator device to communicate with the transmitter. A typical
HART communicator may be modeled as an AC voltage source (along with another HART voltagesensitive
modem for receiving HART data). Connected in parallel with the HART transmitter, the
complete circuit looks something like this:

The HART digital/analog hybrid standard

A technological advance introduced in the late 1980’s was HART, an acronym standing for Highway
Addressable Remote Transmitter. The purpose of the HART standard was to create a way for
instruments to digitally communicate with one another over the same two wires used to convey a
4-20 mA analog instrument signal. In other words, HART is a hybrid communication standard, with
one variable (channel) of information communicated by the analog value of a 4-20 mA DC signal, and
another channel for digital communication whereby many other variables could be communicated
using pulses of current to represent binary bit values of 0 and 1.
The HART standard was developed with existing installations in mind. The medium for digital
communication had to be robust enough to travel over twisted-pair cables of very long length and
unknown characteristic impedance. This meant that the data communication rate for the digital
data had to be very slow, even by 1980’s standards.
Digital data is encoded in HART using the Bell 202 modem standard: two audio-frequency
“tones” (1200 Hz and 2200 Hz) are used to represent the binary states of “1” and “0,” respectively,
transmitted at a rate of 1200 bits per second. This is known as frequency-shift keying, or FSK. The
physical representation of these two frequencies is an AC current of 1 mA peak-to-peak superimposed
on the 4-20 mA DC signal. Thus, when a HART-compatible device “talks” digitally on a two-wire
loop circuit, it produces tone bursts of AC current at 1.2 kHz and 2.2kHz. The receiving HART
device “listens” for these AC current frequencies and interprets them as binary bits.

Advantages and disadvantages of Pneumatic Instruments

The disadvantages of pneumatic instruments are painfully evident to anyone familiar with both
pneumatic and electronic instruments. Sensitivity to vibration, changes in temperature, mounting
position, and the like affect calibration accuracy to a far greater degree for pneumatic instruments
than electronic instruments. Compressed air is an expensive utility – much more expensive per
equivalent watt-hour than electricity – making the operational cost of pneumatic instruments far
greater than electronic. The installed cost of pneumatic instruments can be quite high as well, given
the need for special (stainless steel, copper, or tough plastic) tubes to carry supply air and pneumatic
signals to distant locations. The volume of air tubes used to convey pneumatic signals over distances
acts as a low-pass filter, naturally damping the instrument’s response and thereby reducing its ability
to respond quickly to changing process conditions. Pneumatic instruments cannot be made “smart”
like electronic instruments, either. However, pneumatic instruments actually enjoy some definite technical advantages which secure their continued use in certain applications even in the 21st century. One decided advantage is the intrinsic safety of pneumatic field instruments. Instruments that do not run on electricity cannot generate electrical sparks. This is of utmost importance in “classified” industrial environments where

explosive gases, liquids, dusts, and powders exist. Pneumatic instruments are also self-purging.
Their continual bleeding of compressed air from vent ports in pneumatic relays and nozzles acts as a
natural clean-air purge for the inside of the instrument, preventing the intrusion of dust and vapor
from the outside with a slight positive pressure inside the instrument case. It is not uncommon to
find a field-mounted pneumatic instrument encrusted with corrosion and filth on the outside, but
factory-clean on the inside due to this continual purge of clean air. Pneumatic instruments mounted
inside larger enclosures with other devices tend to protect them all by providing a positive-pressure
air purge for the entire enclosure.
Some pneumatic instruments can also function in high-temperature and high-radiation
environments that would damage electronic instruments. Although it is often possible to “harden”
electronic field instruments to such harsh conditions, pneumatic instruments are practically immune
by nature.
An interesting feature of pneumatic instruments is that they may operate on compressed gases
other than air. This is an advantage in remote natural gas installations, where the natural gas
itself is sometimes used as a source of pneumatic “power” for instruments. So long as there is
compressed natural gas in the pipeline to measure and to control, the instruments will operate. No
air compressor or electrical power source is needed in these installations.

Pneumatic Instruments - Proper Care and Feeding

Perhaps the most important rule to obey when using pneumatic instruments is to maintain clean
and dry instrument air. Compressed air containing dirt, rust, oil, water, or other contaminants will
cause operational problems for pneumatic instruments. First and foremost is the concern that tiny
orifices and nozzles inside the pneumatic mechanisms will clog over time. Clogged orifices tend to
result in decreased output pressure, while clogged nozzles tend to result in increased output pressure.
In either case, the “first aid” repair is to pass a welding torch tip cleaner through the plugged hole
to break loose the residue or debris plugging it.
Moisture in compressed air tends to corrode metal parts inside pneumatic mechanisms. This
corrosion may break loose to form debris that plugs orifices and nozzles, or it may simply eat
through thin diaphragms and bellows until air leaks develop. Grossly excessive moisture will cause
erratic operation as “plugs” of liquid travel through thin tubes, orifices, and nozzles designed only
for air passage.
A common mistake made when installing pneumatic instruments is to connect them to a generalservice
(“utility”) compressed air supply instead of a dedicated instrument-service compressed air
system. Utility air systems are designed to supply air tools and large air-powered actuators with
pneumatic power. These high-flow compressed air systems are often seeded with antifreeze and/or
lubricating chemicals to prolong the operating life of the piping and air-consuming devices, but
the same liquids will wreak havoc on sensitive instrumentation. Instrument air supplies should be
sourced by their own dedicated air compressor(s), complete with automatic air-dryer equipment,
and distributed through stainless steel, copper, or plastic tubing (never black iron or galvanized iron
pipe!).
Once i read an article that  Someone on the operations staff decided they would use 100 PSI instrument air to purge a process pipe filled with acid. Unfortunately, the acid pressure in the process pipe exceeded 100 PSI, and as a result acid flushed backward into the instrument air system. Within days most of the pneumatic instruments in that section of the refinery failed due to accelerated corrosion of brass and aluminum components inside the instruments. The total failure of multiple instruments over such a short time could have
easily resulted in a disaster, but fortunately the crisis was minimal. Once the first couple of faulty
instruments were disassembled after removal, the cause of failure became evident and the technicians
took action to purge the lines of acid before too many more instruments suffered the same fate.
Pneumatic instruments must be fed compressed air of the proper pressure as well. Just like
electronic circuits which require power supply voltages within specified limits, pneumatic instruments
do not operate well if their air supply pressure is too low or too high. If the supply pressure is too
low, the instrument cannot generate a full-scale output signal. If the supply pressure is too high,
internal failure may result from ruptured diaphragms, seals, or bellows. Many pneumatic instruments
are equipped with their own local pressure regulators directly attached to ensure each instrument
receives the correct pressure despite pressure fluctuations in the supply line.

Analysis of a practical pneumatic instrument

A photograph of one with the cover removed is shown here:

A functional illustration of this instrument identifies its major components:

Analogy to OPAMP circuits

Self-balancing pneumatic instrument mechanisms are very similar to negative-feedback operational
amplifier circuits, in that negative feedback is used to generate an output signal in precise proportion
to an input signal. This section compares simple operational amplifier (“opamp”) circuits with
analogous pneumatic mechanisms for the purpose of illustrating how negative feedback works, and
learning how to generally analyze pneumatic mechanisms.
In the following illustration, we see an opamp with no feedback (open loop), next to a baffle/nozzle
mechanism with no feedback (open loop):

For each system there is an input and an output. For the opamp, input and output are both
electrical (voltage) signals: Vin is the differential voltage between the two input terminals, and Vout
is the single-ended voltage measured between the output terminal and ground. For the baffle/nozzle,
the input is the physical gap between the baffle and nozzle (xin) while the output is the backpressure
indicated by the pressure gauge (Pout).
Both systems have very large gains. Operational amplifier open-loop gains typically exceed
200,000 (over 100 dB), and we have already seen how just a few thousandths of an inch of baffle
motion is enough to drive the backpressure of a nozzle nearly to its limits (supply pressure and
atmospheric pressure, respectively).
Gain is always defined as the ratio between output and input for a system. Mathematically, it is
the quotient of output change and input change, with “change” represented by the triangular Greek
capital-letter delta:

Normally, gain is a unitless ratio. We can easily see this for the opamp circuit, since both output
and input are voltages, any unit of measurement for voltage would cancel in the quotient, leaving a
unitless quantity. This is not so evident in the baffle/nozzle system, with the output represented in
units of pressure and the input represented in units of distance.

If we were to add a bellows to the baffle/nozzle mechanism, we would have a system that inputs
and outputs fluid pressure, allowing us to more formally define the gain of the system as a unitless
ratio of

The general effect of negative feedback is to decrease the gain of a system, and also make that
system’s response more linear over the operating range. This is not an easy concept to grasp,
however, and so we will explore the effect of adding negative feedback in detail for both systems.
The simplest expression of negative feedback is a condition of 100% negative feedback, where the
whole strength of the output signal gets “fed back” to the amplification system in degenerative
fashion. For an opamp, this simply means connecting the output terminal directly to the inverting
input terminal:

This “negative” or “degenerative” feedback is because its effect is counteractive in nature.
If the output voltage rises too high, the effect of feeding this signal to the inverting input will be to
bring the output voltage back down again. Likewise, if the output voltage is too low, the inverting
input will sense this and act to bring it back up again. Self-correction typifies the very nature of
negative feedback.