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.