Page 220 - Instrumentation Reference Book 3E

204 Microprocessor-based and intelligent transmitters

before onward transmission. For example, the very useful. With FOUNDATIONTM Fieldbus
primary sensor signal may have a known non- there is a move towards further field-mounted
linear relationship with the measured variable. computation and data reduction with gas mass
The most common examples of this are resistance flow calculations; for example, being carried out
temperature detectors, thermocouples, and the in situ. In a control system based on the
square law inherent in the relation between flow FOUNDATION Fieldbus protocol such as
rate and differential pressure developed across an Smar’s SYSTEM302, the regulatory control is
orifice plate or other head producing primary primarily performed by the Fieldbus control
element. In addition, however, actual calibration valve positioner. such as the Smar FY302.
information for an individual transmitter may be
stored in memory and used to enhance the accur-
acy of the output signal. 13.4.1.3 Expression oj. tlze inensuveinent in
engineering units
In those cases where the measurement system
comprises two discrete units, such as a primary There are several ways in which this can be
sensor and a separate transmitter which incorpor- effected.
ates the computation unit, provision would be
(i) Adjusrineizt of the 4-20 n7A range. Using the
made to enter the constants of the primary sensor span and zero setting functions, it is possible to
into the computation unit of the transmitter, so set the values for the measured quantity at which
that either unit could be changed. the output shall be 4 mA and 20 mA respectively.
As well as these purely mathematical manipu-
lations of the original data, auxiliary internal This is particularly useful in matching existing
recorders or indicators.
measurements may be made of line pressure or (ii) Digital conznnmication. It is generally pos-
temperature to permit the output to be corrected sible to set the scaling of an instrument so that the
for the effect of these quantities on the transmit-
ter’s performance. That is a separate activity from measured variable (e.g.. flow rate) shown on the
hand-held communicator or on the operator dis-
correcting the resulting measurement for changes
in the fluid properties with temperature or pres- play appears in the desired or specified units of
sure, although some instruments may permit measure, with the abbreviation for the unit along-
side (e.g., 7.5 kg/s). This can be set quite independ-
access to these auxiliary measurements over the ently of the scaling of the 4-20mA signal and
communication link so that they could be used avoids calculation errors by the operators con-
externally for that purpose.
The provision of linearized and corrected sig- verting from percentage flow rates to engineering
nals to a control system means that it is not units.
(iii) Pulse oirtpurs for totalization. Many flow-
burdened with the need to perform these compu- meters have provision for a pulse output at a
tations and the type or make of transmitter fitted frequency proportional to the flow rate. These
in a particular location is of no significance to the
pulses may be counted externally or in the instru-
control system, since it can assume that the ment itself to provide a totalized flow read-out.
received signal will always be linearly representa- The scaling of this output can be chosen so that
tive of the process parameter.
the interval between pulses represents a specific
volume or mass of fluid. This feature is useful in
13.4.1.2 Inclusion of controljimctions arid orlzer that it permits simple counters to be used to
algoritkins record the total and permits a suitable pulsating
The microprocessors used in microprocessor- rate to be chosen for electromechanical counters.
based or intelligent transmitters are more than (iv) Frril-safe j’krures. All the intelligent flow-
capable of carrying out the relatively simple com- meters provide some internal diagnostic routines
and are capable of flagging problems. In some
putation involved in on-off, two-term, or three- instruments it may be possible to specify what
term control, and some instruments now avail-
able provide this feature. Tuning of the loop is should happen to the output under certain fault
then implemented using the communication link. conditions, e.g., “go to 3.6mA” or “go to 21 mA.”
Until the advent of FOUNDATIONTM Field- These sections are so application-specific that no
bus, relatively few manufacturers were offering general conclusion can be drawn. In some cases it
transmitters with this feature so they were there- may be important to stop the process; in others this
fore rare in large and complex plants, which may not be feasible and an assumed value for the
required more complex and sophisticated con- process parameter may have to be substituted dur-
trol than could be provided in early transmitter ing failure.
mounted devices. For small self-contained appli- There were initially some fears that these more
cations the built-in PID controller in the Smar complex instruments would fail more frequently
LD301 pressure transmitter has proved to be than their analog counterparts. However, more