Page 221 - Instrumentation Reference Book 3E

Attributes and features of microprocessor-based and intelligent transmitters 205

than a decade of operation has proven them reli- to be reduced in the output signal. Most manu-
able, and when they do fail, much more informa- facturers use a filter which may be represented by
tion regarding the nature of the fault will usually a single lag, but some use an adaptive filter, with
be available. different responses to small and large changes.
Selection of a long time constant will produce a
13.4.2 Intelligent features smooth measurement signal but may mask the
onset of instability in the process variable. When
The use of digital communication superimposed the measurement signal is being used as the input
on the 4-3OmA measurement signal not only to a flow control loop, a long time constant may
enables f~dl advantage to be taken of the enhanced make tuning of the loop more difficult. Changing
performance of the sensors but also permits a wide the time constant will almost certainly affect the
range of information to be extracted from the tuning and result in either poor control or
transmitter, on request, and adjustments made to instability.
the mode of operation. Typical examples of these
features are described below.
13.4.2.3 Diclgnostic routines and status
ii forniatioiz
13.4.2.1 Acljmstmenr of spri aid zero
There are two types of information; one concerns
This feature permits the full range of the analog the normal status of the instrument and the other
output signal (usually 4-20 mA) to be used for a provides diagnosis of fault or unusual process
range which is less than the total measurement conditions. Both are primarily of interest in plant
range of the primary sensor. It is usually maintenance and are therefore considered
employed to change the span without change to together.
the zero, but a suppressed zero may be useful Examples of the status information might be:
when the process variable has to be closely con- (1) Transmitter model and serial number
trolled around a specific value and there is no (2) Primary variable and units of measure
interest in values of the process variable outside (3) Transmitter range and damping time constant
that narrow band. (4) Primary variable corresponding to 4 and 20 mA
More commonly. it is used to permit the same
instrument to be used for either large or small (5) Plant tag number
measurement ranges and has special significance (6) Materials of construction of wetted parts
(7) Software revision number
when the transfer characteristic of the sensor is (8) Date of last calibration.
non-linear. This is particularly true of orifice
plate flowmeters in which the flow rate computa- The ability to call up this information assists in
tion involves taking the square root of the signal keeping an up-to-date plant instrumentation
representing the differential pressure. maintenance log which can be checked by actual
Before the advent of intelligent transmitters, an interrogation to ensure that the type, range, etc.,
orifice metering system for a flow turndown of of the plant-mounted equipment is in line with
20:l might have required three differential pres- the plant records as well as the operational
sure transmitters, with overlapping ranges. One requirements. Some of tke status information,
intelligent transmitter can now provide the same such as the model number, serial number, and
analog outputs but with digital communication of materials of construction, where this is critical,
the range setting. In general, this feature is only is permanent. Other details are entered or mod-
needed if an analog output is required, since a ified when the transmitter is first installed or
digital representation of the differential pressure removed from service.
is also available from the transmitter and this does Diagnostic information is concerned with
not require any span and zero adjustment. An actual operation of the transmitter and changes
important result of this versatility which applies much more frequently. Some diagnostic routines
to all intelligent transmitters is that one model can prevent discrepant configuration information
be configured to deal with a wide range of applica- from being entered (e.g., the measured value cor-
tions so that fewer types and ranges of transmitters responding to ImA being set higher than that for
have to br: held as spares at a process plant. 20 mA). Others warn of unexpected situations,
such as reverse flow when the flowmeter is not
set up to measure bi-directional flow. while others
13.4.2.3 d&justiiieizt of damping, time CoIzStaiit, may report internal failures in the electronics, such
or response time as failure to write to a memory location.
Most transmitters provide a selectable damping, Many of these routines run continuously and
time constant, or response time to permit fluctu- set a flag immediately when a fault is identified.
ations of .:he measured variable or electronic noise Others may be requested by the operator via a