Emerson Rosemount 3051S Series, Справочное руководство, Страница: 109 / 240

Reference Manual
00809-0100-4801, Rev FA
October 2010
7-5
Rosemount 3051S Series
Figure 7-3. CV is the ratio of
Standard Deviation to Mean
CV is stable if Mean is proportional to Standard Deviation.
SPM provides statistical information to the user in two ways. First, the
statistical parameters can be made available to the host system directly via
HART communication protocol or HART to other protocol converters. Once
available, the system can make use of these statistical parameters to indicate
or detect a change in process conditions. In the simplest example, the
statistical values may be stored in a data historian. If a process upset or
equipment problem occurs, these values can be examined to determine if
changes in the values foreshadowed or indicated the process upset. The
statistical values can then be made available to the operator directly, or made
available to alarm or alert software.
The second way for SPM to provide statistical information is with software
embedded in the 3051S. The 3051S uses SPM to baseline the process noise
or signature via a learning process. Once the learning process is completed,
the user can set thresholds for any of the statistical parameters. The device
itself can then detect significant changes in the noise or variation, and
communicate an alarm via the 4 – 20 mA output and/or alert via HART.
Typical applications are detection of plugged impulse lines, change in fluid
composition, or equipment related problems.
Overview
A block diagram of the SPM diagnostic is shown in Figure 7-4. The pressure
process variable is input to a module where basic high pass filtering is
performed on the pressure signal. The mean (or average) is calculated on the
unfiltered pressure signal, the standard deviation calculated from the filtered
pressure signal. These statistical values are available via HART and handheld
communication devices like the 375 Field Communicator or asset
management software like Emerson Process Management’s AMS™ Device
Manager. The values can also be assigned as secondary variables from the
device for 4-20 mA communication to the user through other devices like the
Smart Wireless THUM or Rosemount 333 HART Tri-loop.
Mean
Standard
Deviation
Coefficient of
Variation