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Reference Manual
00809-0100-4665, Rev AA
August 2010
Rosemount 8732
D-2
PROCEDURES If the output of your Rosemount 8732 is unstable, first check the wiring and
grounding associated with the magnetic flowmeter system. Ensure that the
following conditions are met:
• Ground straps are attached to the adjacent flange or ground ring?
• Grounding rings, lining protectors, or grounding electrodes are being
used in lined or nonconductive piping?
• Both of the shields are attached at both ends?
The causes of unstable transmitter output can usually be traced to extraneous
voltages on the measuring electrodes. This “process noise” can arise from
several causes including electrochemical reactions between the fluid and the
electrode, chemical reactions in the process itself, free ion activity in the fluid,
or some other disturbance of the fluid/electrode capacitive layer. In such noisy
applications, an analysis of the frequency spectrum reveals process noise
that typically becomes significant below 15 Hz.
In some cases, the effects of process noise may be sharply reduced by
elevating the coil drive frequency above the 15 Hz region. The Rosemount
8732 coil drive mode is selectable between the standard 5 Hz and the
noise-reducing 37 Hz. See “Coil Drive Frequency” on page 4-26 for
instructions on how to change the coil drive mode to 37 Hz.
Auto Zero To ensure optimum accuracy when using 37 Hz coil drive mode, there is an
auto zero function that must be initiated during start-up. The auto zero
operation is also discussed in the start-up and configuration sections. When
using 37 Hz coil drive mode it is important to zero the system for the specific
application and installation.
The auto zero procedure should be performed only under
the following conditions:
• With the transmitter and sensor installed in their final positions. This
procedure is not applicable on the bench.
• With the transmitter in 37 Hz coil drive mode. Never attempt this
procedure with the transmitter in 5 Hz coil drive mode.
• With the sensor full of process fluid at zero flow.
These conditions should cause an output equivalent to zero flow.
Signal Processing If the 37 Hz coil drive mode has been set, and the output is still unstable, the
damping and signal processing function should be used. It is important to set
the coil drive mode to 37 Hz first, so the loop response time is not increased.
The 8732 provides for a very easy and straightforward start-up, and also
incorporates the capability to deal with difficult applications that have
previously manifested themselves in a noisy output signal. In addition to
selecting a higher coil drive frequency (37 Hz vs. 5 Hz) to isolate the flow
signal from the process noise, the 8732 microprocessor can actually
scrutinize each input based on three user-defined parameters to reject the
noise specific to the application.