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Reference Manual
00809-0100-4444, Rev AC
Section 7: Digital Signal Processing
January 2015
Digital Signal Processing
signal may be compromised. The transmitter performs the same analysis around the 37.5 Hz
coil drive frequency using the 32.5 Hz and 42.5 Hz values to establish a noise level.
7.5 Optimizing flow reading in noisy applications
If the flow reading of the 8732EM is unstable, first check the wiring, grounding, and process
reference 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 a process reference electrode are being used in
lined or non-conductive piping
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 electro-
chemical 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 8732EM coil drive mode is selectable
between the standard 5 Hz and the noise-reducing 37 Hz.
7.5.1 Coil drive frequency
This parameter changes the pulse rate of the magnetic coils.
5 Hz
The standard coil drive frequency is 5 Hz, which is sufficient for nearly all applications.
37 Hz
If the process fluid causes a noisy or unstable flow reading, increase the coil drive frequency to
37 Hz. If the 37 Hz mode is selected, perform the auto zero function for optimum performance.
7.5.2 Auto zero
To ensure optimum accuracy when using 37 Hz coil drive mode, there is an auto zero function
that should be initiated. When using 37 Hz coil drive mode it is important to zero the system for
the specific application and installation.
LOI menu path
Device Setup, Detailed Setup, Additional Params, Coil Drive Freq
Traditional fast keys 1,4,1,1
Device dashboard 2,2,8,3
LOI menu path Device Setup, Diagnostics, Trims, Auto Zero
Traditional fast keys 1,2,5,4
Device dashboard
2,2,8,4