141
Reference Manual
00809-0300-4750, Rev CA
March 2016
7.5
Optimizing flow reading in noisy applications
If the flow reading of the Rosemount 8750W is unstable, first check the wiring, grounding,
and process reference associated with the magnetic flowmeter system. Ensure 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 elec-
trochemical 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 8750W coil drive mode is
selectable between the standard 5 Hz and the noise-reducing 37 Hz. The 37 Hz coil drive
frequency should only be used on sensor sizes 16-in. and smaller.
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
Field Mount: Device Setup, Detailed Setup, Additional Params, Coil Drive
Freq
Wall Mount: AUX. FUNCTION
Traditional Fast Keys
1, 4, 1, 1
Device dashboard
2, 2, 8, 3
LOI menu path
Field Mount: Device Setup, Diagnostics, Trims, Auto Zero
Wall Mount: AUX. FUNCTION
Traditional Fast Keys
1, 2, 5, 4
Device dashboard
2, 2, 8, 4
Summary of Contents for Rosemount 8750W
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