140
Reference Manual
00809-0300-4750, Rev CA
March 2016
7.3
Process noise profiles
7.3.1
1/f noise
This type of noise has higher amplitudes at lower frequencies, but generally degrades over
increasing frequencies. Potential sources of 1/f noise include chemical mixing and slurry
flow particles rubbing against the electrodes.
7.3.2
Spike noise
This type of noise generally results in a high amplitude signal at specific frequencies which
can vary depending on the source of the noise. Common sources of spike noise include
chemical injections directly upstream of the flowmeter, hydraulic pumps, and slurry flows
with low concentrations of particles in the stream. The particles bounce off of the electrode
generating a “spike” in the electrode signal. An example of this type of flow stream would
be a recycle flow in a paper mill.
7.3.3
White noise
This type of noise results in a high amplitude signal that is relatively constant over the
frequency range. Common sources of white noise include chemical reactions or mixing that
occurs as the fluid passes through the flowmeter and high concentration slurry flows where
the particulates are constantly passing over the electrode head. An example of this type of
flow stream would be a basis weight stream in a paper mill.
7.4
High process noise diagnostic
The transmitter continuously monitors signal amplitudes over a wide range of frequencies.
For the high process noise diagnostic, the transmitter specifically looks at the signal
amplitude at frequencies of 2.5 Hz, 7.5 Hz, 32.5 Hz, and 42.5 Hz. The transmitter uses the
values from 2.5 and 7.5 Hz and calculates an average noise level. This average is compared
to the amplitude of the signal at 5 Hz. If the signal amplitude is not 25 times greater than
the noise level, and the coil drive frequency is set at 5 Hz, the
high process noise diagnostic
will trip indicating that the flow 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.
Содержание Rosemount 8750W
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