10 Digital Signal Processing
10.1 Introduction
Magnetic flow meters are used in applications that can create noisy flow readings. The
transmitter has 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 (High Frequency vs. Low Frequency) to isolate the flow signal from the process
noise, the microprocessor has Digital Signal Processing (DSP) that is capable of rejecting the
noise specific to the application. This section explains the different types of process noise,
provides instructions for optimizing the flow reading in noisy applications, and provides a
detailed description of the digital signal processing functionality.
10.2 Process noise profiles
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.
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 on 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.
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 flow meter 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.
10.3 High process noise diagnostic
Note
See also High process noise detection.
The transmitter continuously monitors signal amplitudes over a wide range of frequencies.
For the high process noise diagnostic, the transmitter specifically looks at the noise
amplitude adjacent to the Low and High coil drive frequencies. The noise values are used
to calculate a signal-to-noise ratio for both drive frequencies. If the signal-to-noise ratio is
less than 25 at the currently selected coil drive frequency, the high process noise
diagnostic will trip, indicating that the flow signal may be compromised.
Reference Manual Digital Signal Processing
00809-0100-8782 November 2019
Reference Manual 107
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