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Reference Manual
00809-0100-4444, Rev AC
Section 7: Digital Signal Processing
January 2015
Digital Signal Processing
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.
Set the loop to manual if necessary and begin the auto zero procedure. The transmitter
completes the procedure automatically in about 90 seconds. A clock symbol will appear in the
lower right-hand corner of the display to indicate that the procedure is running.
Note
Failure to complete an auto zero may result in a flow velocity error of 5 to10% at1 ft/s (0.3 m/s).
While the output level will be offset by the error, the repeatability will not be affected.
7.5.3 Digital signal processing (DSP)
The 8732EM contains several advanced functions that can be used to stabilize erratic outputs
caused by process noise. The signal processing menu contains this functionality.
If the 37 Hz coil drive frequency 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 frequency to 37 Hz
to increase the flow sampling rate.
The 8732EM provides an 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 8732EM microprocessor can actually scrutinize each input
based on three user-defined parameters to reject the noise specific to the application.
Operating mode
The operating mode should be used only when the signal is noisy and gives an unstable output.
Filter mode automatically uses 37 Hz coil drive mode and activates signal processing at the
factory set default values. When using filter mode, perform an auto zero with no flow and a full
sensor. Either of the parameters, coil drive mode or signal processing, may still be changed
individually. Turning signal processing off or changing the coil drive frequency to 5 Hz will
automatically change the operating mode from filter mode to normal mode.
This software technique, known as signal processing, “qualifies” individual flow signals based on
historic flow information and three user-definable parameters, plus an on/off control. These
parameters are described below.
LOI menu path Device Setup, Detailed Setup, Signal Processing
Traditional fast keys 1,4,4
Device dashboard
2,2,8,6
LOI menu path
Device Setup, Detailed Setup, Signal Processing, Operating Mode
Traditional fast keys 1,4,4,1
Device dashboard 2,2,8,5