Emerson Rosemount 8732 manual Procedures, Auto Zero

Rosemount 8732

Reference Manual

00809-0100-4665, Rev AA August 2010

PROCEDURES	If the output of your Rosemount 8732 is unstable, first check the wiring and
	grounding 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 grounding electrodes are being
	used in lined or nonconductive piping?
	• Both of the shields are attached at both ends?
	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 electrochemical 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
	8732 coil drive mode is selectable between the standard 5 Hz and the
	noise-reducing 37 Hz. See “Coil Drive Frequency” on page 4-26for
	instructions on how to change the coil drive mode to 37 Hz.
Auto Zero	To ensure optimum accuracy when using 37 Hz coil drive mode, there is an
	auto zero function that must be initiated during start-up. The auto zero
	operation is also discussed in the start-up and configuration sections. When
	using 37 Hz coil drive mode it is important to zero the system for the specific
	application and installation.
	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.

Signal Processing

These conditions should cause an output equivalent to zero flow.

If the 37 Hz coil drive mode 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 mode to 37 Hz first, so the loop response time is not increased.

The 8732 provides for a very 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 8732 microprocessor can actually scrutinize each input based on three user-defined parameters to reject the noise specific to the application.

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