16.7.2 Operation
Start of Auto-Zero Sequence
The Auto-Zero function is activated by a Valve Off signal from pin J on the MFC. This
signal can be triggered by either manual shutoff or the Auto-Shutoff.
Start Delay
When the off signal is received, the Auto-Zero initiates a 90 second flow stabilization
delay.
Operating Limits
Following the 90 second delay, the Auto-Zero continuously compares the output of the
MFC meter with DC Common and makes any necessary alignment of the MFC meter by
adjusting a digital potentiometer. This comparison/adjustment process typically requires
30-60 seconds for completion.
Operating Limits
If the variation between the MFC meter output and DC Common is greater than 4 to 5%,
the Auto-Zero will not zero but will display the differential between the true zero and the 4
to 5% limit. This difference and the Auto-Zero alarm indicates the need to check
calibration or service of the MFC.
Auto-Zero Hold
When the valve is turned on to resume flow, the Auto-Zero reference is stored in memory
and maintained through subsequent operations until the MFC is turned off again. (Any
time the MFC is unplugged the Auto-Zero procedure should be re-initiated before use in
process control.)
System Standby
Most systems are programmed to shutoff the MFC during Standby conditions. This
activates the auto zero continuously until the process is turned on. In addition, most
systems return to standby if there is a loss of power and the system is powered back up.
This standby condition automatically activates the auto zer
o to zer
o the MFC for pr
ocess
running.
Automatic Zero Frequency Requirements
It is recommended that the MFC with Auto-Zero be turned off and taken through above
Steps #2-3 at a typical frequency of daily to weekly to ensure its best performance.
Automatic Zero Alarm
The alarm indicates that zero has drifted beyond the operating limits of the Auto-Zero. The
Auto-Zer
o alar
m will indicate to the system that ther
e is a need to check the calibration.
29
©2005 Celerity
, Inc.
#199-001-0006 REV
.F 4/05
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
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