Celerity UNIT 122 Series User Manual

16.7.3 Theory of Auto-Zero Operation
The Auto-Zero feature in CELERITY Mass Flow Controllers overcomes zero drift by
automatically adjusting the MFC meter to electrical zero during off-cycles between process
runs. A stable zero enables operators to achieve repeatability of ±1% even in very low flow
operations (
±0.2% of full scale). The following tabulation and graph demonstrate the
critical effect of zero drift on repeatability:
*Such drift may vary from MFC to MFC
Figure 3. Simplified Logic of the Auto-Zero Circuitry. Flow Diagram
The Auto-Zero circuitry is triggered by a signal from Valve-Off command (pin J) or zero
setpoint for automatic shutoff (A) feature models of the MFC which initiates a 2 minute
delay
. After the delay the Auto-Zer
o compar
es the output of the MFC meter to DC
Common, determining the direction and degree of variance. The Auto-Zero then executes
an incr
emental or decr
emental step of its digital potentiometer (as determined) and
compares the new reading to zero. This continuous incremental step-and-compare cycle
continues indefi
nitely.
The operator may observe the progress of this cycle by monitoring the readout of the MFC
meter. A stable zero readout is typically achieved in 30-60 seconds, at which point the
MFC is ready for operation. After a stable zero (±0.2%) is achieved, the operator may turn
on the MFC. This action disconnects pin J, disabling the Auto-Zero circuitry, and
maintaining the new zer
o setting.
If the meter output never achieves zero, the MFC zero variance may have exceeded the
cor
r
ection limit of the Auto-Zer
o cir
cuitry (±4-5% of full scale). The Automatic Zero alarm
will then indicate the need for service or re-calibration of the MFC.
30
Error if 1%
Error if 2% Maximum with
Setpoint
Zero Offset Zero Offset Auto-Zero
100% FS 1% 2% 0.2%
50% FS
2% 4% 0.4%
20% FS 5% 10% 1.0%
10% FS
10% 20% 2.0%
1% FS 100% 200% 20.0%
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©2005 Celerity
, Inc.
#199-001-0006 REV
.F 4/05
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com

Celerity UNIT 122 Series Specifications

General

Brand	Celerity
Model	UNIT 122 Series
Category	Measuring Instruments
Language	English