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RPM4™ OPERATION AND MAINTENANCE MANUAL
© 2004-2007 DH Instruments, a Fluke Company Page 60
AutoZ in gauge measurement mode with a Gxxx or BGxxx Q-RPT
Q-RPTs with the designation Gxxx or BGxxx are intrinsically gauge. They have an atmospheric
(TEST(-)) reference port and measure gauge pressure (difference from atmosphere).
In gauge measurement mode the value of P
std,0
is always zero gauge pressure. Zero gauge
pressure, by definition, is applied to the Q-RPT when it is vented to atmosphere.
When using a Gxxx Q-RPT in gauge measurement mode, with AutoZ ON, gauge pressure is
calculated as:
P
gauge
= P
u
- P
offset,G
In gauge measurement modes, AutoZ is run when initiated by the operator pressing [AutoZ]
when the RPM4 is vented. This assures the regular automated updating of the P
offset,G
value.
AutoZ in differential measurement mode
RPM4s with two Q-RPTs of the same type (Axxx, Gxxx, BGxxx) support differential
measurement mode (see Section 3.3.3.1) in which the measured pressure is the difference
between the pressure measured by the Hi Q-RPT and the Lo Q-RPT (Hi – Lo).
In differential measurement mode, the purpose of AutoZ is to compensate for the offset
between the Hi and Lo Q-RPT indications at the “line pressure” at which the differential
pressure is being measured. The value of P
std,0
is the pressure indicated by the Lo Q-RPT at
the time of AutoZ.
When using an RPM4 with two Q-RPTs in differential measurement mode, with AutoZ ON,
differential pressure is calculated as:
P
differential
= P
Hi
- P
Lo
- P
offset,D
In differential measurement mode, AutoZ is run when initiated by the operator pressing
[AutoZ] when both Q-RPTs of the RPM4 are connected to a common pressure..
AutoZ ON/OFF
The AutoZ function can be turned ON and OFF, separately for gauge, absolute and
differential measurement modes. Table 7 summarizes the effect of AutoZ ON and OFF.
Table 7. AutoZ ON and OFF
Q-RPT
TYPE
MEASUREMENT
MODE
AutoZ
STATUS
P
offset
APPLIED
ΔP
atm
APPLIED
AutoZ
ROUTINE
RUNS
ON YES
When initiated
by operator
Absolute
OFF NO
Not Applicable
Not available
ON YES
Axxx
Gauge or
negative gauge
OFF
YES
NO
When initiated
by operator
ON YES
When initiated
by operator
Gxxx
or
BGxxx
Gauge or
negative gauge
OFF NO
Not Applicable
Not available
ON YES
When initiated
by operator
Any, Hi Q-RPT
in dual Q-RPT
RPM4
Differential
OFF NO
Not Applicable
Not available
Recommendations for the Use of the AutoZ Function
The AutoZ function provides a powerful and easy to use tool for improving the stability over
time of RPM4 Q-RPTs and maximizing the recalibration interval by compensating for change
in zero between full recalibrations. In differential measurement mode, it minimizes the
uncertainty in the differential pressure due to the offset between the Hi and Lo Q-RPTs at the