DH Instruments PPC4, Руководство по эксплуатации и обслуживанию

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PPC4™ OPERATION AND MAINTENANCE MANUAL
© 2009 DH Instruments, a Fluke Company Page 38
Note
RPM4 has its own, independent overpressure function. When a
Q-RPT on an RPM4 is active, both the PPC4 and the RPM4 may
execute overpressure routines in response to an overpressure
condition of the RPM4. See the RPM4 Operation and
Maintenance Manual for information on the RPM4 overpressure
function.
3.2.22 AutoZero (AutoZ)

PURPOSE
To offset the PPC4 system Q-RPTs relative to a reference value in order to compensate for
possible changes in Q-RPT zero between full recalibrations.

PRINCIPLE
AutoZero Purpose and Principle
The main component of the change over time of PPC4 Q-RPTs is change in zero or offset,
independent of span. Offsetting or “rezeroing” PPC4 Q-RPTs relative to a reference between
recalibrations allows measurement uncertainty specifications to be maintained with less
frequent full calibrations. The PPC4 AutoZero function (AutoZ) provides full on-board support
for the rezeroing process to simplify its application by the user.
The AutoZero function uses three values:
1. P
std,0
: The pressure value indicated by the AutoZ reference, the device that is acting as
the reference relative to which to offset the Q-RPT.
For Axxx (absolute) Q-RPTs in absolute measurement mode, the pressure at which
AutoZ is performed is normally atmospheric pressure and P
std,0
value can be supplied by
manual entry.
For Gxxx and BGxxx (gauge) Q-RPTs or Axxx (absolute) Q-RPTs in gauge
measurement mode , P
std,0
is always zero (atmospheric pressure) which is supplied by
definition when the Q-RPT is vented to atmosphere.
2. P
u,0
: The pressure reading of the Q-RPT, with no AutoZ offset, at the time AutoZ is
performed.
3. P
offset,G
and P
offset,A
: The difference between the reading of the Q-RPT with no AutoZ
offset ( P
u,0
) and the indication of the AutoZ reference ( P
std,0
) for gauge (
G
) or absolute
measurement mode (
A
):
P
offset
= P
u,0
- P
std,0
P
offset
represents the change in zero of the Q-RPT relative to the AutoZ standard ( P
std,0
).
The AutoZ function manages the determination, storage and application of P
offset
for PPC4
system Q-RPTs in absolute and gauge measurement modes.
When the Q-RPT is calibrated, P
offset
is set to zero. P
offset
is then redetermined at regular
intervals using the AutoZ function. The most recent value of P
offset
is applied to the Q-RPT
reading to correct for change in zero over time.
The AutoZ function can be turned ON and OFF. Table 8 summarizes the effect of AutoZ ON
and OFF for the three Q-RPT types in different measurement modes.
AutoZ in absolute measurement mode with an Axxx Q-RPT
Q-RPTs with the designation Axxx are intrinsically absolute. They have an evacuated and
sealed reference. Axxx Q-RPTs can measure absolute pressure (difference from vacuum).