3. OPERATION
Page 59 © 2002-2007 DH Instruments, a Fluke Company
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 the 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 PPC3
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).
In absolute measurement mode, the source of P
std,0
must be an absolute pressure, nominally
atmospheric pressure, with uncertainty significantly better than that of the Q-RPT that is
being AutoZeroed. For higher range Q-RPTs, this is easily accomplished with a variety of
digital barometers or with a lower range Q-RPT in the PPC3 system. For lower range Q-RPTs, a
piston gauge or other standard may be required to provide P
std,0
with low enough uncertainty.
When using an Axxx (absolute) Q-RPT in absolute measurement mode, with AutoZ ON,
absolute pressure is calculated as:
P
abs
= P
u
- P
offset,A
In absolute measurement mode, execution of the AutoZ function to update the AutoZ value
(P
offset,A
), is initiated by the operator.
AutoZ in gauge and negative gauge measurement modes with an Axxx Q-RPT,
dynamic compensation for atmospheric pressure
Q-RPTs with the designation Axxx are intrinsically absolute but they can also be used in
gauge and negative gauge measurement modes (difference from atmosphere) (see Section
3.3.3, PRINCIPLE). Gauge measurement mode is achieved by subtracting the value of
atmospheric pressure from the Q-RPT’s absolute reading using AutoZ and by dynamically
compensating for changes in atmospheric pressure between opportunities for AutoZ to
execute (see Section 3.2.3).
In gauge measurement modes on an Axxx Q-RPT, 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. In gauge measurement modes, the AutoZ routine is run automatically
whenever the PPC3 is in the vented condition. This assures regular automated updating of
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