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3. OPERATION
Page 59 © 2004-2007 DH Instruments, a Fluke Company
The AutoZ function can be turned ON and OFF. Table 7 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 Lo Q-RPT in the RPM4. 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, running the AutoZ function to update the AutoZ value
(P
offset,A
), is initiated by the operator using [AutoZ].
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 are also 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 3.2.2).
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, AutoZ is run when initiated by the operator
pressing [AutoZ] when the RPM4 is vented. This assures regular updating of the P
offset,G
value corresponding to atmospheric pressure. Gauge pressure is the measured absolute
pressure, P
u
, minus P
offset,G
.
P
gauge
= P
u
- P
offset,G
When AutoZ is ON, dynamic compensation for atmospheric pressure is also applied to
compensate for changes in atmospheric pressure as measured by a barometer (ΔP
atm
) between
AutoZ updates (see Section 3.2.2). The measured gauge pressure is calculated using ΔP
atm
to correct the value of P
offset.G
.
P
gauge
= P
u
- P
offset.G
- ΔP
atm
In gauge measurement mode on an Axxx RPT, running the AutoZ function to update the
value of P
atm,0
, is initiated by the operator using [AutoZ]. Updating of ΔP
atm
occurs
automatically and continuously.
When gauge or negative gauge mode is first activated on an Axxx Q-RPT, the value of
P
offset.G
subtracted from the absolute pressure reading to achieve gauge pressure is
standard atmosphere of 101.325 kPa. This can cause a large zero error when gauge mode
is activated if atmospheric pressure is significantly different from standard atmospheric
pressure. When gauge mode is activated, always press [AutoZ] to run AutoZ and obtain
an actual value of P
offset.G
.