Fluke Calibration RPM4, Operation And Maintenance Manual

Page: 69 / 150

3. OPERATION
Page 59
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 (
∆ P
atm
) between
AutoZ updates (see Section 3.2.2). The measured gauge pressure is calculated using ∆ P
atm
to correct the value of P
offset.G
.
P
gauge
= P
u
- P
offset.G
- ∆ P
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 ∆ P
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
.