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RPM4/HPMS A70M/A20M-AF OPERATION AND MAINTENANCE MANUAL
© 2007 DH Instruments, a Fluke Company Page 56
being AutoZeroed. Given the high pressure range of the RPM4 Q-RPTs, adequate
uncertainty is easily available from a variety of digital barometers or, for the Hi Q-RPT, by the
Lo 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] (see Section 3.3.9.1).
AutoZ in gauge measurement mode, dynamic compensation for atmospheric pressure
The RPM4 A70M/A20M-AF Q-RPTs are intrinsically absolute but they are also used in gauge
measurement mode (difference from atmosphere). 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, PRINCIPLE).
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. 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 RPM4’s on-board 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, running AutoZ is initiated by the operator pressing [AutoZ]
(see Section 3.3.9.1) when the RPM4 is vented. This assures regular updating of the value
of P
offset,G
and P
atm,0
. ΔP
atm
is continuously updated automatically.
When gauge mode is first activated, 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 vent the
RPM4 Q-RPT and press [AutoZ] to run AutoZ and obtain an actual value of P
offset.G
.
AutoZ ON/OFF
The AutoZ function can be turned ON and OFF separately for gauge and absolute
measurement modes. Table 5 summarizes the effect of AutoZ ON and OFF. Generally,
AutoZ should always be left ON.
Table 5. AutoZ ON and OFF
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
Gauge
OFF
YES
NO
When initiated
by operator