Fluke RPM4-AD, Руководство по эксплуатации и обслуживанию

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RPM4-AD™ OPERATION AND MAINTENANCE MANUAL
© 2005-2007 DH Instruments, a Fluke Company Page 58
3. P
offset,G
, P
offset,A
and P
offset,D
: 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
),
absolute(
A
) or differential (
D
) measurement mode:
P
offset
= P
u,0
- P
std,0
P
offset
represents the difference between the Q-RPT reading ( P
u,0
) and the AutoZ standard
( P
std,0
). This value is used to correct the RPM4-AD Q-RPT so it agrees with the AutoZ
standard. The AutoZ function manages the determination, storage and application of P
offset
to correct the Q-RPT in absolute, gauge and differential 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.
The AutoZ function can be turned ON and OFF. Table 11 summarizes the effect of AutoZ
ON and OFF.
AutoZ in absolute measurement mode
RPM4-AD Q-RPTs are intrinsically absolute. They have an evacuated and sealed reference.
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.
When a Q-RPT is 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 3.3.9.2).
AutoZ in gauge and negative gauge measurement modes, dynamic compensation for
atmospheric pressure
RPM4-AD Q-RPTs are intrinsically absolute but they are also used in gauge and negative
gauge (difference from atmosphere) measurement modes (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, 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 a barometer (
Δ P
atm
) between
AutoZ updates (see Section 3.2.2). The measured gauge pressure is calculated using Δ P
atm
to correct the value of P
offset.G
.
P
gauge
= P
u
- P
offset.G
- Δ P
atm
In gauge measurement mode, running the AutoZ function to update the value of P
atm,0
, is
initiated by the operator using [AutoZ] (see Section 3.3.9.1). Δ P
atm
resets when AutoZ is run
is then updated continuously as new atmospheric pressure measurements are made by the
barometer.