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3. OPERATION
Page 55 © 2007 DH Instruments, a Fluke Company
<7intern> Access internal functions including on-board barometer viewing, read rate adjustment, incident
log viewing (see Section 3.5.7).
<8cal> View and adjust RPM4 Q-RPT and barometer calibration coefficients (see Sections 3.5.8, 5.2, 5.3).
<9reset> Access RPM4’s various reset functions (see Section 3.5.9).
3.5.1 <1AUTOZ>
PURPOSE
To turn ON and OFF the AutoZ function that offsets the RPM4 Q-RPT readings relative to a
reference value in order to compensate for possible Q-RPT zero drift between full
recalibrations. To view and adjust the value of the AutoZ offset.
To RUN the AutoZ routine that rezeroes the Q-RPT, use [AutoZ] (see Section 3.3.9).
PRINCIPLE
AutoZ Purpose and Principle
The main component of the change over time of the RPM4 Q-RPTs is change in zero or
offset, independent of span. Offsetting or “rezeroing” RPM4 Q-RPTs relative to a reference
between recalibrations allows measurement uncertainty specifications to be maintained with
less frequent full calibrations. The RPM4 AutoZero function (AutoZ) provides full on-board
support for the rezeroing process to simplify its regular application by the user.
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 RPM4 Q-RPT.
In absolute measurement mode, the pressure at which AutoZ is performed is normally
atmospheric pressure and the P
std,0
value can be supplied a) by manual entry, b)
automatically from a DHI RPMx connected to RPM4’s COM2 communications port, or c)
automatically from the Lo Q-RPT for the Hi Q-RPT.
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 AutoZ standard (P
std,0
) for gauge (
G
) and absolute(
A
) 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
). The AutoZ function manages the determination, storage and application of P
offset
to
correct the zero offset 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 zero drift.
The AutoZ function can be turned ON and OFF. Table 5 summarizes the effect of AutoZ ON
and OFF in absolute and gauge measurement modes.
AutoZ in absolute measurement mode
The RPM4 A70M/A20M-AF Q-RPTs are intrinsically absolute. They have an evacuated and
sealed reference and naturally 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