Fluke MOLBOX RFM Operation And Maintenance Manual page 64

molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL
AutoZ Purpose and Principle
The main component of the change over time of the molbox RFM RPTs is
change in zero or offset, independent of span. Offsetting or "rezeroing" molbox
RFM RPTs relative to a reference between recalibrations allows measurement
uncertainty specifications to be maintained with less frequent full calibrations.
The molbox RFM AutoZero function (AutoZ) provides full on-board support for
the rezeroing process to simplify its application by the user.
The AutoZero function uses three values:
1. P
2. P
3. P
P
(P
The AutoZ function manages the determination, storage and application of P
for both molbox RFM RPTs in absolute mode. The AutoZ handles both molbox
RFM RPTs simultaneously as they are of the same range and always used
together.
The source of P
pressure, with uncertainty significantly better than that of the RPT that is being
AutoZeroed (see Section 1.2.2). This can be accomplished with a variety of
digital barometers or with a piston gauge able to set absolute pressure.
When the RPTs are used with AutoZ ON, absolute pressure is calculated as:
When RPTs are used with AutoZ OFF, P
When the RPT is calibrated, P
regular intervals using the AutoZ function. The most recent value of P
applied to the RPT reading to correct for change in zero over time.
Recommendations for the Use of the AutoZ Function
The AutoZ function provides a powerful and easy to use tool for improving the
stability over time of molbox RFM RPTs and maximizing the recalibration interval
by compensating for change in zero between full recalibrations. The following
simple recommendations will help assure that you use this feature to best
advantage.
•
•
© 1998-2007 DH Instruments, a Fluke Company
The AutoZ function has no effect on the microrange (differential) RPT
measurement (if present).
PRINCIPAL
: The absolute pressure value indicated by the AutoZ reference, the
std,0
device that is acting as the reference relative to which to offset the RPT.
The pressure at which AutoZ is performed is normally atmospheric pressure
and the P value can be supplied a) by manual entry, or b) automatically
std,0
from a DHI RPMx Reference Pressure Monitor.
: The absolute pressure reading of the RPT, with no AutoZ offset, at the
u,0
time AutoZ is performed.
: The difference between the absolute pressure reading of the RPT
offset
with no AutoZ offset (P
represents the change in zero of the RPT relative to the AutoZ standard
offset
).
std,0
must be an absolute pressure, nominally atmospheric
std,0
Always leave AutoZ ON when operating if the AutoZ routine has been run
regularly using a valid atmospheric reference.
Run AutoZ to update P
uncertainty is known to be significantly better than that of the molbox RFM
Page 54
) and the indication of the AutoZ reference (P
u,0
P = P - P
offset u,0 std,0
P = P - P
abs u,0 offset
is ignored.
offset
is set to zero. P
offset
only when a reference whose measurement
offset
):
std,0
offset
is then redetermined at
offset
offset
is