3. OPERATION
Page 65
© 1995 - 2010 Fluke Corporation, DH Instruments Division
PRINCIPAL
AutoZ Purpose and Principle
The main component of the change over time of the RPTs is change in
zero or offset, independent of span. Offsetting or “rezeroing” RPTs
relative to a reference between recalibrations allows measurement uncertainty
specifications to be maintained with less frequent full calibrations. The
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
std,0
:
The absolute pressure value indicated by the AutoZ reference, the
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
std,0
value can be supplied a) by manual entry, or b) automatically
from a
DHI
RPMx Reference Pressure Monitor.
2.
P
u,0
:
The absolute pressure reading of the RPT, with no AutoZ offset, at the
time AutoZ is performed.
3.
P
offset
:
The difference between the absolute pressure reading of the RPT
with no AutoZ offset (
P
u,0
) and the indication of the AutoZ reference (
P
std,0
):
P
offset
= P
u,0
- P
std,0
P
offset
represents the change in zero of the RPT relative to the AutoZ standard
(
P
std,0
). The AutoZ function manages the determination, storage and application of
P
offset
for both RPTs in absolute mode. The AutoZ handles both
RPTs simultaneously as they are of the same range and always used together.
The source of
P
std,0
must be an absolute pressure, nominally atmospheric
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:
P
abs
= P
u,0
- P
offset
When RPTs are used with AutoZ OFF,
P
offset
is ignored.
When the 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 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 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.
•
Always leave AutoZ ON when operating
if
the AutoZ routine has been run
regularly using a valid atmospheric reference.
•
Run AutoZ to update
P
offset
only when a reference whose measurement
uncertainty is known to be significantly better than that of the
RPTs is available. Though it may not be practical and generally is not
necessary, the best possible reference with which to run AutoZ in absolute
measurement mode is a gas operated piston gauge (such as a
DHI
PG7601)
applying an absolute pressure near atmospheric pressure to the
test port. The best day to day reference is a properly calibrated
DHI
RPM4
with a BA100K RPT interfaced directly as an external device to the
COM2 port.
