E-DWT-10000-AF OPERATION AND MAINTENANCE MANUAL
© 2008 DH Instruments, a Fluke Company
Page 54
3.9.1
<1A
UTO
Z>
PURPOSE
To turn ON and OFF the AutoZ function that allows the Q-RPT reading to be zeroed and the
dynamic compensation for atmospheric pressure changes feature. Also, view and adjust the
value of the AutoZ offset.
To RUN the AutoZ routine that zeroes the Q-RPT, use [AutoZ] (see Section 3.7.10).
PRINCIPLE
AutoZ Purpose and Principle
The RPM4-E-DWT AutoZero function (AutoZ) is used to achieve gauge measurement mode
with the RPM4-E-DWT’s intrinsically absolute Q-RPTs and to eliminate the influence of zero
drift on measurement uncertainty. AutoZ can also tare out fluid heads between the E-DWT
and the device under test.
The AutoZ functions provides full on-board support for regular zeroing including
determination, application and storage of offset values as well as dynamic compensation for
changes in atmospheric pressure between zeroing opportunities. The operator initiates
Q-RPT zeroing by pressing the
[AutoZ]
key from the main run screen when zero
(atmospheric pressure) is applied to the RPM4-E-DWT Q-RPT.
The AutoZero function uses four values:
1.
P
std,0
:
Zero (atmospheric pressure) which is supplied by definition when the Q-RPT is
active, the reservoir shut off valve is close, the TEST shut off valve is open and the TEST
port is disconnected and filled with oil to the weep hole (reference level).
2.
P
u,0
:
The absolute pressure reading of the Q-RPT, with no AutoZ offset, at the time
[AutoZ]
is pressed.
3.
P
offset,G
:
The difference between the reading of the Q-RPT with no AutoZ offset when
[AutoZ]
is pressed (
P
u,0
), and zero gauge pressure (atmospheric gauge pressure) (
P
std,0
):
P
offset,G
= P
u,0
- P
std,0
4.
Δ
P
atm
:
The difference between atmospheric pressure (absolute) as measured by the
RPM4-E-DWT’s on-board barometer when
[AutoZ]
was last pressed and the current
measured value.
Δ
P
atm
is used to compensate real time for the evolution of atmospheric
pressure between updates of
P
offset,G
by
[AutoZ]
.
Δ
P
atm
is dependent only upon the ability of
the barometer to measure small changes in atmospheric pressure over short time periods,
not upon the barometer’s absolute accuracy or long term stability.
The gauge pressure measured by the RPM4-E-DWT Q-RPT at any given moment is:
P
gauge
= P
u
- P
offset.G
-
Δ
P
atm
The AutoZ function can be turned ON and OFF. When the AutoZ function is OFF, the
[AutoZ]
function to rezero the RPT is no longer available and dynamic compensation for
changes in atmospheric pressure using the RPM4-E-DWT’s on-board barometer is no longer
active. AutoZ should be left on at all times unless the RPM4-E-DWT’s on-board barometer is
inoperable.
OPERATION
The AutoZ function and values are Q-RPT specific.
To RUN the AutoZ routine that rezeroes the Q-RPT, use [AutoZ] (see Section 3.7.10).
