DH Instruments PPCH, Руководство по эксплуатации и обслуживанию

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3. OPERATION
Page 53 © 2009 DH Instruments, a Fluke Company
 PRINCIPLE
PPCH is calibrated at the reference level marked on the instrument rear panel. Frequently,
when performing a calibration or test, the device or system under test is at a different height
than the PPCH’s
TEST
port. This difference in height, commonly called
head
, can cause a
significant difference between the pressure measured by the PPCH at its
TEST
port height
and the pressure actually applied to the device under test which is at a different height.
In this case, it may be useful to make a head correction to the pressure measured by the
PPCH in order to predict the pressure actually applied at a DUT at a different height.
PPCH can calculate
head
pressure over its working pressure range for oil, water or a user
defined liquid as well as a variety of gasses. The HEAD function allows the height difference
to be specified and causes the resulting head pressure to be added to the pressure measured
at the
TEST
port.
The
[HEAD]
function key is used to specify the height difference between the PPCH
reference level and another height. Entering a height of zero turns the function off.
The height units and the test fluid are specified by pressing
[SPECIAL]
,
<3Head>
(see Section 3.5.3).
Head influences in gauge and absolute mode with a liquid as the test fluid can be confusing,
particularly when using an external reservoir. Follow the recommendations below for
consistent results in various setups and measurement modes.
Head Effect Considerations when Using the Internal Reservoir
When the PPCH is vented, it is open to the return tube in the internal reservoir. The top of
this tube is intentionally above the normal surface of the liquid in the reservoir and is aligned
with the Q-RPT reference level marked on the rear panel. This assures that a consistent
head height is applied to the system connected to the PPCH
TEST
port when the system is
vented by the VENT function of PPCH and that its height is the same as the reference level
of the Q-RPT.
•
In both gauge and absolute measurement modes
, enter the difference between the
PPCH reference level (marked on the rear panel of PPCH) and the DUT height as the
head height in
[HEAD]
. In gauge measurement mode, if you “zero” the device under
test (DUT) with the PPCH vented before the test, always enter the head height in
[HEAD]
as zero.
•
In gauge measurement mode
, be sure AutoZ is ON for the active Q-RPT (see Section
3.5.1). Allow PPCH to AutoZero when vented before running a test. Note that if the
DUT head height entered in
[HEAD]
is not zero, it is normal for PPCH to display a non-
zero value of pressure after AutoZeroing.
Head Effect Considerations when Using an External Reservoir
The external reservoir is normally mounted at a level higher than the PPCH internal reservoir
(see Section 2.3.6.2).
•
The reference level is not the same in gauge and absolute measurement modes.
•
In gauge measurement mode
, enter the difference between the top of the return tube
or liquid level in the external reservoir (
not
the level marked on the back of PPCH) and
the DUT height as the head height in
[HEAD]
. But, in gauge measurement mode, if you
“zero” the device under test (DUT) with the PPCH vented before the test, always enter
the head height in
[HEAD]
as zero.
•
In gauge measurement mode
, be sure AutoZ is ON for the active Q-RPT (see Section
3.5.1). Allow PPCH to AutoZero when vented before running a test. Note that if the
DUT head height entered under
[HEAD]
is not zero, it is normal for PPCH to display a
non-zero value of pressure after AutoZeroing.
•
In absolute measurement mode
, enter the difference between the PPCH reference level
(marked on the rear panel of the PPCH) and the DUT height as the head height in
[HEAD]
.