5. SYSTEM OPERATION
Page 43
© 2007 DH Instruments, a Fluke Company
5.7.1
SETTING 0 PRESSURE
Setting zero pressure using the VLPC has a different meaning in the different measurement
modes of the FPG. See Section 5.4 for information on how to set a measurement mode.
Below is a list of measurement modes and the corresponding actions that FPG Tools takes
when setting zero pressure. In all cases, the VLPC does not control zero pressure.
Gauge Mode
The FPG bypass valve is opened (upper and lower FPG chambers
are connected together) and the VLPC pressure is vented.
No special actions are taken with the optional DUT manifold.
This is a true zero condition (see Section 5.2.1). The
lubrication flow in the FPG does not induce a measurable
pressure.
Absolute Differential Mode
The FPG bypass valve is opened (upper and lower FPG
chambers are connected together) and the VLPC pressure is
vented. If the optional DUT manifold is enabled, the manifold
bypass valve is closed. Due to asymmetry in the flow in the
manifold and the FPG, a differential DUT would not agree with
the FPG pressure if both bypass valves are open. The measured
FPG pressure will not be zero. A pressure results from the
lubrication flow from the upper chamber through the bypass
valve of the FPG to the reference vacuum pump. This pressure
should be approximately the same in all ranges of the VLPC.
Absolute Mode
The FPG bypass valve is closed (upper and lower FPG
chambers are isolated) and the VLPC pressure is vented.
If the optional DUT manifold is enabled, the manifold bypass
valve is closed. This is not the zero condition of the FPG
therefore the pressure will not be zero. The lubrication flow
through the active range restriction of the VLPC will induce a
pressure in the upper FPG chamber. The higher the VLPC
range, the higher the range restriction and corresponding
zero pressure. In the lowest VLPC range, the pressure is
typically less than 5 Pa and is typically greater than 100 Pa
in the highest VLPC range.
5.8
LOGGING DATA WITH THE FPG
There are two data acquisition options provided by FPG Tools:
[Run w/Point Log]
and
[Run
Test Sequence]
. Both options provide features for automatically gathering data from the FPG and any
DUT that is set up. FPG Tools automatically acquires data from DUTs with an RS232 or IEEE-488
interface or prompts for the output of manual DUTs output. All data is written to a data file in a consistent
format that is easily imported into other applications (see Section 9.2).
A separate data file is created for every active DUT. When no DUTs are active, only 1 data file is created.
The
[Run w/Point Log]
option allows discrete data points to be taken whenever desired, averaged for a
specified amount of time, or taken automatically at fixed intervals by using the
[Take Point]
,
[Average
Data]
, and
[Auto Point Log]
functions toolbar options respectively (see Sections 6.4.2 and 6.5.3). It is
up to the test operator to set pressures and define control times for each point.
When a fixed sequence of points are required, use the
[Run Test Sequence]
option (see Section 6.5.4).
Prior to selecting this menu choice make sure a test sequence is defined by using the
[Config], [Test
Sequence Editor]
option. This allows a list of target pressures to be specified along with averaging and
dwell times (see Section 6.6.5).
