DH Instruments FPG8601, Руководство по эксплуатации и обслуживанию

Страница: 32 / 166

FPG8601™/VLPC™ OPERATION AND MAINTENANCE MANUAL
© 2007 DH Instruments, a Fluke Company Page 20
The block diagrams below illustrate absolute and gauge mode pressure and flow.
Figure 9.
Gauge Mode VLPC Block Diagram
Figure 10
. Absolute Mode VLPC Block Diagram
The breakdown of the system is in the following areas: supply and regulation, flow control, range
restrictions, connections and control.
3.2 SUPPLY AND REGULATION
The FPG and VLPC must use the same gas medium. Typically this is bottled N2, however dry air is also
supported. There are additional considerations for initial setup and operation of the FPG in gauge mode
with a nitrogen source. Whenever changing gas mediums, the FPG and VLPC must be thoroughly
purged (see Section 4.12).
The supply gas to the VLPC must be clean, dry air between 700 and 840 kPa gauge (100 and 120 psig),
with stability of ± 1 %. For the VLPC to provide stable pressures, the inlet pressure to the flow controllers
must be stable. After passing through a 0.5 micron filter, the VLPC supply is regulated down to 200 kPa
gauge by a large diaphragm 350 kPa Tescom regulator. The output of the Tescom regulator feeds an
LNI regulator set at 100 kPa above the LNI reference. The LNI reference is downstream of the range
restrictions – a location that is effectively atmosphere for gauge control and vacuum for absolute control.
A 100 kPa differential pressure has been found to produce the most stable flow across the MFCs.
3.3 FLOW CONTROL
The flow is controlled by two mass flow controllers (MFC) in parallel. The full scale flow for the VLPC is
500 sccm. A coarse MFC (500 sccm) is used in parallel with a fine MFC (25 sccm) to achieve a wide
range of flow with very fine control resolution. After controlling to a pressure near the target, the high flow
MFC is set to a constant flow while the low flow, 25 sccm, MFC performs the fine control to achieve and
maintain pressure within specifications.