DH Instruments FPG8601, Руководство по эксплуатации и обслуживанию

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3. VLPC THEORY OF OPERATION
Page 21 © 2007 DH Instruments, a Fluke Company
3.4 RANGE RESTRICTIONS
Given a stable controlled flow, the pressure is defined as the drop across a given range restriction. The thermal
and mechanical stability of the restrictions is critical to the stability and controllability of pressure.
The five lowest conductance (highest pressure range) restrictions are accomplished with a piston and
block configuration very much like a 5 slm
DHI
molbloc. The highest conductance (lowest pressure
range) restriction (labeled R1) is used in absolute mode only. R1 is accomplished by flowing through a
short section of 16 mm ID tubing and a large vacuum valve in a manifold before attaching to KF-25
vacuum tube and a vacuum pump.
The following tables list the pressures available for ranges in gauge and absolute modes.
Table 2.
Nominal VLPC Ranges, 15 kPa Unit
ABSOLUTE RANGES
RANGE
LOWEST
PRESSURE*
[PA]
LOWEST
CONTROLLABLE
[PA]
RANGE
MAXIMUM
[PA]
R0
R1
R2
R3
R4 0.8
0.8
50
75
100
1
3
100
280
650
5
150
1
500
5 000
15 000
*Note: The lowest pressure for a given range occurs when the pressure is set to
0 in that range. The pressure is generated by the flow of lubricating
gas from the upper chamber through the range restriction selected.
GAUGE RANGES
RANGE
LOWEST
PRESSURE*
[PA]
LOWEST
CONTROLLABLE
[PA]
RANGE
MAXIMUM
[PA]
R2
R3
R4
R5
R6 0
0
0
0
0 0.1
0.5
4
9
50
15
150
1
500
3 000
15 000
*Note: The lowest pressure for each range in gauge mode is 0. The bypass
valve is opened whenever the pressure is set to 0 in any gauge
range. The conductance of the bypass in gauge mode is low enough
that there is no measurable pressure difference due to the flow of
lubricating gas from the upper chamber.
3.5 CONTROL
VLPC control is designed to set the pressure to the target value and control continuously to keep
pressure within the hold limit and as close to the target value as possible. The advantage of this type of
control scheme is that the final pressure achieved will be very close to the target value. However, control
noise can lead to errors unless the FPG pressure (and DUT pressure if available) is averaged. For this
reason FPG Tools contains an averaging feature for running tests and logging data (see Sections 6.4.2,
6.7.4).
The control of both the coarse (high flow) and fine (low flow) MFCs is accomplished with PID control
based on pressure readings. The FPG pressure is used for all control readings all ranges for optimum
control and resolution.