DH Instruments ADCS-601, Operation And Maintenance Manual

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ADCS-601™ OPERATION AND MAINTENANCE MANUAL
© 2005 DH Instruments, Inc. Page 60
4.1.4 DIFFERENCE BETWEEN REQUESTED AND ACTUAL SET
PRESSURE
ADCS-601 is not designed to set exactly the pressure that has been requested. Rather, it
sets a value of pressure very close to the requested pressure and returns the exact pressure
value it has set.
The majority of the ADCS-601 pressure range (15 kPa (4.5 inHg) and up) is covered by an
automated PG7601 piston gauge (see Section 2.2.1). The automated mass handling system
of the PG7601 can load mass in steps of 0.1 kg. On the PG7601’s 10 kPa/kg piston-cylinder,
0.1 kg corresponds to increments of 1 kPa (0.3 inHg). When a pressure is requested of the
PG7601, it calculates the mass to load to reach the pressure. It then loads the mass as
closely as possible using its 0.1 kg resolution, calculates the actual pressure achieved and
displays that value. Therefore, when using the PG7601, the actual pressure set may be up
to 0.5 kPa (0.15 inHg) different from the pressure requested.
The FPG8601 pressure standard used to handle pressure requests under 15 kPa (4.5 inHg) (see
Section 2.2.2) is normally used in a similar manner. The FPG8601’s PPC3 pressure controller is
commanded to set and stabilize the requested pressure. Due to the possible difference between
the PPC3’s pressure transducer and the FPG8601, the final pressure set by the PPC3 will not be
exactly the requested pressure as read by the FPG8601. Since the FPG8601 is the pressure
standard and has much lower measurement uncertainty than the PPC3’s transducer, the
FPG8601’s reading is displayed as the ADCS-601 pressure value.
When using the FPG8601 (pressure points under 15 kPa (4.5 inHg), it is possible to set exactly
the requested pressure by using
Regulation mode. In Regulation mode, the PPC3 pressure
target is iteratively adjusted by the difference between the PPC3 pressure transducer reading
and the FPG8601 reading to cause the FPG8601 to read the requested value of pressure within
the FPG8601 pressure hold limit. See Section 4.1.2.3 for information on pressure
Ready/Not
Ready criteria when using FPG8601 with Regulation mode ON and OFF. Regulation mode
can be turned ON and OFF in the ADCS Tools Run Screen Control Panel when the FPG8601
is the active pressure standard (see Section 4.3.5.1).
4.1.5 ASOLUTE AND GAUGE MEASUREMENT MODES
ADCS-601 can operate in two measurement modes.
In absolute mode pressure is defined relative to a vacuum. Vacuum is provided by the
system reference vacuum pumps and is applied to the reference circuit of the pressure
standards (see Figure 29). The residual vacuum level is ready by thePG7601 and FPG8601
reference vacuum CDGs.
In gauge mode pressure is defined relative to atmosphere. The reference vacuum pumps
are turned OFF or isolated and the reference circuit of the pressure standards is opened to
atmosphere. The DUT may be connected to the reference circuit on the
TEST(-) port to
assure that the DUT and pressure standards are at a common reference pressure.
See Section 4.2.3 for additional information on changing measurement modes.
There is a well defined order of operation for transitioning the ADCS-601 system between
measurement modes. The order is controlled by the ADCS Tools software. Mode
transitions should only be performed under ADCS Tools control. Vacuum pumps must
always be left ON in absolute mode. Before turning OFF vacuum pumps or disconnecting
vacuum supplies, transition to gauge mode. Once a measurement mode change is
initiated, it should be allowed to complete uninterrupted.