PG7000™ OPERATION AND MAINTENANCE MANUAL
© 1998-2008 DH Instruments, a Fluke Company
Page 54
PRINCIPLE
Differential mode is designed to allow PG7601 to define pressures which piston
gauges have typically not covered or for which they have been difficult to use.
This includes positive and negative gauge pressures right around zero, for
example ± 2.5 kPa (10 in. H2O); as well as low differential pressures at low
absolute static pressures, for example 0 to 2.5 kPa (20 Torr) at 15 kPa absolute
(2.2 psia) static pressure.
The principal of differential mode is to define a differential pressure as the
difference between an absolute pressure relative to vacuum and a second
absolute pressure (the static pressure) measured by a precision Reference
Pressure Monitor (RPM). An offsetting procedure “tares” the RPM so that the
uncertainty contributed to the differential pressure by the RPM’s measurement is
a function of the RPM resolution and short term repeatability rather than its
absolute accuracy and long term stability.
For differential mode operation, the static pressure is set to the desired value as
measured by the RPM (atmospheric pressure for gauge pressure
measurements). Then RPM offset at the static pressure is determined by
applying the static pressure as defined by the PG7601 following:
RPM
offset
= PGRPM
start
- RPM
indicate
Where:
PGRPM
start
=
Pressure applied by PG7601 to RPM
(nominally equal to
RPM
start
)
RPM
indicate
=
RPM reading when
PGRPM
start
is applied
to RPM test port
Once the RPM offset has been determined, differential pressures are defined by
subtracting the static pressure measured by the RPM from an absolute pressure
defined by the PG7601 following:
PG
dif
= PG
abs
- RPM
current
- RPM
offset
Where:
Pg
dif
=
Differential pressure defined by PG7601
Pg
abs
=
Absolute pressure defined by PG7601
RPM
current
=
Current RPM indication of static pressure
RPM
offset
= Disagreement
between PG7601 and RPM at
the operating static pressure
For differential mode operation, the PG7000 Differential Mode Interconnections
Kit should be used to provide the DUT high and low test connections and support
the RPM offsetting procedure (see Figure 9 and Section 3.9.4.1). A precision
Reference Pressure Monitor, such as a
DHI
RPM, must be selected for the AtmP
source in SETUP (see Section 3.10, 3.11.5.4).
PG7601 differential measurement mode manages the data acquisition and
handling to support differential mode operation without operator effort.
Differential mode operation requires setting the static pressure at which
differential measurements are to be made and regular determination of the RPM
offset. These functions are supported under
[MODE]
,
<4dif>
.