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3. GENERAL OPERATION

Page 61

PRINCIPLE

DHI Technical Note 0080TN03 provides more detailed information on high line

differential mode principles including a complete uncertainty analysis. Consult DHI or
visit our website, www.dhinstruments.com to obtain a copy.

High line differential mode is designed to allow two PG7102, PG7202 or PG7302
piston gauges to be used together to define differential pressures relative to line
pressures significantly above atmospheric pressure. This capability is most often
used to test or calibrate differential pressure devices designed to operate at elevated
line pressures under their normal operating line pressure conditions.

The main challenge of defining differential pressures at elevated line pressures
comes from the very high ratio of line pressure to differential pressure. Relatively
small errors and instabilities in the line pressure are very large relative to the
differential pressure.

The principal of high line differential mode is to “crossfloat” two PG7000 piston
gauges so that they define a common line pressure. After the crossfloat is
completed, a BYPASS valve is closed isolating one PG7000 from the other. The
“tare” PG7000 maintains the line pressure. Mass is added to the “reference”
PG7000 to define differential pressures “on top of” the line pressure. The very high
precision of the PG7000 gas operated piston gauges allows them to set and
maintain a common line pressure with uncertainty much lower than the overall
measurement uncertainty on either piston gauge. The two PG7000s are
crossfloated at the line pressure prior to making differential measurement to
minimize the contribution of line pressure errors to differential pressure. The
crossfloating procedure consists of making mass adjustments on the tare
PG7000 so that both pistons, when connected together at the line pressure, fall
at their “natural” drop rate. PG7000’s embedded piston fall rate measurements
and calculations capabilities are used to assist the operator in performing the
crossfloat.

The PG7000 that is used only to maintain the line pressure on the low side of the
device under test (DUT) is designated the

tare

PG7000. The

tare

PG7000 is a

standard PG7000 but it is normally used with a

tare

piston-cylinder and a

tare

mass

set (a standard piston-cylinder and mass set may also be used). A

tare

piston-

cylinder effective area is entered as the nominal effective area and its
characteristics include k(P), which must be entered in the piston-cylinder
definition file (see Section 3.11.1.1). A

tare

mass set has the same configuration

as a standard mass set but the exact values of the masses are not measured
and the mass set is set up using nominal values for each mass measured value
(see Section 3.11.1.6). The

tare

piston mass is adjusted to be slightly under the

nominal piston mass. This assures that the

tare

side will always be the light side

when crossfloated with the

reference

side. If a

tare

piston-cylinder and mass

set is

not

used on the

tare

PG7000, the

tare

side may not be the light side in a

crossfloat. In this case, the

tare

mass load on the

reference

side should be

increased slightly (generally < 1 g) to ensure that it is heavier, taking care not to
confuse the added tare mass with the subsequent differential mass load.

The PG7000 that is used to apply the added pressure to create the differential
pressures on the high side of the DUT is designated the

reference

PG7000.

The

reference

PG7000 is a standard PG7102, PG7202 or PG7302 with a

standard piston-cylinder and mass set. The

reference

must be a PG7102,

PG7202 or PG7302. The

tare

may be a PG7102, PG7202, PG7302 or a