DH Instruments PG7000 Operation And Maintenance Manual Download Page 80

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PG7000™ OPERATION AND MAINTENANCE MANUAL

Page 70

It is not possible to operate in differential mode with leaks at any point in the

test system. Thoroughly leak check the system before operation and correct any
leaks detected.

High line differential mode is often used to calibrate DUTs whose differential

pressure range is very low relative to the line pressure. These DUTs can easily be over
pressured by inadvertently applying excessive pressure to one port. When operating
in high line differential pressure mode, OPEN the system BYPASS valve to zero the
differential across the DUT when there is any risk of overpressure.

Before operating in differential mode the line pressure must be set (see Section
3.9.4.2,

Setting a Line Pressure ([MODE], <3HLdif>, <1run>, <2yes>)

Once the line pressure is set, from a practical standpoint, operating in high line
differential mode is very similar to operating in gauge mode (see Section 3.9.11).

All operator interaction is with the PG Terminal of the

reference

PG7000. The PG

Terminal of the

tare

PG7000 is for display only. The

tare

PG7000 PG Terminal

keypad is locked out during high line differential mode operation.

Operation is in either

pressure to mass

mass to pressure

mode

(see Section 3.9.12).

To operate in differential mode the system BYPASS valve must be closed (see
Figure 10).

All mass loading and unloading to define differential pressures is made on the

reference

PG7000. Mass loading and unloading instructions follow conventional

PG7000 mass loading protocol (see Section 3.6) but the differential mass loading
instructions are “on top of” the line pressure mass load. The mass load
corresponding to the line pressure on the

tare

and

reference

PG7000s must

remain exactly the same throughout the differential measurements.

While in high line differential mode, the piston position and piston rotation

Ready/Not Ready

indicators on the

reference

PG7000 reflect the status of both

the

tare

and the

reference

PG7000s. For the indicator to indicate

Ready

, both

the

tare

and the

reference

PG7000s must meet their current

Ready/Not Ready

criteria (see Section 3.4). A

<T>

in a

Ready/Not Ready

status indicator indicates

that the tare PG7000 is

Not Ready.

When operating in high line differential mode, the measurement mode

indicating character directly to the right of the pressure unit of measure in the main
run screen is <d> (see Section 37).

Zero differential pressure (BYPASS valve closed) can be set by the system.

In some cases, the differential mass to load to set zero is not zero. This is normal
and can occur due to differential changes in the temperature of the tare and
reference piston-cylinders after the original line pressure crossfloat. A differential
piston-cylinder module temperature change while running in high line differential
mode affects the line pressure causing an offset to the differential pressure. The
offset to the differential pressure causes a non-zero differential mass load to be
needed to set zero differential pressure (see Section 7.2.1). Conversely, and for the
same reason, a differential mass load of zero may not result in a differential
pressure of zero.