DH Instruments PG7000, Руководство по эксплуатации и обслуживанию

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3. GENERAL OPERATION
Page 41 © 1998-2008 DH Instruments, a Fluke Company
3.5 PISTON POSITION
When operating a PG7000 piston gauge, reference pressure values are defined by loading known mass
values on the piston and adjusting the pressure to float the piston. Piston position is measured and
displayed real time on the MAIN run screen (see Section 3.7) and in the first SYSTEM run screen (see
Section 3.9.5). Piston position is used as a criterion for the
Ready/Not Ready indication as valid
measurements can only be made when the piston is in the correct position (see Section 3.4.1).
The full piston stroke is
±
4.5 mm from the midstroke position. The stroke is divided into different
positions and zones as illustrated in Figure 8. These zones are:
•
High and low stops: The piston is at the minimum or maximum end of stroke. The pressure
applied to the piston is higher (at high stop) or lower (at low stop) than that corresponding to the
mass loaded on the piston. The high and low stop positions are fixed.
•
High and low spring zones: The combination of pressure and the force of the high or low stop
springs have caused the piston to leave the stop. The pressure applied to the piston is within the
equivalent of 2 kg of the pressure corresponding to the mass loaded on the piston. The high and low
spring zones indicate the piston is
about to float . The spring zone positions are fixed.
•
High and low measurement zones: The piston is within the zone in which a Ready measurement
can be made (see Section 3.4.1). The pressure applied to the piston is the pressure corresponding
to the mass loaded on the piston. The default value of the high and low measurement zones is
± 2.5 mm around midstroke position. This value can be adjusted in the SETUP file (see Section 3.10).
•
Midstroke: The piston is at the middle of its stroke. The bottom of the piston (or its equivalent
for hollow pistons) is at the reference level marked on the mounting post (see Section 3.9.7).
There is equal stroke available to the high and low stops.
The piston position monitoring system is driven by the internal ring in the base of the mass loading bell.
When the bell is not installed, piston position cannot be measured. If the current mass load is less than the
mass of the bell, PG7000 will display < ---- > where piston position is normally displayed. If the
current mass load is greater than the mass of the bell but the bell is not actually installed, the piston
position indication is not valid.
Figure 8. Piston Stroke and Zones