7. PISTON-CYLINDER MAINTENANCE
Page 115
© 2007 DH Instruments, a Fluke Company
Verify that both retaining pins are tight.
Verify that the silicon O-ring is in place on the upper mounting post (between the upper and lower
mounting post).
Attach the lower mounting post, aligning it with the cylinder.
Place a hand under the lower mounting post and firmly press up.
With the other hand, thread the mounting post nut mating the upper mounting post to the lower
mounting post.
Restart the FPG by pressing the
[ON]
button on the
FPG Run Screen
. Wait until the start up
procedure is complete.
Wait at least five minutes with the FPG at zero gauge pressure. If the pressure (FPG load cell output
<N>
) is noisy, the piston and cylinder may require lubrication restoration procedure (see Section 7.7)
or re-cleaning (see Section 7.4).
7.7
RESTORING PISTON-CYLINDER LUBRICATION AND
MOBILITY
7.7.1
OVERVIEW
For the FPG piston-cylinder to operate properly, the piston must be well centered in the
cylinder so that it is free and mobile. The piston is lubricated and centered by the flow of
lubricating gas from the center of the piston-cylinder through its upper and lower conical gaps
(see Section 2.2). While the lubricating gas flow can maintain the piston centered in the
cylinder, for its action to be effective, the piston must first be well lubricated with gas and
centered in the cylinder.
A piston-cylinder that has just been installed in the FPG mounting post may not be well
enough centered and lubricated for the lubricating gas flow to center. it. The piston centering
may also be lost if the lubricating gas flow is interrupted. The symptom of this condition is
noisy output of the pressure even when the pressure is stable, in particular when the FPG is
being zeroed or when the lower mounting post has been removed.
When the FPG output is noisy it often is not because the piston-cylinder is dirty. It is because
it is not well centered and lubricated.
The four procedures described in Section 7.7.2 should be used to center the piston in the
cylinder and lubricate it before applying the lubricating pressure.
If the centering methods do not reestablish stability, the piston-cylinder may need to be
cleaned (see Section 7.4).
It is not unusual to need to repeat the procedures multiple times.
7.7.2
PISTON CENTERING AND LUBRICATING METHODS
The four procedures below are used to prepare the piston-cylinder for application of the
lubrication pressure.
These procedures are used with the piston-clinder installed in the mounting post but with the
lower mounting post removed so that the piston-cylinder can be accessed and manipulated.
Remove the lower mounting post following the instruction in Section 7.2, but do not remove
the piston-cylinder.
Observe the value of
<N>
on the
FPG Tools Run Display Screen
(see Section 6.3.2). This is
the output of the FPG8601 load cell. Alternate the four methods until the output of the load cell is
consistently stable ± 1 or 2 counts. Then install the lower mounting post and start the FPG8601
by pressing the
[On]
button on the
FPG Tools Run Display Screen
. The FPG8601 output
(display of
<N>
) should increase and then stabilize again ± 1 or 2 counts. If it does not, remove
the lower mounting post and repeat the process.
