7. PISTON-CYLINDER MAINTENANCE
Page 109
© 2007 DH Instruments, a Fluke Company
7.2
REMOVING THE PISTON-CYLINDER FROM THE
MOUNTING POST
Unless the piston-cylinder is obviously contaminated, consider using in situ methods to
restore mobility of the piston in the cylinder before removing it and cleaning it (see
Section 7.7).
The following procedure describes the steps to be taken in order to safely remove the piston-
cylinder from the mounting post. Great care must be taken when removing the assembly.
Permanent damage can occur if the recommended steps are not specifically followed.
Start with the FPG in gauge mode. If the FPG is in absolute mode, use FPG Tools to
transition to gauge mode (see Section 6.9.2.1).
Shut down the FPG system by pressing the
[OFF]
button on the
FPG Run Screen
. Wait
until the shut down procedure is complete (FPG Pressure is
< --- SHUTDOWN --- >
and
indicates
Ready
).
The FPG reference vacuum CDG is delivered with an isolation valve connected to it. This
valve is normally closed and serves to maintain vacuum on the CDG at all times as
exposing the CDG to atmospheric pressure will cause zero shifts. Do not break the valve
fittings or operate the valve without establishing vacuum first.
Remove the vacuum isolation valve (valve 9) on the bottom of the lower mounting post
and remove the vacuum reference CDG and valve.
Detach the bottom mounting post PRT by unscrewing the knurled nut on the back of the
mounting post.
Apply a slight upward pressure with one hand under the lower mounting post (see Figure 60).
Loosen the nut at the center of mounting post.
The upper and lower mounting post are held together by a large surface area retaining nut
that compresses an O-ring between the mounting post halves. Pressing upward on the
lower mounting post allows the mounting post nut to be easily unscrewed with one hand.
If pressure is not applied to the lower mounting post, the mounting post nut can remove
small particles from the mounting post and possibly cause contamination.
With the other hand, completely unscrew the mounting post nut.
Lower the mounting post down as straight as possible.
If the lower mounting post is not lowered in vertical descent, it will make contact with the
external surface of the cylinder. If this happens, contaminants can be introduced into the
low-pressure chamber of the mounting post.
Fully remove the lower mounting post and set it on a clean surface.
