3. OPERATION
Page
9
© 2008 DH Instruments, a Fluke Company
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3.1
CONTROLLED CLEARANCE PRESSURE (CCP)
In normal operation, the CCP pressure is the measured pressure. The pressure applied to the TEST port
should also be applied to the CCP port.
PG7607 allows an independent pressure to be applied to the inside of the piston. The purpose of this feature
is to allow the piston to be artificially expanded, varying the size of the gap between the piston and cylinder.
The ability to predictably vary the size of the gap between the piston and the cylinder can be useful in
analyzing piston-cylinder behavior.
The controlled clearance pressure is introduced through the CCP port on the back of the PG7607 platform.
The pressure is applied through the mounting post to the inside of the piston (see Figure 1, Ref. 8). The
pressure is contained by the two piston sealing O-rings (see Figure 1, Refs. 10a, 10b).
The piston sealing O-rings are positioned on the ends of the piston so that the deformation due to pressure
along the active zone of the piston is constant and parallel.
The expansion of the piston with pressure can be predicted using thick walled cylinder stress/strain relations
and the mechanical properties of the piston material (tungsten carbide). The result is 0.16 micron on the
diameter/MPa.
A typical 50 mm piston-cylinder assembly has a radial gap of 0.9 micron so a CCP of 3 MPa will reduce the
gap by about 50 %. Around 5 MPa, the CCP may close the gap completely causing mechanical contact
between the piston-cylinder and damage to the parts.
A controlled clearance pressure (CCP) of about 5 MPa may close the gap between the piston and cylinder
completely causing metal to metal contact between the two parts and possible damage.
3.2
[HEAD]
The natural reference level for the PG7607 piston-cylinder assembly is near the top of the cylinder cap. So
that, in practice, the actual reference level is at a conveniently identified location, a reference level offset is
applied. The reference level offset places the reference level at the bottom of the piston. This point is defined
by the horizontal line where the bottom of the piston meets the mounting post when the piston is mounted.
The reference level offset is included in the piston-cylinder file (see PG7000 Operation and Maintenance
Manual).
When using a AMH-38 with the PG7607, the PG7607 reference level is 3 mm below the reference level
marked on the AMH vacuum chamber. This is because the AMH-38 is marked for the PG7601.
