X2.2.7 Piston Parameters—The diameter of the piston foot
should be checked. If it is worn, material can flow back past the
tip. This would result in erroneous data.
X2.2.7.1 The piston tip, or foot, is sometimes screwed into
the end of the piston and can be changed easily. However,
because it is easily unscrewed, it can work itself loose. The
piston tip must be checked frequently and kept tightly screwed
into the piston.
X2.2.7.2 Care must be taken not to bend a piston. Even a
slight, almost non-detectable curvature in the piston can result
in the force not being applied directly to the vertical position,
resulting in excessive pressure on the wall of the barrel. This
will cause low results and can also scratch the barrel wall.
X2.2.7.3 The piston has two reference marks. The lower
scribe mark on the piston must be at the reference start position
(top of the guide ring or barrel) at 7.0 6 0.5 min, as stated in
9.8 and 10.9. Starting each test at a different position can give
variable results.