MTS Model 286-30 Pore Pressure Intensifier - Manual (Page 26)

286.30 Pressure Intensifier 26
Pore Pressure System Operation Considerations
Operation
Effects of Thermal Expansion of Pore Fluid
When heaters are used to heat the triaxial cell and the specimen and pore fluid,
the pore fluid expands.
Thermal expansion
when operating in
pressure control mode
If the pore pressure system is being operated in the pressure control mode while
the triaxial cell is being heated, pressure inside the triaxial cell normally will not
increase as temperature rises. This is because any tendency for pressure to
increase, as a result of thermal expansion or any other cause, will be
automatically corrected by the controller. The piston of the PPI’s pressure
intensifier will simply retract slightly, as required to maintain the correct
pressure, in spite of increasing thermal expansion, assuming that the pressure
intensifier’s piston does not reach the physical end of its stroke in the process. If
the piston cannot retract sufficiently to maintain the programmed pressure, pore
pressure will increase rapidly with further thermal expansion.
Thermal expansion
when operating in
displacement control
mode
If the pore pressure system is being operated in the displacement control mode,
while the triaxial cell is being heated, pore pressure will increase rapidly as
thermal expansion occurs, because, except for various monitoring devices such
as limit detectors, the controller is insensitive to pressure changes while in
displacement control.
Properly executed operating procedures can avoid either of the situations that are
described under this heading, where pressure increases undesirably because of
thermal expansion. They are described here to indicate that such situations can
develop and to provide a basis for describing the use of the controller’s error
detectors and limit detectors.