MTS Model 286-20 Confining Pressure Intensifier - Manual (Page 24)

286.20 Pressure Intensifier 24
Confining System Operation Considerations
Operation
Effects of Thermal Expansion of Confining Fluid
When heaters are used to heat the triaxial cell, the specimen and the confining
fluid, the confining fluid expands.
Thermal expansion
when operating in
pressure control mode
If the confining 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 CPI’s pressure
intensifier will simply retract, 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, confining
pressure will increase rapidly with further thermal expansion.
Thermal expansion
when operating in
displacement control
mode
If the confining pressure system is being operated in the displacement control
mode, while the triaxial cell is being heated, pressure inside the triaxial cell will
increase rapidly as thermal expansion occurs, because, except for various
monitoring devices such as limit detectors, the controller is insensitive to
pressure changes.
(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, which follows.)