MTS Model 286-20 Confining Pressure Intensifier - Manual (Page 23)

Confining System Operation Considerations
286.20 Pressure Intensifier Operation 23
Confining System Operation Considerations
The following paragraphs provide some general information about operation of
the CPI.
Closed-Loop Control Modes Used For Pressurizing Confining Fluid
When pressurizing the confining fluid, the pressure intensifier inside the CPI
operates under closed-loop control principles. External electronic controller
equipment receives signals from a pressure transducer in the CPI, and also from a
linear displacement transducer, which indicates current pressure intensifier piston
position relative to a reference position. The amplitudes of the two signals are
proportional to their respective variables.
Pressure control mode
Calibration of the electronic controller equipment is such that the signal from the
pressure transducer relates directly to pressure inside the triaxial cell, in psi or
MPa, depending on how the controller equipment was calibrated. When the
output of the pressure transducer is chosen as a feedback signal for the controller,
pressure becomes the controlled, or independent, variable, and displacement
becomes a dependent, or uncontrolled, variable. This is usually referred to as the
pressure control mode.
The pressure control mode is the preferred control mode.
Displacement control
mode
The signal from the CPI’s linear displacement transducer. When the output of the
displacement transducer is chosen as a feedback signal for the controller,
volumetric displacement becomes the controlled, or independent, variable and
pressure becomes a dependent, or uncontrolled, variable. This is usually referred
to as the displacement control mode.
The displacement control mode is less desirable for use than the pressure control
mode because very small changes in displacement cause very large changes in
pressure. Also, if the displacement control mode were used while the triaxial cell
was being heated, the controller would be insensitive to fluid expansion due to
increasing temperature and very high pressures could result.)
If the triaxial cell is equipped with heaters, it is important to remain aware of the
effects of thermal expansion of the fluid when the Confining Pressure Intensifier
is in operation. This is described under the following heading.