THEORY OF POWER-COMPENSATED DSC

Theoretical analysis of two systems of power-compensated DSC is presented; one is a system with proportional temperature control of the sample holder and the other is with PID (proportional, integral and differential) temperature control of the sample holder. Power-compensated DSC is designed to measure directly and exactly the enthalpy change of the sample holder as a function of time. However, the measured output signal in both systems is not exactly equal to the enthalpy change of the sample holder. When there is an input signal, the enthalpy change of the sample holder is a step function of time, and the output signals in both systems are examined. The output signal in the system with proportional temperature control of the sample holder gradually approaches the input signal as time increases but does not converge uniformly to the input signal. The output signal in the system with PID temperature control of the sample holder shows time delay and an overshoot or damped vibration depending on the values of control parameters. A proportionality relation between total enthalpy change of the sample holder and peak area of the output signal is valid in both systems.