Digital control of a differential scanning microcalorimeter

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Abstract
A detailed description of the digital control for a differential scanning microcalorimeter to be used in the study of biomolecules is given. Three control loops and an adequate control program ensure that the instrument works under adiabatic conditions and the temperature difference between the sample and reference cells is as close as possible to zero. A static and dynamic characterization of the instrument is also described. The gain of the control loop can be selected by the operator, thus changing the calibration constant, signal/noise ratio (SNR) and time constant of the instrument according to experimental requirements. Some characteristics of the instrument are: (a) any scan rate between 0.1 and 2 K min-1, with deviations from linearity and reproducibility about 0.2-0.3%, can be selected; (b) its calibration constants at each scan rate are highly reproducible (about 0.4%); (c) the reproducibility of the instrument upon refilling of the cells is about 10 mu J K-1; (d) the noise in the sensor is about 2 nV; and (e) the time constant of the instrument changes from 7 to 70 s depending on the control loop gain used.