Automatic Measurement and Data Collection

A constant-current power supply with potential-limiting circuits is used for supplying current to the calorimeter heater which is switched into 

The total time of a measurement cycle (heating plus equilibration periods) for an organic compound varies from about four minutes to several hours. For example, at temperatures below 50 K, heat capacities are small and the sample usually equilibrates rapidly, so that a heating period of 120 s followed by a similar equilibration period is typical. In contrast, equilibration may require many hours for crystals in the region of a phase transition. 

The equipment described is capable of unattended operation for periods of many hours. At temperatures above 50 K, only occasional manual adjustments to the shield controls and to the level of the power input to the sample are necessary, but at lower temperatures frequent adjustments of the controls are usually required. 

The molar heat capacities of the compound are smoothed graphically or by computer. For computer smoothing Chebyshev polynomials in temperature, usually up to order 15 or 20, are fitted to the experimental values, and the order which best represents the data is selected as follows  

By use of these criteria, an order of the polynomial can usually be selected to represent the experimental data for the compound in a particular condensed state. If it is necessary to divide the data for one state into more than one temperature range, the fitting is calculated to make the heat capacity and its first differential the same at the common point. 

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Automatic Measurement and Data Collection.—X description of an automatic data acquisition system for calorimetry has recently been published. The instrumentation for low-temperature calorimetry now used in the Division of Chemical Standards, N.P.L., is illustrated by Figure 2. It is used in conjunction with cryostats and sample containers similar to published designs (Figures 2 and 9, respectively, of ref. 3). 

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