Measuring system, calorimeter proper

In calorimeters, heat loss by radiation, therefore, depends largely on temperature even in the presence of a constant temperature difference between the measuring system and its surroundings. This is in effect a temperature-dependent heat leak. Losses by radiation can only be ruled out when the temperature difference between the system and its surroundings has been ehminated. This can be achieved by enveloping the measuring system with temperature-controlled radiation shields with a temperature equal to that of the system proper ... 

This brief overview of offline measurements can be concluded by considering the measurements of the heat released by chemical reactions, which can be obtained via calorimetric measurements [7, 18]. The most diffused industrial calorimeters are the so-called reaction calorimeters, basically consisting in jacketed vessels in which the reaction takes place and the heat released is measured by monitoring the temperature of the fluid in the jacket. A class of alternative instruments are the scanning calorimeters (differential or adiabatic), in which the analysis is performed by linearly increasing the sample temperature with respect to time, in order to test the reactivity of potentially unstable chemical systems in a proper temperature range by measuring the released heat. 

Examples of systematic error are a calibration error in the instrument, failure to establish properly a zero reading of the instrument scale, improper graduation or ahgmnent of an instrumental scale, uncompensated instrumental drift, leakage of material (e.g., of gas in a pressure or vacuum system) or of electricity (in an electrical circuit), and incomplete fulfillment of necessary experimental conditions (e.g., incomplete reaction in a calorimeter, incomplete dehydration of a sample prior to weighing). An example of another kind is faulty theoretical treatment of the results of the measurements to obtain the desired result, perhaps through a faulty approximation in the phenomenological theory involved. 

Wolf, Bohmhammel, and Wolf (1998) described the constmction of a fully automated adiabatic calorimeter for the temperature range of 15-300 K with a helium refrigerator system. A computer program controls the complete measurement, calculates Cp for each heating step, and displays the respective temperatures of the sample and radiation shield as a function of time and Cp/Tas a function of temperature. The heater current is automatically adjusted to the respective heat capacity to get the proper temperature step. 

It is common that commercial calorimeters have internal, automatic calibration. Although this makes a calorimeter user friendly, it is problematic if the user does not know whether the calibrations are accurate. One way to check whether the instrument is working properly and whether the user is performing the measurement in a correct way is to run a validation procedure, i.e. an experiment with a known outcome (proficiency test). A number of such chemical calibration systems are described by Wadso and Goldberg (2001) however, none is similar to cement hydration measurements. It is therefore of interest to establish reference cements - or other similar systems, for example, based on calcium hemihydrate - that can be used to validate the quality of calorimetric cement measurements in a laboratory. 

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