Semi-adiabatic calorimeter

For an ideal adiabatic case, the radiation-induced temperature rise of the absorber of the calorimeter is a linear function of time during irradiation at constant dose rate. The temperature rise of a semi-adiabatic calorimeter during irradiation as a function of time is shown in O Fig. 49.5. 

A common design of semi-adiabatic calorimeters contains thin or thick disc-shape absorbers used mainly in monodirectional beams both for low-energy (McDonald et al. 1972) and for higher-energy electron beams (Bewley 1969). Water calorimeters of the same shape were designed by Brynjolfsson et al. (1963) and Fielden and Holm (1970). 

Semi-adiabatic calorimeters have been designed for dosimetry at high-energy electron accelerators (1-10 MeV) both for calibration and for routine process control (Humphreys and McLaughlin 1989 Miller and Kovacs 1985 Burns and Morris 1988) and also for low energies between 100 keV and 500 keV (Janovsky and Miller 1987). The disc-shape absorber is either water or graphite containing thermistors for temperature measurement placed in the center of the absorber. The absorber is placed in polystyrene foam insulation. 

Isothermal calorimeters measure thermal power (heat production rate), while (semi)adiabatic calorimeters measure temperature (change). It is possible to calculate one of these from the other, but to do so we need to take the derivative (to go from semiadiabatic to isothermal) or integrate (to go from isothermal to semiadiabatic) and in both cases we need the heat capacity of the sample. The thermal power signal from an isothermal calorimeter shows more details than the temperature signal from an adiabatic calorimeter as the former directly assesses the rate of the process, while an adiabatic calorimeter measures the integral of the rate. Isothermal calorimetry is thus a more generally useful analytical method than semiadiabatic calorimetry. 

There are a number of different types of adiabatic calorimeters. Dewar calorimetry is one of the simplest calorimetric techniques. Although simple, it produces accurate data on the rate and quantity of heat evolved in an essentially adiabatic process. Dewar calorimeters use a vacuum-jacketed vessel. The apparatus is readily adaptable to simulate plant configurations. They are useful for investigating isothermal semi-batch and batch reactions, and they can be used to study ... 

Any chemical reaction is accompanied by an energy conversion, in the most cases heat production, and normally this heat is proportional to the amount of substance converted. It can therefore be a measure of its amount. In an insulated adiabatic system of defined heat capacity (calorimeter), the heat produced leads to a proportional temperature rise, and even in open semi-adiabatic systems proportional temperature changes are observed, however, these systems must be calibrated for substance determinations. Very sensitive devices for the measurement of temperature changes are thermistors, which are semiconductor resistances with high temperature coefficients, eg, 3-4% °C . ... 

Adiabatic calorimeters include here, all at once, the Ideal adiabatic and the Semi-adiabatic The same general meaning for the term adiabatic was kept to define the adiabatic family in sections 4.2. and 4.3. 

Figure 2.4 An example of how (a) temperature, (b) thermal power and (c) heat develop over time in an Isothermal, a semiadiabatic and an adiabatic calorimeter. Note that paste or mortar is generally used in isothermal calorimetry, while mortar or concrete is used in (semi)adiabatic calorimetry.

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