Adiabatic Measurements

The viscoelastic functions as defined refer to isothermal changes of state, and indeed the control of constant temperature is an important feature of all the experimental methods described in this chapter. It is evident, however, that the dynamic measurements described in some of the preceding sections must in fact be adiabatic rather than isothermal, because of failure to reach thermal equilibrium within the period of deformation. This distinction has usually been ignored, and reasonably so since the difference between the adiabatic and isothermal quantities is in most cases negligible. For the sake of completeness, however, some general features of the problem are mentioned here. 

---

An important feature of the adiabatic measuring technique is the determination of the adiabatic induction time, Ti. The influence of the temperature on the adiabatic induction time is illustrated in Figure 2.23. 

However, if the thermokinetic evaluation procedure is applied, which was explained in Section 4.3.3.3, very reliable kinetic models can be obtained from adiabatic measurements. The parameters thus determined can reliably be used for the prediction of the reactor behaviour in other maloperation scenarios. In a first approximation, other adiabatic induction times may be estimated with the help of the following equation... 

As an independent experiment to verify the findings of the TDBS we also performed quasi adiabatic measurements of the specific heat capacity Cp (74) using modulated differential scanning calorimetry (N SC) (20-22). Usually the glass transition is characterized by a step like behavior of Cp(T). The thermal glass transition... 

Adiabatic measurements and kinetic parameters. The experimental temperature measured in a semi adiabatic box and the adiabatic temperature rise, obtained from a paste with w/c=0,45, is shown in the Fig. 1. Once the adiabatic temperature is known the global heat transfer coefficient (U) can be determinate with a good correlation coefficient (Fig. 2). 

The heat capacity of solids and melts can be determined by adiabatic calorimetry and various methods of DSC and MTDSC, where with modem methods, the latter may reach a precision equal or better than the adiabatic measurement, so that curves like in Figures 4.1 and 4.2 are available for an... 

The measurement can be done both in isothermal or adiabatic calorimeters, the latter being preferred. For isothermal measurement (see ASTM D3286), the temperature of the calorimeter jacket is held constant and a correction for heat transfer from the calorimeter is applied, while in the adiabatic measurement (see ISO 1928 and ASTM D2015), the temperature of the calorimeter jacket is continuously adjusted to approximate that of the calorimeter itself. 

In the realm of classical experiments, the adiabatic and the isothermal elastic moduli of solids are related through the specific heats of the materials. It would be helpful to have reliable isothermal measurements of the moduli of metals at low temperatures to compare with adiabatic measurements which have been made [5] and thus verify this relationship. A convenient tensile cryostat would be helpful here, also. 

The thermocouple method involves adiabatic measurement of the heat evolved from the start of a photo-chemically initiated autoxidation. Radical lifetimes are derived from the non-stationary state region before the onset of the steady state rate of reaction. 

A variety of experimental devices similar to those described in this chapter have been devised for study of metals and other nonpolymeric solids. - In principle, the distinction between isothermal and adiabatic measurements should again be drawn, but in practice the difference between the two will be small. 

---