Thermal discharge

Despite the immensity and complexity of known and suspected roles of temperature in aquatic ecosystems, certain thermal criteria have been especially useful in minimizing risks from thermal discharges. More data have been organized at the physiological level than at higher levels of organization. 

In the methods reported above, the temperature change AT used to measure the heat capacity C(T) was supposed to be so small that the time constant r = R C could considered constant in the AT interval. Let us consider, for example, the thermal discharge of a system with heat capacity C(T) a T and thermal conductance to the bath G(T) a T3 (e.g. a metal sample and a contact resistance to the bath at rB). A AT/TB = 10% gives a At/t = 20% over the interval AT, that is a time constant definitely not constant. 

We repeated the same procedure at different values of TB, between 0.06 K and 0.28 K, in order to obtain a set of thermal discharges. 

To measure G(T), an electrical power PR was supplied to the heater (H), keeping the frame temperature constant. G(T) was obtained as the derivative of the PH(T) curve. The heat capacity of the sample was evaluated simply as C = r x G, where r was obtained by the fit of the thermal discharges around a set of fixed temperatures. A single time constant was always found. An example of thermal discharge at T 160mK is shown in Fig. 12.11. 

In fact, since the relaxation constant is t = C/G and C is linear in the higher temperature range, while G a T2-5, r scales as T h5. Hence, at higher temperatures, the number of useful data points, obtained in a single thermal discharge measurement, vanishes and the error in the determination of r increases. For example, at the highest temperatures, t was as low as 2 3 seconds. 

Drost-Hansen, W. (1973a). Aspects of the relationship between temperature and aquatic chemistry (with special reference to biological problems). U.S. Senate Committee on effects and methods of control of thermal discharges. Part II, Serial 93-14, November 1973, pp. 847-1140. Available from U.S. Government Printing Office, Washington, DC. 

Figure 1. Block layout of electret thermal analysis instrument. The sample is polarized at high temperature, cooled, and the thermal discharge current monitored at a programmed rate of heating.

Thermal discharge currents exhibit behavior parallel to low-frequency dielectric loss as a function of temperature. This is shown clearly in Figure 7 for the case of the 2 mol % zinc chloride content PPO. Both i and c" show three relaxation regions, with the temperature locations normally displaced to lower values in the case of the current curve because of the dc nature of this measurement. However, this generalization is not always true as can be seen from a comparison of Tables II and III. At low salt concentrations the lowest temperature relaxation... 

Figure 7. Comparison of thermal discharge currents (-) with e" for PPO containing 2% ZnCl2 ...

Figure 8. Thermal discharge currents for a series of ZnCl2-containing PPO samples. Comparison of relaxation locations are given in Tables II and III. Field strength 2 kVj cm Tp = 40°C heating rate 5°C/min mol % ZnCl2 (— —) 0 ( ) 2.2 (---------------) 4.4 (----) 18.0 (— — ) 27.0.

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