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System Model

Previous: Introduction.

The system considered in this document comprises an electrical heater of heat capacity Ch connected via a thermal resistance Rho to the oven, heat capacity Co. The oven loses heat to the environment, at temperature Te, through the thermal resistance Ro of its insulation. The temperature controller adjusts the power dissipated in the heating elements, W, by comparing the oven temperature, To, with the set-point temperature Ts.


Figure 1. Equivalent circuit representation of system model.

The red symbols on the diagram are thermal components; the black ones are electrical devices. Dashed lines represent transducers: a thermometer in one case, conversion of electrical current flowing through the heater into heat (thermal current W) in the other. With the notable exception of cryogenic systems, the thermometer time constant is usually very small so its effects will be tacitly assumed to be negligible during much of the following discussion. However, it will be mentioned in the final section which discusses the frequency domain behaviour of the system.


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