Feedback controlled temperature filament

Modern equipment uses a feedback controlled temperature filament. The electrical circuit of such a system is given in Figure 4.3.1. 

Figure 4,3.1. Circuit diagram of a pyroiyser with boosted heated filament and with feedback controlled temperature.

The true temperature of a sample heated using a filament pyrolyzer can be quite different from the above profile temperature, significantly lower temperatures being recorded inside the samples [4]. In order to obtain a correct Teq, modern equipment uses a feedback controlled temperature system (see e g. [5] for a more detailed description of this type of pyrolyzer). Several other procedures for a precise temperature control of the filament are available, such as the use of optical pyrometry or thermocouples [6, 7], Special pyrolysis systems that allow programmed heated rates at different time intervals also are available [8]. 

It is essential in mechanistic work to exert precise control over the pyrolysis temperature. However, quoted temperatures may be average values over the whole filament. A pyrolysis unit that uses a refined feedback system was developed at the University of Birmingham to overcome this problem (Figure 4). The filament temperature is measured near its central point by means of a very fine thermocouple spot-welded to the rear of the filament. The thermocouple output is compared to that of a reference voltage preset to correspond to the required T q. A large current ( 40A) drives a temperature increase and, as the two voltages converge, the filament current is reduced rapidly to a maintenance level. TRTs of 50 ms to 800 0.5°C were obtained with 15 ms to... 

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