Temperature Detection and Recording Systems

The thermocouple is in close proximity to the sample in (b) and is positioned inside, but not in contact with, the sample holder. This arrangement is better than (c) because the thermocouple will respond to small changes of sample temperature. The best method of sample temperature detection is to have the thermocouple either in contact with the sample or with the sample container, as shown in (c). In the latter, the temperature detected will be the integrated temperature. However, the main problem is that with sensitive recording balances the thermocouple leads can cause weighing errors, or at least interference with the balance mechanism. One way to detect the actual sample temperature and yet not interfere with the balance mechanism is to suspend an electronic device near the sample holder which will transmit the sample temperature to a fixed receiver located near the sample container. Manche and Carroll (13) described a unijunction transistor relaxation oscillator which used a thermistor as the temperature detector. The frequency of oscillation, which is a function of sample temperature, was transmitted via a mutual inductance between two suspended coils to a receiver and counter. The device was limited, however, to a maximum temperature of about 150°C. 

The calibration of the temperature of the furnace and/or sample chamber has been discussed by Stewart (12) and Norem et al. (14, 15). Stewart (12) used a conventional thermobalance which contained a thermocouple mounted external to the sample, while Norem et al. (14, 15) calibrated a furnace which used a resistance element for lemperature detection. 

Stewart (12) discussed three approaches to temperature calibration  

The use of standard materials with reproducible mass-loss points that could be referred to the temperature. 

The use of materials having known reproducible (and reversible) temperature transitions and direct measurement of temperature. 

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