TY - JOUR
AU - Fleming, D. M.
AB - The Heat Amplifier—A New Technic in Calorimetry 1 I. T. Myers , Ph.D. and D. M. Fleming , B.A. Excerpt Semiconductor resistance thermometers, or thermistors, have been used in calorimetry for many years. The simplicity of their resistance readout technics and their high sensitivity have made this method of measuring temperature attractive in a great many applications, not the least of which is calorimetry. Like other semiconductors, thermistors have a negative temperature coefficient. If the voltage applied to a thermistor is constant, the power dissipated in the thermistor will increase as the temperature increases. For example, if a source of power P is added to a calorimeter containing a thermistor connected to a constant voltage, the temperature rise will be greater than if the thermistor were not there. This heat amplification is given by the expression The change in thermistor power is simply ( E 2 / R 0 ) — ( E 2 / R ) where R is the thermistor resistance with the power P applied to the calorimeter, and R 0 is the thermistor resistance when P = 0. Normally, this picture is too simple, since there is always some lead resistance in series with the thermistor. In fact, it may be desirable to add series resistance to prevent thermistor burnout if the supply voltage is inadvertently turned too high. For this case: where R 8 is the resistance in series with the thermistor. Heat amplification can also be accomplished with positive temperature coefficient resistors, if a constant current source is used instead of a constant voltage source. The derivation of the amplification factor is similar to that given above. A heat amplifier was set up in the form of an isothermal calorimeter. The submarine jacket was made of brass and the insulation was Styrofoam. The calorimeter itself consisted of ten washer thermistors stacked together and held in a polystyrene holder. The individual thermistors had a resistance of about 30 ohms at 25° C. and a temperature coefficient of 3.8 per cent per degree centigrade. Amplification was measured as a function of the voltage applied to the thermistor stack. The amplification increased with increasing voltage up to 6.5 at 10 volts. Above this voltage, measurements were hampered by instabilities in the power supply. Present plans call for the investigation of what effect changing the series resistance will have on both the amplification factor and the stability. It is also hoped that the system stability can be improved by using a highly regulated power supply.
TI - The Heat Amplifier—A New Technic in Calorimetry
JF - Radiology
DO - 10.1148/78.1.111b
DA - 1962-01-01
UR - https://www.deepdyve.com/lp/radiological-society-of-north-america-inc/the-heat-amplifier-a-new-technic-in-calorimetry-jdNuzG4Zt0
SP - 111
VL - 78
IS - 1
DP - DeepDyve
ER -