APPENDIX B: OPTIMUM PARAMETERS FOR 
LINEARIZING THERMISTOR TEMPERATURE 
BRIDGES 
INTRODUCTION 
One of the simplest ways of utilizing a thermistor for the 
measurement of the temperature is to measure the voltage drop 
across it due to the passage of current through it.* If the current 
is constant, the voltage drop is nonlinear as can be seen from the 
empirical expression R =@ exp (0/T ) for thermistor resistance, 
where a and 5 are constants, and J’ is absolute temperature. ** 
However, by using a constant potential source to drive current 
through a resistor of proper value in series with the thermistor, 
a certain degree of linearity in the variation of the voltage drop 
across the thermistor with change in temperature is achieved. 
The purpose of this appendix is to show how to choose the optimum 
value of series resistance, and the resultant degree of linearity 
which can be achieved with typical thermistors. 
The following is a short review of some of the work that has 
been done in this field. 
Burke, during temperature studies at NEL worked with 
Wheatstone bridges and found an expression to determine the 
optimum value of resistance, ” (fig. Bl). The condition for 
linearity was such that two equal increments of voltage output 
corresponded to two equal temperature increments. This answer 
was found in terms of the thermistor resistance at three discrete 
equally spaced temperatures (7g, FR p» and #,***; the thermistor 
resistance at the low, middle, and high end respectively of the 
desired temperature range) (fig. B2) and is shown below: 
* The maximum current through the thermistor should be small 
enough so as to produce negligible elevation in temperature 
relative to that of the fluid in which it is immersed. 
** This is equation (A1) of Appendix A. 
***Symbols have been changed from some of the original papers 
quoted, for consistency. 
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