Tom Handley says
a common method to characterize a thermistor is to use the SteinhartHart equation:1/T = A + [B * ln(R)] + [C * ln(R)^3]Where:
T = Degrees Kelvin R = Resistance A,B,C = Curvefitting constants.You typically measure three data points; min(A), middle(B), and max(C). Then solve the three simultaneous equations for A, B, and C. The accuracy is related to the span. So a span of 100 degrees is more accurate than a span of 500 degrees.
In one case, I used this to characterize a common cooking temperature probe (Polder) for characterizing the thermal response of my oven between 100 to 300 deg F. I ended up writing a MathCAD sheet to simplify things. Once I had the constants, I fed them to an Excel 97 sheet to analyze and graph the data. From there you can generate a surprisingly accurate curve (again, depending on span). Then you can either implement the equation in FP or scaled fixpoint math or use a lookup table.
Scott Dattalo says:
With the SH approach {calculated "on the fly" in the processor rather than precalculated into a table} you have to compute these really nasty logarithms and arithmetic operations. All of that just to get 1 or 2% accuracy. However with a lookup table it only takes about 20 CPU cycles to get an answer that's accurate to with in a 0.5%. However, I found that the look up table can only give you this accuracy after the thermistor has been carefully calibrated. In other words, with the SH equation I found my compuatations suffered from cumulative round off errors but the lookup table is accurate as the data in the table.
See also:
file: /Techref/io/sensor/thermist/charsheq.htm, 2KB, , updated: 2002/3/13 11:33, local time: 2018/9/23 06:30,

©2018 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions? <A HREF="http://www.piclist.com/techref/io/sensor/thermist/charsheq.htm"> Characterizing a thermistor with the SteinhartHart equation</A> 
Did you find what you needed? 
PICList 2018 contributors:
o List host: MIT, Site host massmind.org, Top posters @20180923 RussellMc, Van Horn, David, Sean Breheny, David C Brown, Bob Blick, Isaac M. Bavaresco, Neil, Denny Esterline, Harold Hallikainen, John Gardner, * Page Editors: James Newton, David Cary, and YOU! * Roman Black of Black Robotics donates from sales of Linistep stepper controller kits. * Ashley Roll of Digital Nemesis donates from sales of RCL1 RS232 to TTL converters. * Monthly Subscribers: Gregg Rew. ongoing support is MOST appreciated! * Contributors: Richard Seriani, Sr. 
Welcome to www.piclist.com! 
.