On Fri, 3 Dec 1999, Tom Handley wrote: > Imre, a common method to characterize a thermistor is to use the > Steinhart-Hart equation: > > 1/T = A + [B * ln(R)] + [C * ln(R)^3] > > Where: > > T = Degrees Kelvin > R = Resistance > A,B,C = Curve-fitting constants. --- snip --- > > BTW, my best to your wife. She sounds like a very patient lady ;-) Yes, she is. She is learning actually electrical engineering ;-) I gave her this small lesson as a practice. Here is the report of the analysis of the measured data. I have chosen this formula because of it can be implemented relatively easy to the PIC, as no transcendental functions are used. -------------------------------Polynomial Ratios------------------------------- Date/Time 12-06-1999 16:54:13 Data Base Name C:\erika\thermist Description Data base created at 11:06:08 on 09-18-1998 Estimation Summary Report Model: Y = (A+BX1+CX2)/(1) Y:1/TEMP X:RESIST Itn's: 0 Term Coefficient Estimate Std. Error T-Value Prob(|t|>T) R-Squared A 9.233908932517655D-03 4.651098E-04 19.9 0.0000 0.99592817 B 6.195481889641157D-04 4.85661E-05 12.8 0.0000 C 2.351109127751624D-06 8.53004E-07 2.8 0.0135 Source df Sum-Sqr Mean Square SQR(M.S.) F-Ratio Prob(f>F) Model 2 2.755776E-03 1.377888E-03 3.711991E-02 2079.0 0.0000 Error 17 1.126692E-05 6.627597E-07 8.141006E-04 Total 19 2.767043E-03 1.456338E-04 1.206788E-02 -------------------------------Polynomial Ratios------------------------------- Date/Time 12-06-1999 16:54:13 Data Base Name C:\erika\thermist Description Data base created at 11:06:08 on 09-18-1998 Residual Analysis Actual Actual Predicted Lower95% Upper95% Row X Y Y Value Value Residual 1 52.3 20.3 20.80419 19.96934 21.71189 -.5041882 2 43 25 24.86221 23.75899 26.07289 .1377875 3 34.6 30 29.86418 28.28189 31.63402 .1358194 4 28 35 35.18089 33.00754 37.66061 -.1808835 5 22.9 40 40.56054 37.73282 43.8464 -.5605342 6 18.8 45 46.05665 42.50026 50.2626 -1.056651 7 15 50 52.47655 47.9753 57.90992 -2.476548 8 13.1 55 56.32703 51.20144 62.59299 -1.327036 9 10.8 60 61.73121 55.64604 69.31068 -1.731204 10 8.8 65 67.25853 60.08178 76.38235 -2.258529 11 7.7 70 70.70223 62.78568 80.90321 -.702221 12 5.5 75 78.66245 68.85303 91.73126 -3.662455 13 4.6 80 82.41591 71.62411 97.03669 -2.415906 14 4.25 85 83.9669 72.75224 99.26908 1.033106 15 3.9 90 85.57316 73.91027 101.6065 4.426841 16 5.5 80 78.66245 68.85303 91.73126 1.337545 17 6.9 75 73.41752 64.88421 84.53521 1.582484 18 10.94 65 61.37547 55.35664 68.86279 3.624534 19 17 55 48.91004 44.94741 53.63892 6.089957 20 55 20.2 19.83294 19.03479 20.70095 .3670586 Regards, Imre