Equation to exponential eq using curve fit tool.

Hello all,

I am new to matlab and trying to find the equation of a exponetial from the curve fit tool which looks at the cooling of an object my code looks like where T is 658

636

583

512

490

449

418

365

359

320

306

266

248

228

218

201

186

178

163

149

139

138

125

118

112

102

96

90

87

81

72

and t is 0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

290

300

but the equation dosnt produce the correct values

clc

clear all

load('Project_1_data.mat');

T

t

a=205.9;

x=20;

b=-0.7099;

Teq=a*exp(b*x)

Answer 1

[Star Strider]

It will produce the correct values if it is allowed to. The ‘b’ value is 100 times greater than it should be, according to the fitted parameters here —

T = [  658
       636
       583
       512
       490
       449
       418
       365
       359
       320
       306
       266
       248
       228
       218
       201
       186
       178
       163
       149
       139
       138
       125
       118
       112
       102
        96
        90
        87
        81
        72];

t = [0
    10
    20
    30
    40
    50
    60
    70
    80
    90
   100
   110
   120
   130
   140
   150
   160
   170
   180
   190
   200
   210
   220
   230
   240
   250
   260
   270
   280
   290
   300];

ft = fittype('a*exp(b*x)', 'Dependent','T', 'Independent','x', 'Coefficients',{'a','b'})

ft = 
     General model:
     ft(a,b,x) = a*exp(b*x)

fft = fit(t,T,ft, 'StartPoint',[T(1),-0.01])

fft = 
     General model:
     fft(x) = a*exp(b*x)
     Coefficients (with 95% confidence bounds):
       a =         664  (653.1, 675)
       b =   -0.007808  (-0.008011, -0.007604)

figure
plot(fft, t, T)
grid
xlabel('t')
ylabel('T')

Experiment to get different results.

.