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again: nonlinear fit...


I've posted some days before and i didn't get an answer.
I think my question was not detailed enough and therefor I'll
try it again.
My experimental data have to be fit to this model:
lamkonv[x_, {v_, Dz_, L_}] := 
1/(2*v^2*x^3)*((Dz*x/Pi)^0.5*(L*Exp[-L^2/(4*Dz*x)] - (L + v*x)
*Exp[-(L - 2*v*x)^2/(4*Dz*x)]) + 0.5*(L^2 + Dz*x)*
(Erf[L/(2*(Dz*x)^0.5)] - Erf[(L - 2*v*x)/(2*(Dz*x)^0.5)]))

where Dz,v,L are the parameters.
(It is a 2-dim. convective-diffusion model)

I've tried an own code, nonlinearFit, NonlinearRegress and other
algorithms (quasiNewton..) ad I couldn't work it out.
1.The fit runs in a physically wrong range or 
2.I get imaginary values for the parameters or
3. I get the "nrlnum" -message or
4. The fit does nothing

Especially the parameter Dz behaves very strange. The value of Dz i
about 10^3 times smaller than the other parameters. I've read in a
posting before, that this may cause problems.

Can anybody help me with that?

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