Found it. The curve is exponential not J and the following worked.
fit <- nls(pos ~ a*tmin^b,
data = d,
start = list(a = .1, b = .1),
trace = TRUE)
Thanks due to Jorge I Velez at R Help Oct 26, 2009
Also I used "An Appendix to An R Companion to Applied Regression, second edition" by John Fox & Sanford Weisberg last revision 13: Dec 2010.
Final working settings for me were:
fit <- nls(y ~ a*log(10)^(x*b),pos_curve2,list(a = .01, b = .01), trace=TRUE)
I figured out what the formula should be by using open office spread sheet and testing the various curve fit options until I was able to show exponential was the best fit. I then got the structure of the equation from that. I used the Fox & Sanford article to understand the way to set the parameters.
Maybe I am not alone in this one but I really found it hard to figure out the parameters and there were few references or questions on it that helped me.