I'm a programmer. I'm working with a sensor that has given the following graph. I would like to make a function where i put the value of Rs/R0 in. The outcome of this function is the value of ppm of alcohol.
I understand that precision is imposible so, for the function i'm going for a straigt line from point (200 , 2.9 ) and the second point would be (10000 , 0.68).
I tried to google how you create log functions but i got stuck with the problem that i don't know the value of y (in my case rs/r0) when x (in my case ppm) is 1 . All tuts online i found used that. Is there an other way to calculate the function?
In your graph $\frac {R_s}{R_0}$ is a logarithmic function of the concentration in ppm. You want to go the other way, so your function will be exponential. You are fitting a line through two points, which you do by choosing your functional form and evaluating the coefficients. If you choose your form to be $\log_{10} \frac {R_s}{R_0}=a+b\log_{10}(c)$ where $c$ is the concentration in ppm. Each point gives one equation in $a,b$. You have two equations in two unknowns. $$0.46=a+b\cdot 2.30\\ -0.70=a+b\cdot 4\\ -1.16=1.70b\\ b=-1.47\\ a=5.16$$ You can raise $10$ to the power of each side, getting $$\frac {R_s}{R_0}=10^a\cdot c^b=145000c^{-1.47}$$ where I have shown too many significant figures so you can follow through the calculation.