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O. A. Uyehara and K. M. Wastson, *Nat. Petroleum News, Tech. Section*, 36, 764(Oct. 4, 1944); revised

The above graph represents reduced viscosity as a function of reduced temperature for several values of the reduced pressure.


I am writing a code which will estimate the viscosity, in the following steps :

  1. Calculating critical viscosity($\mu_c$) by using the formula,

$$ \mu_c = 7.70M^{0.5}p_c^{2/3}T_c^{-1/6} $$

  1. Calculating reduced temperature and pressure as,

$$ T_r = \frac{T}{T_c} \\ p_r = \frac{p}{p_c} $$

Now, from the graph at the top, estimating $\mu_r$ by using $T_r$ and $p_r$ values calculated from the previous step.

  1. Finally, calculating the predicted value of $\mu$ as,

$$ \mu = \mu_r\mu_c $$

(this value of $\mu$ is unusually a good agreement with the measured value)


Question: How do I feed/extract the data/equation of the plot (top) which is experimentally generated so that I can also plot/test it in my code?

P.S - All other parameters $T_c$ , $p_c$ , $T$ , $p$, $M$ will be input by the user


REFERED : O. A. Uyehara and K. M. Wastson, Nat. Petroleum News, Tech. Section, 36, 764(Oct. 4, 1944); revised | Transport Phenomena, 2nd edition, R. Byron Bird, Warren E. Stewart, Edwin N. Lightfoot

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You can use Plot Digitizer and extract the data points in your image graph as a xml file and then you can parse it by using Python. It's pretty straightforward. You need to import the image of your graph into the software. Then calibrate the X and Y axes by specifying the $x_{min}$, $x_{max}$, $y_{min}$, and $y_{max}$ which are the min and max of X and Y axes. Note that if X or Y axis is in logarithmic scale, you can specify it in the software during calibration. Finally you just manually touch the points in your graph and software would save the X and Y of points for you and you can easily import it as xml file and you can parse this xml file in Python or even in Microsoft Excel or LibreOffice.

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