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


2 Answers 2


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.

  • $\begingroup$ Apologies for commenting after an eternity! Thank you that worked smoothly $\endgroup$
    – kedarb
    Oct 29, 2020 at 5:56

Dear you can return to the reference "Courtsey of Hougen and Watson " "Chem. Process Principle" for more accurate calculation of reduced volume and reduced pressure at reduced temp. and the the figure content all required data. Best Regards Dr. Kafaa Alani


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.