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Program to create a nomogram for gelcoat/peroxide ratio.
Python Makefile
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Making a nomogram with Python and Postscript

Date: 2018-11-18
tags:python, postscript, nomogram
Author: Roland Smith


At work I needed a suitable way to check the callibration of gelcoat spray equipment. Gelcoat requires an initiator (often called “catalyst”) in the form of a peroxide to cure. The peroxide/gelcoat ratio is important, so it is checked regularly by spraying the separate components into a suitable container and weighing them.

For those familiar with gelcoat spraying, this is not a system with coupled gelcoat and peroxide pumps. But rather an external mixing spray gun where the peroxide is simply fed from a pressurized container to the spray gun.

Since we're handling resins, solvents and peroxide, protective equipment including gloves is a must. That makes it cumbersome to whip out a smartphone to use it as a calculator to check the ratio. Since you don't want to get gelcoat or peroxide on your expensive phone, you have to take off your gloves to handle it. This might have to be repeated several times.

So I decided to make a diagram where one could relatively easy read off the peroxide percentage given the quantities of both compontents. This can be printed and laminated between plastic to make it resistant against stains.


Such a diagram (basically a graphical analog computation device) is called a nomogram. The linked wikipedia article gives and excellent overview.

Python and PostScript

Although PostScript is good at creating graphics, I find it cumbersome for calculations. So in this case, I'm using Python to do most of the calculations and have it generate PostScript commands for drawing.


The supplied Makefile specifies how the Python code is used to create the graphs.

It uses my eps2png script to create PNG images. The link takes you to my github repository for scripts.




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