As difficult is it may seem on the surface, converting PPM to % Vol. is actually very simple! Think of 1 ppm as 1/1,000,000. One divided by one million = 0.000001 or 0.0001%. Dividing 1 by 10,000 and then interpreting the results as a percentage will net you the same result.
Using that same logic in a real world example, a customer recently requested a mixture of 6000 PPM propane in air and like most customers, he required a high degree of accuracy to safisfy safety requirements within his plant.