Fearns Calculator Methane Flow circular slide rule case 1960 England

This unusual high quality 5 layer slide rule was produced by Fearns Calculators from England. Made ca. 1960 specifically for Methane flow calculations with purity correction, pressure to atmosphere, density correction and differential Pressure. On the back it has the calculations for Diameter of orifice to the diameter of the pipe to come to the orifice factor (C.Z.E.). On both sides there are also instructions for use. It comes with the original green sleeve that has kept it in great condition so there are minimal signs of use or age. It is fully functioning and the diameter of the slide rule is 14 cm or 5 1/2 inches. Check the attached video on how to work your vintage man tool: slide rule operation instructions!