Inclined Manometer gauge takes Red Oil

We have one of these on the wall in the back. It measures pressure in inches of WC but it only goes up to 3" of WC maximum. They put it on an incline so that they can measure finer pressures, fractions (decimals) of an inch.  They also put a lighter fluid (not lighter fluid) in it, an oil that won't evaporate and one that has a specific gravity of .826 which means it is easier to lift than water, which means it can measure a much smaller pressure in the same space.