Testing the accuracy of the MityVac cooling system pressure gauge.

It was suggested that the pressure gauge of the MityVac cooling system pressure tester might not be accurate. A valid question for any relatively inexpensive gauge. I was in particular interested in the low pressure end of it, say 3 to 5 psi. As it so happens in my work I sometimes need to measure pressures in the range of 0.5 to 2.0 inches of water column. Water column measurements using a manometer are fairly foolproof as there is no chance of your instrument being out of calibration, it's a simple measurement of inches! Then the conversion to psi is inches H2O divided by 27.708. So I set up a vessel that I could pressurize with the hand pump, lifting the water in a vinyl tubing to be measured with a tape measure. The distance the water is raised in the tubing above the water level in the pressurized vessel is your "inches H2O". As it turns out the MityVac gauge was as close as can be at least in the low range where I tested.
A little side story... Many years ago we tested natural gas or propane piping in commercial buildings with a manometer that used mercury instead of water. The inspector would have us pump up the system until the mercury was somewhere above midscale on our instrument at which point the system was sealed and he would mark the level of the mercury in the manometer with a pencil. Then we wait 15 or 20 minutes or so and recheck the level. If it dropped at all we failed the test. This test eliminated any question about gauge accuracy or mechanical condition. There's no mechanical part to go wrong.