23-04-13, 01:22 PM
(23-04-13, 12:19 PM)Ebme Geek link Wrote: These are not mercury, they have stainless rods in them and actually work by drawing a small bit of air over them so by rights, they are actually a flow meter not a vacuum gauge.A small amount of air does find its way past the rods, but it's not the flow that is being measured... the pressure difference at the two ends of the rod acts against the spring connected at the bottom end. So it is a vacuum gauge, just a slightly leaky one.
If you can get in there it may be possible to match the sensitivity of the four columns by tweaking the springs, but it's not something I've investigated with mine as they were OK.
You can get false readings if a rod jams, that's why they emphasise the need to make sure they're all jiggling slightly as you make the measurement.