First, any scale CAN be used to measure vacuum (negative pressure) as well as positive pressure. The trick is knowing which is best suited for which and the size of the scale. Larger units of measure are better suited for higher pressure and greater differentials, smaller units of measure are better suited for lower pressures or smaller more critical differentials.
A Micron of Mercury (or micron) is a very small/fine unit of measure related to the displacement of a mercury column by atmospheric pressure thus the distance part. In fact, a micron is one-millionth of a meter of mercury displacement. That’s a tiny amount of pressure.
Inches of Mercury is a more rough measure of pressure, usually vacuum or even barometric pressure or altitude. Inches of Mercury is represented by the abbreviation HG
1 HG is equal to .491 PSI or roughly 1/2 of a PSI.
The force of the atmosphere around us is equal to 29.92 inches of mercury or hg or 14.7 PSIA Therefore a perfect vacuum can be thought of as 0 hg although a “perfect” vacuum can never be achieved.
When we read pressure as a tech with a gauge we read it in PSIG which means it is already set to zero at 14.7 PSIA and 29.92 hg.
So in the case of the suction/compound / blue gauge when it goes into a vacuum it reads in the “negative” hg scale down to -29.92 because it is PSIG, not PSIA.
1 inch of mercury (HG) is equal to 25,400 microns (of mercury)
In the micron vacuum scale, we start at 760,000 microns at sea level atmospheric pressure and work down towards a perfect vacuum of 0 microns or 0 hg. This is why a lower # in the micron vacuum scale equals a better / deeper vacuum, a higher number equals a worse / less deep vacuum.
This shows why pulling a deep vacuum is done in microns, it is a very fine measurement that provides very detailed results. This is why very small changes can make such a huge difference in the micron reading on a micron gauge.
It also shows why micron gauges can seem finicky. They are really precise instruments.