I am pretty familiar with high vacuum equipment and techniques but I don't understand the units on my HVAC pressure gauge.

I have a relatively expensive pressure gauge used in labs. I am sure it works and is properly calibrated. I have a pressure in the system of 10^-2 mbar. I know you guys are not very familiar with the units so I do a little conversion.

- 1 bar = 7.5 * 10^5 micron Hg or mtorr

Therefore I have a vacuum of 7.5 micron Hg or mtorr. I trust that number.

Now the confussing part my HVAC pressure gauge shows in green that my pressure is sitting at 25-26 in of Hg. I can get a better pump and get it down more but I do not understand that meter/units!

If I do the conversion into in of Hg...

1 mtorr = 3.9 * 10^-5 in Hg therefore I should have about 30 mtorr or microns...

WHY does my meter show 25-25 in of Hg they must be different units... ! Can anybody explain?

Should I now take one of my vacuum pump stands which gets my equipement down orders of magnitude lower (10^-6 mbar) and try it but I don`t really want to risk contaminating my vacuum pump. What is the vacuum pressure of the compressor oil?