A Vacuum Gauge is a pressure measuring instrument that measures pressure in a vacuum (i.e., in a vessel operating at sub-atmospheric pressure).A positive gauge pressure range would not be suitable for measuring a vacuum, since it will only measure pressures that are greater than atmospheric pressure. Therefore only gauge pressure ranges that include a negative pressure range will be suitable for measuring a vacuum pressure.
Vacuum gauge is measured from ambient air pressure in the negative direction. So for example at ambient air pressure the vacuum reading is 0 bar gauge and if a suction pressure of 0.25 bar is applied, the vacuum reading will be -0.25 bar gauge.