I have vacuum pump connected to quartz tube and vacuum sensor, when the pump reaches it max value the sensor reading (-1 bar) and the analog gauge in the pump reach the max value which is (-1000mbar). what is meant by negative pressure? I know it is below ATM but I need to translate it to positive value, example my experiment I want to reach for example 0.05 mbar does the señor and pump help me or not? can any one explain that to me please?
what is meant by negative pressure?
A pressure that is below zero gauge pressure, i.e, below 1 atm.
To translate it into a positive value, you need to use units of absolute pressure. In general those are Pascals (Pa) or Torr. 1 atm equals an absolute pressure of about +101.325 kPa or +760 Torrs
So 0.05 mbar equals an absolute pressure of about + 5Pa or +0.038 Torr. A gauge pressure of -1 bar is an absolute pressure of 0 Pa, or a theoretical perfect vacuum, which doesn’t exist, even in outer space.
The following link describes various pressure gauges used for measuring absolute (positive) near vacuum conditions that you might find interesting.
Hope this helps.
This answer is a small addendum about a part that has been left out in the answers so far.
While not the case for you (which is about gauge pressure), there can be metastable states with actual negative pressure for materials with internal cohesion forces that adhere to the walls of a container.
Negative pressure can never be a stable thermodynamic equilibrium state (as the system can always reach a lower free energy by separating from the walls, and leaving part of the container empty or, for the case of a liquid, filled with vapour). Then the pressure will be positive and the total system energy less. However, there can be a kinetic barrier to that state, so that the transition can't be easily reached with local fluctuations.
One real world example of this is water transport in trees. Capillary action could not transport water high enough for trees.