Different methods to measure pressure
Since Pressure has no direction there are two possibilities to measure pressure.
Absolute Pressure:
Pressure measured against pressure zero pressure is called absolute pressure. Zero pressure is called Vacuum. Absolute Pressure is always positive since the range starts from Zero.
Relative Pressure:
Relative pressure is measured against the ambient pressure. Relative pressure values can be negative since we are comparing against ambient pressure.
Learn more about Balluff Pressure sensors