Pressure can be measured in pounds per square inch (psi), inches of mercury (inHg), millimeters of mercury (mmHg), kilograms per square centimeter (kg/cm²), kilopascals (kPa), bars (100 kPa), megapascals (MPa) — there are several different scientific, professional, and national conventions.
I think you need to review basic refrigeration fundamentals, measurements using psi and inHg and refrigeration temperature/pressure charts. The t/p charts show chemical characteristics relative to temperatures. Find a t/p chart for r134a. At specific temperatures or pressures in a sealed volume, refrigerant reacts with repeatable results. The vacuum gauge shows the t/p relationship of each refrigerant as pressure varies. You'll need to know service manual lists of normal operating pressures for the low and high sides and whether that's measured at an idle rpm or a higher specified rpm to correlate values in determining if a system is operating correctly.
Sea level pressure is 14.7 psi, 29.9 inHg, etc. Zero on gauges use it as a set reference point for easier use. When a vacuum occurs, measurements are in inHg, not psi. There is no such thing as negative pressure but the term is used loosely sometimes for expedience. In refrigeration, evacuating a system should show the vacuum gauge needle move ccw from '0' towards 29.99 (the '-' connotation is left out) indicating lower than sea level pressure approaching a vacuum. Search for descriptions of how a tube filled with mercury is inverted into a container of mercury; as the heavy mercury tries to empty from the glass tube, a vacuum is created but the column stabilizes as atmospheric pressure pushes down on the container of mercury to equalize liquid in the glass tube trying to flow out. At sea level, that's 29.99 inches of mercury. Measuring vacuum in inHg is much easier than referring to psi.
Low side pressure can be anywhere from 25–40 psi, correlating to whatever temperature is shown on the gauge. Actual vent temps would be some degrees higher and above freezing to prevent creating an ice box blocking airflow. Refrigeration gauges may have service instructions for replacing miscellaneous parts with diagrams. At one time long ago, instructions described how each three-way valve operated. Understanding refrigeration gauges are a must before connecting to any system.
