Final answer:
The primary air pressure gauge on the Beta-Freightliner R-12 Hydrant-Servicing Unit indicates the gauge pressure, which is the pressure relative to atmospheric pressure.
Step-by-step explanation:
The primary air pressure gauge on the Beta-Freightliner R-12 Hydrant-Servicing Unit indicates the gauge pressure. Gauge pressure is the pressure relative to atmospheric pressure. When the gauge reads zero, it means the pressure is equal to the atmospheric pressure. If the gauge reads a positive value, it means the pressure is higher than atmospheric pressure, and if it reads a negative value, it means the pressure is lower than atmospheric pressure.