I see that a number of folks have been looking for a conversion from PSI to Atmospheres, or back.
One Atmosphere is 14.7 PSI. One Atmosphere is also one Bar.
When you look at your air gauge on the air tank which is at zero pressure, the gauge indicates a zero reading.
Actually, the gauge has been set up to “ignore” the fact that it should show about 14.7 PSI when the air tank is empty, as that is the real air pressure of the atmosphere at sea level.