Measuring Vacuum Pressure
The most common type of pressure range for pressure measurement instrumentation is one which has a gauge reference. So if you want to measure vacuum pressure, it would be convenient to use a gauge pressure device which is the most widely available pressure range format. So is it possible to measure vacuum with a gauge pressure range instrument, and if so how do you select the correct pressure range?
There are different levels of vacuum quality, from low partial, to very high, but they we all reside within the confines of a pressure range between local atmospheric air pressure and full vacuum where no pressure exists at all.
A vacuum, regardless of quality, which is generated in vessel or chamber, will always be measured as a negative pressure relative to the ambient environment air pressure surrounding the vessel or chamber.
Here are some definitions:
Absolute Pressure - pressure relative to absolute zero pressure.
Gauge Pressure - pressure relative to local atmospheric pressure
Differential Pressure - pressure difference between two points.
Vacuum - Pressure less than times the local atmospheric pressure.
According to the definitions above, absolute pressure is measured from absolute zero - not pressure.
There is absolutely no pressure in space, but on Earth's surface, the atmosphere exerts a pressure of about 14.7 psia at sea level. The local air pressure is not only dependent on time, but also on altitude. Atmospheric pressure is about half that at sea level at 17,000 feet.
Vacuum is a pressure below the local atmospheric pressure. It is defined as the difference between the local air pressure and the measuring point. A vacuum is correctly measured with a differential pressure gage that has one port open to atmosphere. If, for example, the negative port is connected to a vacuum and the positive port open to atmosphere, the reading will increase as the vacuum increases. It will always indicate the correct vacuum, even when the local atmospheric pressure changes with the weather.
Local altitude affects the vacuum measurement because it affects atmospheric pressure: no matter how powerful the vacuum pump is, you can't help but have a vacuum of 14 psi at 6,000 feet - because the atmosphere there is only about 12.5 psia and the difference between atmosphere and vacuum cannot exceed that pressure.
Vacuum is best measured using a differential pressure gage with the high pressure port open.