About Pressure Metrology

No, they are not. The internationally recognized SI unit for pressure is the Pascal, abbreviated to Pa, and this is the unit realized by the primary measurement standards in the world's national metrology institutes to provide traceability for pressure measurements.

The numerical relationship between the Pascal and some other units is shown on different pressure units table. When converting between these pressure units consideration should be given to the number of significant figures to use, bearing in mind that many of the underlying conversion factors are not themselves exact and cannot be made so. There is little point in expressing the result of a conversion with more significant figures than is warranted either by the underlying conversion factor, the precision of the value to be converted or the measurement uncertainty associated with it; Thus, depending on circumstances, it is not always necessary to use the full precision of the conversion factors and it is never worth using more. Despite these rules you will find conversion figures elsewhere that are published with far more significant figures than is appropriate; they are misleading and are certainly not more accurate.

Yes. Provided that it is under the scope of our measurement range and can be detached from the system itself

When referring to a pressure value, it is important to specify its mode in order to avoid ambiguity. If a vessel were to contain no molecules whatsoever, the pressure would be zero. Pressures measured on a scale which uses this zero value as their reference values are said to be absolute pressures. In everyday life, however, many applications of pressure are not so much dependent on the absolute value of a pressure as the difference between it and the pressure of the atmosphere. These measurements are known as the gauge pressure measurements. Thus, the difference between an absolute pressure value and a gauge pressure value is the variable value of atmospheric pressure: Absolute pressure = gauge pressure + atmospheric pressure. In other applications, where knowledge of the pressure difference between two places or systems is needed, the reference pressure may not necessarily be either zero or atmospheric pressure but some other value. These are known as differential pressures.