Instrumentation Reference Book, 4th Edition

Chapter 15. Measurement of Vacuum

D.J. Pacey

15.1. Introduction

15.1.1. Systems of Measurement

The measurement of vacuum is the measurement of the range of pressures below atmospheric. Pressure is defined as force divided by area. The American National Standard unit is the pound/inch ², or psi; the SI unit is the newton/meter ² (Nm ⁻²) or pascal (Pa). Pressure may also be stated in terms of the height of a column of a suitable liquid, such as mercury or water, that the pressure will support. The relation between pressure units currently in use is shown in Table 15.1.

TABLE 15.1. Relations between pressure units

N/m 2(Pa)	torr	mb	atm
N/m 2 (Pa)	1	7.50 × 10 −3	10 −2	9.87 × 10 −6
torr	133.3	1	1.333	1.316 × 10 −3
mb	100	0.750	1	9.87 × 10 −4
atm	1.013 × 105	760	1.013 × 10 ...