Page 138 - Instrumentation Reference Book 3E

Measurement of pressure

E. H. HIGHAM AND J. M. PAROS

9.1 What is pressure? the unknown pressure against the pressure
produced by a column of liquid of known density.
When a fluid is in contact with a boundary it The second method involves allowing the
produces a force at right angles to that boundary. unknown pressure to act on a known area and
The force per unit area is called the pressure. In measuring the resultant force either directly or
the past. the distinction between mass and force indirectly. The third method involves allowing
has been blurred because we live in an environ- the unknown pressure to act on an elastic niem-
ment in which every object is subjected to gravity ber (of known area) and measuring the resultant
and is accelerated towards the center of the earth stress or strain. Examples of these methods are
unless restrained. As explained in Chapter 8, the described in the following sections.
confusion is avoided in the SI system of units
(Systkme International d'UnitCs) where the unit 9.2.1 Pressure measurements by balancing
of force is the newton and the unit of area is a a column of liquid of known density
square meter so that pressure, being force per unit
area, is measured in newtons per square meter The simplest form of instrument for this type of
and the unit, known as the pascal, is independent measurement is the U-tube. Consider a simple U-
of the acceleration due to gravity. tube containing a liquid of density p as shown in
The relation between the pascal and other Figure 9.2. The points A and B are at the same
units used for pressure measurements is shown in horizontal level, and the liquid at e stands at a
Table 9.1. height 11 mm above B.
There are three categories of pressure measure- Then the pressure at A
ments, namely absolute pressure, gauge pressure, =the pressure at B
and differential pressure. The absolute pressure is = atmospheric pressure + pressure due to
the difference between the pressure at a particular column of liquid BC
point in a fluid and the absolute zero of pressure,
i.e., a complete vacuum. A barometer is one exam- = atmospheric pressure + hp
ple of an absolute pressure gauge because the If the liquid is water the unit of measure is
height of the column of mercury measures the mmHzQ, and if the liquid is mercury then the unit
difference between the atmospheric pressure and of measure is mmHg. The corresponding SI unit
the "zero" pressure of the Torricellian vacuum is the pascal and
that exists above the mercury column. 1 mmHzO = 9.806 65 Pa
When the pressure-measuring device measures 1 mmHg = 133.322Pa
the difference between the unknown pressure and
local atmospheric pressure the measurement is For a system such as this it must be assumed that
known as gauge pressure. the density of the fluid in the left-hand leg of the
When the pressure-measuring device measures
the difference between two unknown pressures,
neither of which is atmospheric pressure, then the
measurement is known as the differential pressure.
A mercury manometer is used in Figure 9.1 to
illustrate these three measurements.

9.2 Pressure measurement
There are three basic methods for pressure meas- Figure 9.1 Comparison of types of pressure
urement. The simplest method involves balancing measurements.