Page 129 - Instrumentation Reference Book 3E

8 Measurement of density

E. H. HIGHAM AND W. H. BOYES

8.1 General 8.2 Measurement of density
using weight
The measurement and control of liquid density is
critical to a large number of industrial processes. The actual weighing of a sample of known
But although density in itself can be of interest, it volume is perhaps the simplest practical applica-
is usually more important as a way of inferring tion of this principle. Various methods for con-
composition, or concentration of chemicals in tinuous weighing have been devised, but the most
solution, or of solids in suspension. In addition, successful involves the use of a horizontal U-
density is often used as a component in the meas- shaped tube with flexible couplings at a pivot
urement of mass flow. Because the density of point.
gases is very small, the instruments for that meas- One example of this type of instrument is the
urement have to be very sensitive to small Fisher-Rosemount mark V gravitrol density
changes. They will be dealt with separately at meter? shown in Figure 8.1. In it, the process fluid
the end of the chapter. passes via flexible connectors into the tube loop
In considering the measurement and control of which is supported towards the curved end on a
density or relative density* of liquids, the units link associated with the force-balance measuring
used in the two factors should be borne in mind. system. In the pneumatic version of the instru-
Density is defined as the mass per unit volume of ment, the link is attached towards one end of the
a liquid and is expressed in such units as kg/m3, weighbeam which itself is supported on cross
g/l or glml. flexure pivots and carries an adjustable counter-
Relative density, on the other hand, is the ratio balance weight on the opposite side. Also
of the mass of a volume of liquid to the mass of attached to the weighbeam is a dash-pot to
an equal volume of water at 4°C (or some other reduce the effect of vibration induced by the flow
specified temperature), the relative density of of the process fluid or by the environment in
water being taken as 1.0. Both density and rela- which the instrument is located.
tive density are temperature-dependent and, for In operation, the counterbalance weight is pos-
high precision, the temperature at which a meas- itioned to achieve balance with the tube loop
urement is made will have to be known, so that filled with fluid having a density within the
any necessary compensation can be introduced. desired working range, and the span adjustment
The majority of industrial liquid-density instru-
ments are based on the measurement of weight, Llquid - Tube loop
buoyancy. or hydrostatic head, but measuring
systems based on resonant elements or radiation -
techniques are also used. In recent years, the
increasing popularity of Coriolis force based Counter balance Wwhi
mass flowmeters has led to their being one of
the most common ways to measure density,
derived, of course, from the mass flow value.

OUWA 10 2- 3 bar1
*The term “specific gravity” is often used for relative n3r IUPPl”
Regulated
density. However, it is not included in the SI System of !l 3 bar)
Units and BS350 points out that it is commonly used Figure 8 .I Gravitrol density meter. Courtesy, Fisher-
when the reference substance is water. “Specific gravity” Rosemount Inc.
is far more commonly used in the United States, where
American Standard Units prevail. +No longer available