Page 436 - Instrumentation Reference Book 3E

Measurement techniques 419

reagent, and the electrolysis current is a measure to give a suitable attenuation. Electronic develop-
of the mass flow of water. ments are causing increased interest in this
technique.
19.3.2.2 Inpared instruments
The same comments apply as for gases (Section 19.3.2.5 Turbiditylnephelometer
19.3.1.3), but sample cell lengths are usually Undissolved water must be detected in aviation
shorter, in the range 1-100mm. It is an attractive fuel during transfer. After thorough mixing the
method for online analysis, but care must be taken fuel is divided into two flows: one is heated to
that other components in the liquid do not inter- dissolve all the water before it passes into a refer-
fere with the measurement. Single-beam instru- ence cell; the other passes directly into the work-
ments are most often used. ing cell. Light beams split from a single source
pass through the cells, and suspended water drop-
19.3.2.3 Vapor pressure methods lets in the cell scatter the light, and a differential
output is obtained from the matched photoelec-
As discussed in Section 19.2.2, the equilibrium tric detectors on the two cells. 0 to 40 ppm mois-
relative liumidity above a liquid can be used to ture can be detected at fuel temperatures of -30 to
determine the moisture content in the liquid. 40 "C.
Either the relative humidity in a closed volume
above the liquid can be measured, or a sensor
which responds to the moisture vapor pressure
in the liquid can be immersed in the liquid. The 19.3.3 Solids
aluminum oxide sensor (Section 19.3.1.4) can be The range of solids in which moisture must be
used, either above the liquid because it does not measured commercially is wide, and many tech-
require a gas flow rate, or within the liquid niques are limited to specific materials and indus-
because though the aluminum oxide pores will tries. In this book just some of the major methods
adsorb water molecules, they will not adsorb the are discussed.
liquid molecules. These techniques are not appro-
priate if suspended free water is present in the
liquid. 19.3.3.1 Equilibrium relative humidity
One manufacturer has developed a system in
which the sensor is a moisture-permeable plastic The moisture level of the air immediately above a
solid can be used to measure its moisture content.
tube which is immersed in the liquid. A fixed Electrical probes as discussed in Section 19.3.1.4
quantity off initially dry gas is circulated through are generally used, and if appropriate can be
the tube, and the moisture in the gas is measured placed above a moving conveyor. If a material is
by an optical dew point meter. When equilibrium being dried its temperature is related to its equilib-
is reached, the dew point measured equals that of rium relative humidity, and a temperature meas-
the moisture in the liquid.
urement can be used to assess the extent of
drying.
19.3.2.4 Microwave instruments
The water molecule has a dipole moment with rota- 19.3.3.2 Electrical impedance
tional vibration frequencies, which give absorption
in the microwave, S-band (2.G3.95 GHz), and Moisture can produce a marked increase in the
X-band (8.2-12.4 GHz) suitable for moisture electrical conductivity of a material and, because
measurement (Figure 19.7). The S-band needs of water's high dielectric constant, capacitance
path lengths four times longer than the X-band for measurements can also be valuable. Electrical
a given attenuation, and therefore the microwave resistance measurements of moisture in timber
band as well as cell dimensions can be chosen and plaster are generally made using a pair of
sharp pointed probes (Figure 19.8(a)) which are
pushed into the material, the meter on the instru-
ment being calibrated directly in percentage
moisture. For on-line measurements of granular
materials, electrodes can be rollers, plates (Figure
19.8(b)), or skids but uniform density is essential.
A difficulty with this and other on-line methods
which require contact between the sensor and the
material is that hard materials will cause rapid
Figure 19.7 The basic concept for measuring moisture
by microwave absorption. erosion of the sensor.