Page 437 - Instrumentation Reference Book 3E

420 Chemical analysis: moisture measurement

15.3.3.5 Neutron moderation
Hydrogen nuclei slow down (“moderate”) fast neu-
trons and therefore if a fast neutron source is placed
over a moist material with a slow neutron detector
adjacent, the detector output can be used to indi-
cate the moisture concentration. The concentration
of any other hydrogen atoms in the material and its
packing density must be known. This technique is
described in Chapter 23. Nuclear magnetic res-
onance can also be used to detect hydrogen nuclei
as a means of measuring moisture content.
(a) (bl
Figure 19.8 Two techniques for electrical measurements 19.4 Calibration
of moisture in solids: (a) pointed probes for insertion in
wood, plaster, etc. to measure resistance, (b) capacitance It will be seen from the above sections that many
plates to measure moisture in flowing powder or granules. moisture measurement techniques are not abso-
lute and must be calibrated, generally at very
15.3.3.3 Microwave instruments regular intervals. It must first be emphasized that
the absolute accuracy of moisture measurement,
Most comments appropriate to liquids also apply
to solids, but, as above, constant packing density particularly in gases, is not usually high. Though
is necessary. For sheet materials, such as paper or it is possible to calibrate moisture detectors for
cloth, measurement is simple, the sheet passing liquids or solids to 0.1 to 1.0 percent, such
through a slot in the waveguide. For granular accuracies are the exception rather than the rule
materials, uniform material density is achieved for gases. Figure 19.10 shows the accuracies of
by design of the flow path; alternatively, extruders some of the techniques discussed in this chapter
or compactors can be useful. compared with the absolute gravimetric standard
of the U.S. National Bureau of Standards.
19.3.3.4 Inpared instruments
19.4.1 Gases
The basic difference from measurements in gases First of all, the difficulties of making accurate
and liquids is that, for solids, reflectance methods moisture measurements must be stressed. This is
(see Figure 19.9) are usually used rather than particularly so at low levels, say, less than
transmission methods. Single-beam operation is 100 vppm because as all materials absorb moisture
used with a rotating absorption and reference to some extent sample lines must come to equilib-
frequency filter to give regular zero readings. rium as well as the detector. At low moisture levels
The calibration of a reflectance method can be this can take hours, particularly at low flow rates.
substantially independent of the packing density
as it measures only the surface concentration. For A rapid-flow bypass line can be valuable. Patience
is mandatory, and if possible the outputs of the
material on a conveyor belt a plough is often used instruments should be recorded to establish when
in front of the sensing position to ensure a meas-
urement more typical of the bulk. The method is
not suitable for poorly reflecting materials, e.g., FROST POINT, “C DEW POINT, ‘C
carbon and some metal powders. u.1 , -60 -50 -40 -30 -20 -10 0 10 20 30 40 50
-70
GRAVIMETRIC
s STANDARD
-
r
5 1.0
c
Y
Y
oz
r
e 10
e e
- 10 101 io3 104 io5 I
J
CONCENTRATION. ppm by volume
MATERIALONCONVEYOR BELT Figure19.10 Theaccuracyofsomeof themajor
Figure 19.9 The principle of infrared reflectance used to techniques for measuring moisture in gases, after Wexler
measure moisture in a solid on a conveyor. (1970).