21.5.2 Falling body viscometer
The falling body viscometer is
particularly recommended for the measurement of high-viscosity fluids. It can
give measurement uncertainty levels down to ±1%. It involves measuring the time
taken for a spherical body to fall a given distance through the liquid. The
viscosity for Newtonian fluids is then given by Stoke’s formula as (in units of
poise):
where R is the radius (m) of the
sphere, g is the acceleration due to gravity (m/s2 ),
ps and pl are the specific gravities (g/m3 ) of the
sphere and liquid respectively and V is the velocity (m/s) of the sphere.
For non-Newtonian fluids, correction
for the variation in shear rate is very difficult.
21.5.3 Rotational viscometers
Rotational viscometers are relatively
easy to use but their measurement inaccuracy is at least ±10%. All types have
some form of element rotating inside the liquid at a constant rate. One common
version has two coaxial cylinders with the fluid to be measured contained
between them. One cylinder is driven at a constant angular velocity by a motor
and the other is suspended by torsion wire. After the driven cylinder starts
from rest, the suspended cylinder rotates until an equilibrium position is
reached where the force due to the torsion wire is just balanced by the viscous
force transmitted through the liquid. The viscosity (in poise) for Newtonian
fluids is then given by:
where G is the couple (Nm) formed by
the force exerted by the torsion wire and its deflection, R1 and R2
are the radii (m) of the inner and outer cylinders, h is the length of the
cylinder (m) and ω is the angular velocity (rad/s) of the rotating cylinder.
Again, corrections have to be made for non-Newtonian fluids.
21.6 Moisture measurement
There are many industrial
requirements for the measurement of the moisture content. This can be required
in solids, liquids or gases. The physical properties and storage stability of
most solid materials is affected by their water content. There is also a
statutory requirement to limit the moisture content in the case of many
materials sold by weight. In consequence, the requirement for moisture
measurement pervades a large number of industries involved in the manufacture
of foodstuffs, pharmaceuticals, cement, plastics, textiles and paper.
Measurement of the water content in
liquids is commonly needed for fiscal purposes, but is also often necessary to
satisfy statutory requirements. The petrochemical industry has wide-ranging
needs for moisture measurement in oil etc. The food industry also needs to
measure the water content of products such as beer and milk.
In the case of moisture in gases, the
most common measurement is the amount of moisture in air. This is usually known
as the humidity level. Humidity measurement and control is an essential
requirement in many buildings, greenhouses and vehicles.
As there are several ways in which
humidity can be defined, three separate terms have evolved so that ambiguity
can be avoided. Absolute humidity is the mass of water in a unit volume of
moist air; specific humidity is the mass of water in a unit mass of moist air;
relative humidity is the ratio of the actual water vapour pressure in air to
the saturation vapour pressure, usually expressed as a percentage.
21.6.1 Industrial moisture
measurement techniques
Industrial methods for measuring
moisture are based on the variation of some physical property of the material
with moisture content. Many different properties can be used and therefore the
range of available techniques, as listed below, is large.
Electrical methods
Measuring the amount of absorption of
microwave energy beamed through the material is the most common technique for
measuring moisture content and is described in detail in Anderson (1989), and
Thompson (1989). Microwaves at wavelengths between 1 mm and 1 m are absorbed to
a much greater extent by water than most other materials. Wavelengths of 30 mm
or 100 mm are commonly used because ‘off-the-shelf’ equipment to produce these
is readily available from instrument suppliers. The technique is suitable for
moisture measurement in solids, liquids and gases at moisture-content levels up
to 45% and measurement uncertainties down to š0.3% are possible.
The capacitance moisture meter uses
the principle that the dielectric constant of materials varies according to
their water content. Capacitance measurement is therefore related to moisture
content. The instrument is useful for measuring moisture[1]content
levels up to 30% in both solids and liquids, and measurement uncertainty down
to ±0.3% has been claimed for the technique (Slight, 1989). Drawbacks of the
technique include (a) limited measurement resolution owing to the difficulty in
measuring small changes in a relatively large standing capacitance value and
(b) difficulty when the sample has a high electrical conductivity. An
alternative capacitance charge transfer technique has been reported (Gimson, 1989)
that overcomes these problems by measuring the charge carrying capacity of the
material. In this technique, wet and dry samples of the material are charged to
a fixed voltage and then simultaneously discharged into charge-measuring
circuits.
The electrical conductivity of most
materials varies with moisture content and this therefore provides another
means of measurement. Techniques using electrical conductivity variation are
cheap and can measure moisture levels up to 25%. However, the presence of other
conductive substances in the material such as salts or acids affects the
measurement.
A further technique is to measure the
frequency change in a quartz crystal that occurs as it takes in moisture.
Neutron moderation
Neutron moderation measures moisture
content using a radioactive source and a neutron counter. Fast neutrons emitted
from the source are slowed down by hydrogen nuclei in the water, forming a
cloud whose density is related to the moisture content. Measurements take a
long time because the output density reading may take up to a few minutes to
reach steady state, according to the nature of the materials involved. Also,
the method cannot be used with any materials that contain hydrogen molecules,
such as oils and fats, as these slow down neutrons as well. Specific humidities
up to 15% (±1% error) can be measured.
Low resolution nuclear magnetic
resonance (NMR)
Low-resolution nuclear magnetic
resonance involves subjecting the sample to both an unidirectional and an
alternating radio-frequency (RF) magnetic field. The amplitude of the
unidirectional field is varied cyclically, which causes resonance once per
cycle in the coil producing the RF field. Under resonance conditions, protons
are released from the hydrogen content of the water in the sample. These
protons cause a measurable moderation of the amplitude of the RF oscillator
waveform that is related to the moisture content of the sample. The technique
is described more fully in Young (1989).
Materials that naturally have a
hydrogen content cannot normally be measured. However, pulsed NMR techniques
have been developed that overcome this problem by taking advantage of the
different relaxation times of hydrogen nuclei in water and oil. In such pulsed
techniques, the dependence on the relaxation time limits the maximum fluid flow
rate for which moisture can be measured.
Optical methods
The refractometer is a
well-established instrument that is used for measuring the water content of
liquids. It measures the refractive index of the liquid, which changes
according to the moisture content.
Moisture-related energy absorption of
near-infra-red light can be used for measuring the moisture content of solids,
liquids and gases. At a wavelength of 1.94 µm, energy absorption due to
moisture is high, whereas at 1.7 µm, absorption due to moisture is zero. Therefore,
measuring absorption at both 1.94 µm and 1.7 µm allows absorption due to
components in the material other than water to be compensated for, and the
resulting measurement is directly related to energy content. The latest
instruments use multiple-frequency infra-red energy and have an even greater
capability for eliminating the effect of components in the material other than
water that absorb energy. Such multi-frequency instruments also cope much
better with variations in particle size in the measured material.
In alternative versions of this
technique, energy is either transmitted through the material or reflected from
its surface. In either case, materials that are either very dark or highly
reflective give poor results. The technique is particularly attractive, where
applicable, because it is a non-contact method that can be used to monitor
moisture content continuously at moisture levels up to 50%, with inaccuracy as
low as ±0.1% in the measured moisture level. A deeper treatment can be found in
Benson (1989).
Ultrasonic methods
The presence of water changes the
speed of propagation of ultrasonic waves through liquids. The moisture content
of liquids can therefore be determined by measuring the transmission speed of
ultrasound. This has the inherent advantage of being a non[1]invasive
technique but temperature compensation is essential because the velocity of
ultrasound is particularly affected by temperature changes. The method is best
suited to measurement of high moisture levels in liquids that are not aerated
or of high viscosity. Typical measurement uncertainty is ±1% but measurement
resolution is very high, with changes in moisture level as small as 0.05% being
detectable. Further details can be found in Wiltshire (1989).
Mechanical properties
Density changes in many liquids and
slurries can be measured and related to moisture content, with good measurement
resolution up to 0.2% moisture. Moisture content can also be estimated by
measuring the moisture level-dependent viscosity of liquids, pastes and
slurries.
21.6.2 Laboratory techniques for
moisture measurement
Laboratory techniques for measuring
moisture content generally take much longer to obtain a measurement than the
industrial techniques described above. However, the measurement accuracy
obtained is usually much better.
Water separation
Various laboratory techniques are
available that enable the moisture content of liquids to be measured accurately
by separating the water from a sample of the host liquid. Separation is
effected either by titration (Karl Fischer technique), distillation (Dean and
Stark technique) or a centrifuge. Any of these methods can measure water
content in a liquid with measurement uncertainty levels down to ±0.03%.
Gravimetric methods
Moisture content in solids can be
measured accurately by weighing the moist sample, drying it and then weighing
again. Great care must be taken in applying this procedure, as many samples
rapidly take up moisture again if they are removed from the drier and exposed
to the atmosphere before being weighed. Normal procedure is to put the sample
in an open container, dry it in an oven and then screw an airtight top onto the
container before it is removed from the oven.
Phase-change methods
The boiling and freezing point of
materials is altered by the presence of moisture, and therefore the moisture
level can be determined by measuring the phase-change temperature. This
technique is used for measuring the moisture content in many food products and
in some oil and alcohol products.
Equilibrium relative humidity
measurement
This technique involves placing a
humidity sensor in close proximity to the sample in an airtight container. The
water vapour pressure close to the sample is related to the moisture content of
the sample. The moisture level can therefore be determined from the humidity
measurement.
21.6.3 Humidity measurement
The three major instruments used for
measuring humidity in industry are the electrical hygrometer, the psychrometer
and the dew point meter. The dew point meter is the most accurate of these and
is commonly used as a calibration standard. The various types of hygrometer are
described more fully in Miller (1975b).
The electrical hygrometer
The electrical hygrometer measures
the change in capacitance or conductivity of a hygroscopic material as its
moisture level changes. Conductivity types use two noble metal electrodes
either side of an insulator coated in a hygroscopic salt such as calcium
chloride. Capacitance types have two plates either side of a hygroscopic
dielectric such as aluminium oxide.
These instruments are suitable for
measuring moisture levels between 15% and 95%, with typical measurement
uncertainty of ±3%. Atmospheric contaminants and operation in saturation
conditions both cause characteristics drift, and therefore the recalibration
frequency has to be determined according to the conditions of use.
The psychrometer (wet and dry bulb
hygrometer)
The psychrometer, also known as the
wet and dry bulb hygrometer, has two temperature sensors, one exposed to the
atmosphere and one enclosed in a wet wick. Air is blown across the sensors,
which causes evaporation and a reduction in temperature in the wet sensor. The
temperature difference between the sensors is related to the humidity level.
The lowest measurement uncertainty attainable is ±4%.
Dew point meter
The elements of the dew point meter,
also known as the dew point hygrometer, are shown in Figure 21.13. The sample
is introduced into a vessel with an electrically cooled mirror surface. The
mirror surface is cooled until a light source-light detector system detects the
formation of dew on the mirror, and the condensation temperature is measured by
a sensor bonded to the mirror surface. The dew point is the temperature at
which the sample becomes saturated with water. Therefore, this temperature is
related to the moisture level in the sample. A microscope is also provided in
the instrument so that the thickness and nature of the condensate can be
observed. The instrument is described in greater detail in Pragnell (1989).
Even small levels of contaminants on
the mirror surface can cause large changes in the dew point and therefore the
instrument must be kept very clean. When necessary, the mirror should be
cleaned with deionized or distilled water applied with a lint-free swab. Any
contamination can be detected by a skilled operator, as this makes the
condensate look ‘blotchy’ when viewed through the microscope. The microscope
also shows up other potential problems such as large ice crystals in the
condensate that cause temperature gradients between the condensate and the
temperature sensor. When used carefully, the instrument is very accurate and is
often used as a reference standard.
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