Page 431 - Instrumentation Reference Book 3E

I9 Chemical analysis: moisture

measurement

D. B. MEADOWCROFT, edited by I.VERHAPPEN

19.1 Introduction weight (e.g., coal, ore, tobacco, textiles) can most
profitably have moisture contents just below the
The measurement and control of the maximum acceptable limit. Some textiles and
moisture content of gases, liquids, and solids is papers must be dried to standard storage condi-
an integral part of many industries. Numerous tions to prevent deterioration caused by excessive
techniques exist, none being universally applic- wetness and to avoid the waste of overdrying as
able, and the instrument technologist must be the moisture would be picked up again during
able to choose the appropriate measurement tech- storage. Finally, many granulated foods must
nique for the application. It is particularly have a defined moisture content.
important to measure moisture because of its The purpose of this chapter is to introduce the
presence in the atmosphere, but it is awkward reader to the major measurement techniques
because it is a condensable vapor which will com- which are available. The three states, gas, liquid,
bine with many substances by either physical and solid, will be treated separately. In addition,
adsorption or chemical reaction. Moisture meas- many commercial instruments measure some
urement may be needed to ensure the level parameter, which changes reproducibly with
remains below a prescribed value or within a moisture concentration, and these instruments
specified band, and the range of concentrations must be regularly calibrated by the user. The
involved can be from less than one part per mil- chapter therefore ends with a discussion of the
lion to percentage values. major calibration techniques which the average
A few examples will illustrate the range of user must be willing to employ when using such
applications: instruments.
First it is necessary to clarify a further aspect
of moisture measurement which can confuse the
Gases In gas-cooled nuclear reactors the moist- newcomer, which is to define the large number of
ure level of the coolant has to be within a pre- units which are used, particularly for gases, and
scribed band (e.g., 250-500 volume parts per show how they are interrelated.
million) or below a certain value (e.g., l0vppm)
depending on the type of reactor. Rapid detection
of small increases due to leaks from the steam 19.2 Definitions
generators is also essential. Moisture must be
excluded from semiconductor device manufac- 19.2.1 Gases
ture, and glove boxes are fitted with moisture Although water vapor is not an ideal gas, for most
meters to give an alarm at, say, 40vppm. Envir- hygrometry purposes, and to gain an understand-
onmental control systems need moisture ing of the units involved, it is sufficient to assume
measurement in order to control the humidity, water vapor does behave ideally. The basic unit of
and even tumble dryers can be fitted with sensors moisture in a gas against which other units can
to automatically end the clothes drying cycle. readily be referred is vapor pressure, and Dalton’s
Law of Partial Pressures can be assumed to hold if
Liquids The requirement is usually to ensure the the saturated vapor pressure is not exceeded.
water contamination level is low enough. Exam- In environmental applications the unit often
ples are the prevention of corrosion in machinery, used is relative humidity which is the ratio in per-
breakdown of transformer oil, and loss of effi- cent of the actual vapor pressure in a gas to the
ciency of refrigerants or solvents. saturation vapor pressure of water at that tempera-
ture. It is therefore temperature-dependent but is
independent of the pressure of the carrier gas.
Solids Specified moisture levels are often neces- For chemical measurements the concentration
sary for commercial reasons. Products sold by of moisture is usually required. The volume