Page 438 - Instrumentation Reference Book 3E

References 421

stable conditions are achieved. Many plastics are rapid evaporation. The method can be used
permeable to moisture and must never be used. At very successfully, syringes in particular allow-
high moisture levels copper, Teflon, Viton, glass, or ing a very wide range to be covered.
quartz can be satisfactorily used, but at low levels (e) If a single humidity level can be generated,
stainless steei is essential. Finally, at high moisture a range can be obtained using a flow mixing
levels it must be remembered that the sample lines system, but to achieve sufficient accuracy
and detectors must be at least 10 Kelvins hotter mass flow meters will probably be necessary.
than the dew point of the gas.
There are two basic calibration methods, which
can, with advantage, be combined. Either a sam- 19.4.2 Liquids
ple gas is passed through a reference hygrometer The basic absolute method is that of the Karl
and the instrument under test, or a gas of known Fischer titration which was described in Section
humidity is generated and passed through the 19.3.2.1.
instrument under test. Obviously it is ideal to
double-check the calibration by using a known
humidity and a reference hygrometer. 19.4.3 Solids
The most suitable reference hygrometer is the
dew point meter, which can be readily obtained There are several methods which allow the abso-
with certified calibration traceable to a standard lute moisture level of a solid to be determined,
but for all of them, samples of the specific sub-
instrument. For many applications, less sophisti- stance being measured by the process technique
cated dew point instruments would be adequate, must be used. The most common technique is of
and coillometric analyzers are possible for low
moisture levels. At high levels gravimetric methods course to weigh a sample, dry it, and then weigh
again. Drying temperature and tine depend on
can be used, but they are slow and tedious and
difficult to make accurate. the material; if necessary, the temperature must be
limited to avoid decomposition, loss of volatile
There are a range of possible humidity sources,
some of which are available commercially, and components. or absorption of gases from the
the choice depends on the facilities available and atmosphere.
Balances can be obtained with a built-in heater,
the application:
which gives a direct reading of moisture content
for a fixed initial sample weight. Other favored
(a) A plastic tube, permeable to moisture, held techniques include measuring the water vapor
in a thermostatically controlled water bath, given off by absorbing it in a desiccant to avoid
will give a constant humidity for a given flow the effects of volatiles; the Karl Fischer method
rate. Some manufacturers sell such tubes pre- again; or mixing the substance with calcium car-
calibrated for use as humidity sources, but
obviously, the method is not absolute, and bide in a closed bomb and measuring the pressure
of acetylene produced. The method must be care-
the ]permeation characteristics of the tubes
may change with time. fully chosen to suit the substance and process
technique being used. Finally, it is worth noting
(5) Gas cvlinders can be Durchased with a ore- that rather than an absolute calibration, calibra-
determined moisture level which does not sig- tion directly in terms of tbe desired quality of the
nificantly drift because of the internal surface substance in the manufacturing process may be
treatment of the cylinder. However, to prevent the most appropriate.
condensation in the cylinder the maximum
moisture level is limited to about 50Ovppm
even with a cylinder pressure Qf only 10 bar. 19.5 References
They ase most suitable for spot checks of
instruments on site. Mitchell, J. and D. Smith: Aquametry. Part 1, A Trea-
If an inert gas containing a known concen- tise on Methods for the Determination of Water, Chem-
tration of hydrogen is passed through a bed ical Analysis Series No. 5, Wiley, New York (1977)
of copper oxide heated to -350°C the hydro- Mitchell, J. and D. Smith, Aquametry. Part 2, The Karl
gen is converted to water vapor. This method Fischer Reagent, Wiley, New York (1980)
relies on the measurement and stability of the Verdin, A,, Gas Analysis Instrumentation, Macmillan,
hydrogen content which is better than for London (1973)
mmisture. The generated humidity is also Wexler, A., “Electric hygrometers,” National Bureau of
Standards Circular No 586 (1957)
independent of flow rate. Wexler, A. (ed.), Humidity andbfoistzwe (3 volumes), papers
(d) Wafer can be continuously injected into a presented at a conference, Reinhold, New York (1965)
gas stream using either an electrically driven Wexler, A.; “Measurement of humidity in the free
syringe pump or a peristaltic pump. The atmosphere near the surface of the Earth,” Meteor-
injection point should be heated to ensure ological Monographs, 11, 262-282 (1970)