Page 434 - Instrumentation Reference Book 3E

Measurement techniques 417

low moisture levels as it is possible to use a
flow rate of 100ml min-' at the measuring
pressure, which does not increase the velocity
of gas along the element and hence does not
impair the absorption efficiency of the P2Q5.

19.3.1.3 Infruaved instruments
Water vapor absorbs in the 1-2pm infrared
range, and infrared analyzers (see Chapter 22)
can be successfully used as moisture meters. For
concentrations in the vppm range the path length
has to be very long, and high sample flow rates of
several liters per minute can be necessary to
reduce the consequent slow response time. Both
single-beam instruments, in which the zero base-
line is determined by measuring the absorption at
a nearby non-absorbing wavelength, and double-
beam instruments, in which a sealed parallel cell
is used as reference, can be used. Single-beam
instruments are less affected by deposits on the
cell windows and give better calibration stability
in polluted gases.

Figure 19.4 A schematic diagram of a sensor of a 19.3.1.4 Electrical sensor instruments
coulometric instrument.
There are many substances whose electrical
impedance changes with the surrounding mois-
proportional to the mass of water absorbed ture level. If this absorption process is suffi-
(Faraday's law). Thus the current depends on ciently reproducible on a thin film the
the flow rate, which must be set and controlled impedance, measured at either an audio fre-
accurately at a predetermined rate (usually quency or a radio frequency, can be calibrated
100ml min-') so that the current meter can iE terms of moisture concentration or relative
be calibrated directly in ppm. Several points are humidity. Materials used in commercial instru-
worth making: - ments include polymers, tantalum oxide, silicon
oxide, chromium oxide, aluminum oxide,
The maximum moisture concentration lithium chloride mixed with plastic, and car-
measurable by this technique is in the range bon-loaded plastics which change length and
1000-3000 vppni but care must be taken to
ensure surges of moisture level do not wash hence resistance with moisture level. Many
such instruments are available commercially,
off the P2Qj. particularly using an anodized aluminum oxide
There is generally a zero leakage current layer which has a very narrow columnar pore
equivalent to a few ppm. To allow for this structure (Figure 19.51, but aging and other
error, when necessary, the current should be deterioration processes can occur so that regu-
measured at two flow rates and the difference lar calibration is essential. A major advantage
normalized to the flow for 100ml min-'.
Platinum electrodes are not suitable for use
in gases containing significant amounts of POROUSGOLD CONTACT
hydrogen. The platinum can catalyze the
recombination of the electrolyzed oxygen,
and this water is also electrolyzed giving in-
accurate measurements. Gold or rhodium ALUMINA
elements reduce this effect.
In the absence of recombination and gas
leaks the response of a coulometric instru- ALUMINUM
ment can be regarded as absolute for many Figure 19 5 An idealized representation of the pore
purposes. structure of anodized alumina The pores are typically less
than 20 nm in diameter and more than 1 00 pm deep A
Cells, which work at pressure, can be porous gold layer IS deposited on the alumina for electrical
obtained. This can increase the sensitivity at contact when used as a hygrometer sensor