United States
Environmental Protection
Agency
Environmental Sciences Research'
Laboratory _
Research Triangle Park NC 2771
Research and Development
EPA-600/S2-81-162 Jan. 1982
Project Summary
A Water Vapor Monitor
Using Differential
Infrared Absorption
D. E. Burch and D. S. Goodsell
A water vapor monitor has been
developed with adequate sensitivity
and versatility for a variety of applica-
tions. The instrument has been
designed for the continuous
monitoring of ambient air and the
measuring of the mass of H2O desorbed
from aerosol filters. The' sample gas
may be held static, or it may flow con-
tinuously through the 56 cm3 sample
cell, which is temperature controlled
at 45°C. Infrared energy from a
tungsten-iodide bulb passes through a
rotating filter wheel and the sample
cell to a PbS detector. The infrared
beam passes through the sample gas
twice to produce a total optical path of
40 cm. As the filter wheel rotates at
1800 rpm, the infrared beam passes
alternately through two semicircular
narrow bandpass filters; one is
centered in a spectral region of strong
absorption and the other is centered
nearby in a region of weak absorption.
Absorption by the water vapor in the
sample produces a 30-Hz modulation
of the detector signal that is propor-
tional to the HgO concentration. The
zero-setting of the monitor is stabi-
lized by controlling the temperatures
of the detector and the filters. The
r.m.s. noise level corresponds to ~3
ppm of HaO. The maximum concen-
tration that can be measured
accurately is -»5%; higher concentra-
tions could be measured by shortening
the sample cell.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The U.S. Environmental Protection
Agency (EPA) would like to obtain the
mass of H20 associated with ambient
air particulate matter that has been
collected on Teflon filters. This meas-
urement is needed to estimate the
contribution of H2O to the deficit in mass
between the total measured'aerosol
mass and the mass that can be attri-
buted to currently measurable particu-
late components. This deficit is as much
as 30% in some cases.
A positive and quantitative identifica-
tion of H20 poses an analytical problem
for several reasons: (1) the amount of
H2O is small — i.e., 30% of a typical
loading of 500 mg of aerosol per filter is
only 150 mg; (2) the H20 associated
with the particulate matter is not acces-
sible; (3) volatilization of H2O from the
filter by heating also volatilizes other
aerosol components so that a simple
measurement of weight loss is not
acceptable as an indication of H20 on
the filter. However, volatilization of the
H20 coupled with a sensitive H2O
monitor capable of a real-time output
was conceived as a tenable approach
provided that the H20 vapor concentra-
tion was sufficiently high (parts-per-
thousand range) to achieve a
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reasonable signal to noise as an output
signal.
Previous research for the EPA con-
cerned with the design and fabrication
of prototype instruments for the detec-
tion of CO and CH4 at ambient air
concentrations resulted in highly
successful procedures for using
infrared absorption as the basis for
sensitive and specific ambient air
monitors. Considerations based on this
analogous work indicated that the H20
could be detected if thermal desorption
over a period of one to two minutes
occurred into a carrier gas volume of 50-
100 cc. In other work, a prototype unit
capable of effecting the thermal
desorption of volatile aerosols into a
sulfur detector (for H2SO4 detection)
was developed. This prototype was to be
used for the detection of aerosol-
associated H20 by replacing the sulfur
detector with an H2O monitor.
Materials and Methods
The monitor consists of a radiation
source (tungsten-iodide lamp), an
optical absorption cell, a PbS detector
and associated optical and electronic
components. The unit is analogous in
many ways to the family of so-called gas
filter correlation (GFC) monitors that
have been previously designed and
fabricated at Ford Aerospace and
Communications Corp.
The concentration of H2O vapor in the
sample cell of the monitor is determined
by electronically comparing the trans-
mittance of-the sample in two narrow
infrared spectral intervals. One interval
occurs near 2.59 fjm where H2O vapor
absorbs much more than in the other
interval centered near 2.51 /um.
Samples of gas containing H2O vapor
may be introduced into the 20-cm long
sample cell and kept static during the
measurement, or they may flow contin-
uously through the cell. Good stability of
the zero-setting and the sensitivity is
achieved by controlling the tempera-
tures of the sample cell, spectral filters
and detector. Water vapor concentra-
tions as high as 5% can be measured
with good accuracy. The minimum
detectable concentration in air samples
at one atm total pressure is less than 10
ppm. Discrimination against interfer-
ence by other gases, particularly those
in the normal atmosphere, is excellent
due to the spectral position and width of
the spectral filters.
A calibration curve is provided as part
of the main report text. Based on the
performance of the monitor during
calibration, the monitor appears to be an
excellent ambient air monitor, giving a
real-time output proportional to the
ambient H2O content. Combined with
the temperature and pressure sensors
that are included in the instrument, the
absolute and relative humidities can be
obtained. This type of system may offer
significant advantages over other
currently available H20 monitors.
Recommendations
Variations of the monitor are recom-
mended for consideration for many
applications in which the concentration
of H20 vapor is to be measured. The
required complexity of a monitor of this
type depends on the sensitivity,
stability, accuracy and time response
involved. Therefore, it is not practical to
design a single H20 monitor for all
applications. Some of the design fea-
tures that should be considered for a
given application are as follows:
1. The two spectral intervals (A =
2.59 and 2.51 //m) passed by the
two filters are appropriate for
many applications. If a gas species
in the samples absorbs strongly
within one of these intervals, a
different set of intervals should be
selected. It may be advisable to
select another spectral interval to
replace the 2.59jum interval if the
absorption is too great. A different
interval is recommended if pL is
greater than ~2 atm cm (p is the
H2O partial pressure and L is the
optical path length through the
sample cell).
2. A PbS detector designed for long
wavelength sensitivity is inexpen-
sive, durable, convenient, and
adequate for most applications.
However, a cryogenically cooled
PbS, PbSe, or InSb detector could
provide higher sensitivity and
possibly better stability when
improved performance is required.
Good stability of the zero-setting
can be achieved with a PbS
detector near room temperature
only if the detector is temperature
controlled.
3. Controlling the temperature of the
filters improves the stability of the
zero-setting and the span calibra-
tion. However, this is not required
for many monitoring applications.
4. The electronics that process the
detector signal should automat-
ically account for small variations
in source radiance, detectoi
sensitivity, window transmittance,
etc.
5. The optical path in air outside ol
the sample cell should be kepi
short, and if very good stability is
required, should be enclosed foi
convenient purging or drying with
dessicant.
6. Gas lines leading to the sample
cell should be heated to reduce
adsorption or condensation of H2C
on the walls.
7. The same basic design may be
used to monitor gases withoui
drawing them into a sample cell
For example, the monitoring bean-
may traverse an open volume
such as an atmospheric pathorar
exhaust vent.
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D. E. Burch and D. S. Goodsell are with Ford Aerospace & Communications
Corporation, Newport Beach, CA 92660.
William A. McClenny is the EPA Project Officer (see below).
The complete report, entitled"A Water Vapor Monitor Using Differential Infrared
Absorption," (Order No. PB 82-114 422; Cost: $ 7.50, subject to change} will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
- US GOVERNMENT PRINTING OFFICE, 1982—559-017/7436
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