Measurement of the generated water vapor will usually be necessary.
This may be done by a wet-bulb thermometer or by one of several
commercially available electronic humidity sensor-readout devices
(Table 12). The wet-bulb thermometer technique is satisfactory if
only an approximate or occasional check of the relative humidity is
needed, whereas the electronic sensor is necessary for the most
accurate continuous readout and if it is desirable to have a
permanent record of the humidity level for a given time.

Concentration Monitoring Systems

An independent monitor is necessary to determine the actual
concentration of the gas or vapor being produced. The selection of
a monitor or independent sampling method to use will depend upon
many factors including the kind of gas or vapor being generated, the
concentration level, and if it is a single component or a mixture.
In general, monitors may be classified as continuous or
intermittent, and these may be further classified as specific or
non-specific.

Continuous monitors are those whose sample lines are attached
directly to the generation system sample manifold and provide a
continuous direct readout and/or a permanent record of the
concentration level at any time during the operation. Continuous
monitors include non-specific instruments such as flame ionization
detectors, photoionization detectors and infrared detectors and
specific instruments such as CO detectors, mercury vapor detectors,
and NO/NOz detectors (Table 13). Continuous monitors are most
useful where only a single contaminant is present to be measured, as
would be the case whenever a sampling method or procedure was being
evaluated. However, continuous monitors may also be useful for
monitoring mixtures, either alone or in combination with one or more
other continuous monitors, in certain situations.

Intermittent monitors are instruments or devices which can be used
to measure the concentration level at intervals throughout the
operation, either directly, as is the case when a gas chromatograph
with an automatic gas sampling valve is attached to the sample
manifold; or indirectly, as when a series of sorbent tubes or
sorbent solution samples are collected throughout the sampling
period and later analyzed.

Flame ionization detectors will respond to almost all hydrocarbons,
and have been used by our laboratory more than any other instrument
for monitoring the concentration of generated gases and vapors.
Flame ionization detectors are durable, reliable, simple to operate
and require very little maintenance. Depending upon the model
chosen, they are able to monitor a wide range of concentrations.

-14-


TABLE 7 (cont'd.)

Calculations for Vapor Pressure Generation
Thus, by adjusting Req, the final concentration, Cp, can be
produced with a manageable amount of dilution air.

Cq x Req

Ed? le cd
Cy

"

1000 x 50 - 50
100

Fa2

= 50 mL (1000 ppm - 1)
min 100 ppm
Fqg2 = 450 mL/min

This calculation is also used for generating dynamic concentrations
from cylinders or bags containing known, higher concentrations.

-35-


TABLE 5

Sources of Permeation Tubes

Analytical Instruments Development, Inc.
250 South Franklin Street
West Chester, Pennsylvania 19380

Metronics Associates, Inc.
3201 Porter Drive

Stanford Industrial Park
Palo Alto, California 94304

Tracor Instruments
Tracor Incorporated
6500 Tracor Lane,
Austin, Texas 78721

Varian Instrument Service Center
670 East Arques
Sunnyvale, California 94086

TABLE 6

Sources of Vapor Pressure Information

Vapor Pressure of Organic Compounds
T. Earl Jordan

Interscience Publishers, Inc.

250 Sth Avenue

New York, New York

52-11414

The Vapor Pressure of Pure Substances
T. Boublik, V. Fried, E. Hola
American Elservier Publishing Company
52 Vanderbilt Avenue

New York, New York 10017

72-97420

Handbook of Chemistry and Physics
Robert C. Weast, Editor

Chemical Rubber Company Press
18901 Cranwood Parkway

Cleveland, Ohio 44128

13-11506

Section D., pp. 162-188

SSi=


TABLE 11

Some Sources of Air Cleaning Equipment

Aadco, Inc.
2257 Lewis Avenue
Rockville, Maryland 20852

Bolston, Inc.

Box C

703 Massachusetts A
Lexington, Massachusetts 02173

Puregas Heatless Air Drier
Curtin Matheson Scientific
4220 Jefferson Avenue
P.O. Box 1546

Houston, Texas 77001

Matheson Gas Products

P.O. Box 85

932 Patterson Plank Road

East Rutherford, New Jersey 07073

-40-


SYSTEMS USED FOR VAPORS

Dynamic Vapor Pressure Systems

There are several systems which may be used to generate known
concentrations of vapors. (10) The system most often used in our
laboratory is the dynamic vapor pressure generation system.
Basically, this system consists of a reservoir of the contaminant
liquid held at some elevated temperature, e.g., 35°C, over which a
small stream of air or nitrogen is passed, e.g., 5 - 100 mL/min,
into a tube type condenser maintained at some subambient temperature
(Figure 4.a. & b.). Thus, the stream from the condenser is
saturated with the contaminant vapor at the lower temperature. By
knowing the vapor pressure of the substance at that temperature, it
is possible to calculate the concentration in mole fraction of the
substance in the stream (Table 6). Then it is possible to determine
the exact amount of dilution air needed to produce the desired
concentration of the substance (Table 7).

A dynamic vapor pressure generation system offers three variables
which may be used to change the concentration: the contaminant
stream flow rate, the dilution air flow rate, and the condenser
temperature. Within certain limits, it is possible to produce a
wide range of concentrations of many contaminant vapors. The limits
for dilution air are: maximum; the amount available and the amount
the mixing and sampling chambers can allow to pass through at
approximately atmospheric pressure; and minimum, the smallest amount
needed for the test or experiment. The limits for contaminant
stream flow are: maximum; the greatest amount (normally about

100 mL/min) that can be passed through the condenser without
changing the temperature; and minimum, the smallest amount that can
be accurately measured. The limits for condenser temperature are:
maximum; at least 2°C under ambient air and minimum, the lowest
temperature at which the substance exhibits a vapor pressure and/or
the lowest temperature that can be maintained without the coolant
liquid freezing. For most situations it is easier to change the
concentration by changing the dilution air flow rate than by
changing the temperature because of the time required for the vapor
to reach equilibrium after a temperature change.

Dynamic vapor pressure systems can be used for generating a wide
range of concentrations (e.g., 1 - 1000 ppm) of most organic
solvents and many other compounds (see Table 8).

Drawbacks to using this system include the cost of the subambient

temperature control device and the fact that for some substances,
vapor pressure information is difficult to find, or very limited,

-8-


10.

FIGURES

Static Generation System Components
Cylinder Dilutions System
Permeation Tube Generation System

A. Vapor Pressure Generation System
B. Vapor Pressure Generation System

Syringe Pump Generation System

Diffusion Tube Generation System

Ozone Generation System

Air Purification System and RH Generation System
Soap Film Flow Meters

Orifice Flow Diagram

-50-


TABLE 18

Sources of Constant Temperature Bath-Circulators

Forma Scientific, Inc.
P.O. Box 649
Marietta, Ohio 45750

Neslab Instruments
871 Islington Street
Portsmouth, New Hampshire 08801

Lab-Line Instruments, Inc.
Lab Line Plaza
Melrose Park, Illinois 60160

B. Barun Instruments
805 Grandview Drive
South San Francisco, California 94080

GCA Precision Scientific
3737 West Cortland Street
Chicago, Illinois 60647

Blue M Electric Company
138th & Chatham Streets
Blue Island, Illinois 60406

Fisher Scientific Company

711 Forbes Avenue
Pittsburgh, Pennsylvania 15219

=h8=


CONTAMINANT

STREAM
PURIFIED AIR
FROM
DILUTION AIR CYLINDER GENERATOR
STREAM
PURIFIED AIR |MOLECULAR
FROM VENT SEIVE
GENERATOR MIXING
CHAMBER
SAMPLING BULB
OR MANIFOLD
H>0 OUT=|( )|CONDENSOR
1
CONSTANT ROTAMETER |IOR MASS
TEMP. BATH FLOWM
CIRCULATOR pel in J
CJ
HEATING NEEDLE
MANTLE VALVE

VAPOR PRESSURE GENERATION SYSTEM

FIGURE 4a


LAMP

BUSHING [J

L

c QUARTZ TUBE

il SLOTTED LAMP TUBE

IL.

COLLAR ———— —
/ ] UV LAMP MOVABLE
LAMP SLEEVE
BASE
AIR FROM
AIR PURIFICATION
SYSTEM
| POWER
SUPPLY
\ GENERATOR
x
ee ee © ee
SAMPLING MANIFOLD

AIRFLOW MEASUREMENT
AND CONTROL

OZONE GENERATION SYSTEM
FIGURE 7


TABLE 16

Manufacturers of Manometers

Dwyer Instruments, Inc.
P.O. Box 373
Michigan City, Indiana 46360

The Meriam Instrument Company
10920 Madison Avenue
Cleveland, Ohio 44102

6


A needle valve or other fine metering valve may be used as a
variable limiting orifice or as a critical orifice if precautions
are taken to prevent the stem from moving from its set position.
Any vibration or motion in the area of the valve should be avoided.
Because of the small opening in any orifice, care must be exercised
to keep dust from entering to change or stop the flow.

When using an orifice to measure or control the flow rate, it is
necessary to provide a means of indicating the amount of pressure
drop across the orifice and ahead of the orifice. For lower
pressure measurements, the most satisfactory means to measure it is
with a manometer. Manometers which are designed to read pressures
in inches of water are available which can detect changes in
pressure as small as 0.01" H20 (Table 16). Manometers may also be
used to monitor the pressure inside a generation system, which
should be maintained at just above (0.1 - 0.8" H90) atmospheric
pressure.

In addition to manometers, there are diaphragm activated gauges
available which are capable of measuring a wide range of pressures
including the low inches of water range. These gauges provide a
direct readout on a dial, and may be permanently mounted in a panel
if desired. This type gauge can also be electronically activated
and coupled with a flow controller to provide unattended
compensation for line pressure changes.

Temperature - Measurement and Control

For generation systems utilizing the vapor pressure of a liquid or
permeating tube, it is necessary to be able to measure and provide
control of the temperature within close tolerances (+ 0.2°C) in
order to obtain a uniform output of the contaminant.

Many devices are available which will provide the controlled level
of power necessary for those systems which operate at above ambient
temperatures. For example, heating mantles for flasks, controlled
by a variable autotransformer, will provide a constant elevated
temperature for liquids in glass containers. Also, hot plates,
heating tapes, strip heaters and immersion heaters may be used so
long as adequate control is provided. Some experimentation will be
necessary to determine the voltage required to maintain a certain
temperature in a vessel or circulating bath, but once this is
established the temperature will remain constant. The different
heating devices and variable autotransformers are available from
most laboratory supply houses at rather modest cost (see Table 17).
Systems which must operate at subambient temperatures should be
purchased as a unit for best results. Constant temperature

=19=


TABLE 8

List of Some Substances Generated by the
Vapor Pressure Method and Range of Concentrations

Substances Generated
Trichloroethylene
Methylene Chloride
Toluene

Acetone

Hexane

PCB's

Styrene

Benzene

MEK

Epichlorohydrin
Trichloronitromethane
DBCP

2-Nitropropane
Carbon Disulfide
Methyl Alcohol

Mercury

-36-

Concentrations, ppm

10

eS

25

43)

250

0.5

25)

10

25

300
250
300
1000
2000
10
200
200
500
25
10
10
10
50
400

-5 (mg/m3)


TABLE OF CONTENTS

ane.

Basic Systems Used - Static and Dynamic. ............-.-. 2
A. Static Systems ............-.-220-2.2250088- 2

B. Dynamic Systems... 2... 2-1 1 ee ee ee ee 4
Systems Used for Vapors. ........-.+++++++++s+6-28 8
A. Dynamic Vapor Pressure Systems. ....-+-+--+++ ++ +es 8

B. Syringe Pump Systems... .... 2... 2 1 ee eee eee 9

(Hy Diffusion Tube Systems. .........2.2..4.4..... #10
Other Generation Systems we ee ee ee ee eee ee ee ee ee «10
Systems Used in Conjunction with Basic Generation Systems... ... 12
Air Purification Systems 2 2. ee eee eee eee)
Humidity Generation, Control, and Measurement. ......... 13
Concentration Monitoring Systems. ................ 14
Air Flow Rate Measurement and Control. ..........2.2.2. «15
Temperature Measurement and Control See de ee ee ee 18
Laboratory Ventilation and Safety Considerations. ........ 19
GIR 6 og 6 4 BS eo eee Oo eK eH OK a AO
References ees ec seen ee ase eat
Mawes ocsoa boas ocon docs oacoo aos oo coe oo

Figures... ee 50


TABLE 4

Calculations for Permeation Tube Generation Systems

Permeation Tubes can be used to generate concentrations of
contaminant gases and vapors due to the permeability of fluorocarbon
resins to these gases and vapors. (8) The degree of permeability

is given by the permeability coefficient P.

Wo x Te

Po eet

Ap xT, x P

where We = weight of the permeating contaminant

Teas film thickness
Ag = area of film
Te = contact time
Pee pressure difference

The permeation (or diffusion) of the contaminant gas or vapor
increases non-linearly with temperature, pressure and surface
contact area. The permeation decreases with the wall thickness of
the tube and the density of the fluorocarbon resin used to make the
tube.

To calculate the concentration of contaminant from the permeation
tube, the permeation rate, expressed in ng/cm/min, of the
contaminant must be known. This information can be obtained in two
ways:

1) from the manufacturer's calibration

2) self - calibration. -- After a 2 - 5 day equilibration
period in the constant temperature oven, the permeation
tube should be weighed in a periodic manner, at preferable
constant time intervals. The key here is to maintain the
permeation tube at a given temperature. A plot of weight
versus time should be linear and the slope of the line will
be the permeation rate.

Once the permeation rate is accurately known, the concentration that
is generated is given by:

=29=


Photoionization detectors will respond to a wide variety of organic
compounds and some inorganic compounds. The range of concentrations
that can be monitored include most that would be encountered in
industrial hygiene monitoring situations. This detector, although
relatively new, has been found to be durable, reliable, simple to
operate, and relatively maintenance free.

Infrared analyzers may be used to monitor a single contaminant
continuously or may be used to monitor mixtures of contaminants
intermittently. Many contaminants have characteristic absorption
bands in the infrared region of the spectrum, and the amount of
infrared radiation absorbed by a sample can be directly related to
the concentration of the contaminant. Some infrared analyzers are
equipped with a specific wavelength filter and can be used for that
substance only, while some have a variable wavelength guide and will
monitor for several components by scanning the spectrum.

Although less frequently used in this laboratory, UV visible
absorption monitors with long path cells are available to monitor
some contaminants.

Chemiluminescent detector systems for ozone, ammonia and oxides of
nitrogen are available and are used by this laboratory. This
detector principle has been found to be accurate and stable and is
especially useful when monitoring very low levels of these
contaminants.

Other monitoring systems are available for both specific and general
applications. The choice of which system to use for a particular
application depends upon many considerations: availability, cost,
range and level of response, accuracy, ease of operation, frequency
of use - to mention a few. Also important to consider is the
required sample flow rate into the monitor when the generation
system is set up. While most monitoring systems function quite
satisfactorily with a sample flow rate of 1 Lpm or less, some may
require up to 5 Lpm or more, which would seriously limit the
usefulness of a concentration generation system designed to produce
only 4 Lpm total flow, for example.

Air Flow Rate Measurement and Control

The importance of measuring and controlling the quantity of air
flowing in a generation system cannot be overemphasized. Small
variations or errors in air flow can produce larger errors in the
final concentration of contaminant produced. Knowing the exact
amount of air placed in a static system is also very important to
both the accuracy and reproducibility of the concentration.

pls =


REFERENCES

1.

10.

Roper, C.P., An Evaluation of Perchloroethylene Detector Tubes.
Amer. Ind. Hyg. Assoc. J. 32:12 (December 1971) pp. 847-849.

. Ash, R.M. and J.R. Lynch, American Industrial Hygiene

Association Journal 32, 490 (1971).

. Willy, Maurine A. and McCammon, Charles S., Jr., Evaluation of

Portable, Direct-Reading Hydrocarbon Meters. HEW (NIOSH)
Publication No. 76-166.

. Hill, R.H., Jr., McCammon, C.S., Jr., Saalwaechter, A.T., Teass,

A.W., and Woodfin, W.J., "Gas Chromotographic Determination of
Vinyl Chloride in Air Sampler Collected on Charcoal,” Journal
of Analytical Chemistry, Vol. 48, Page 1935, August 1976.

. McCammon, Charles, S., Jr., and William J. Woodfin: Evaluation

of Portable, Direct-Reading Mercury Vapor Meters. HEW (NIOSH)
Publication No. 76-167.

Saalwaechter, A.T., McCammon, C.S., Teass, A.W., and Woodfin,
W.J., "Toluene Breakthrough Studies on Activated Charcoal
Sampling Tubes". Presented at the American Industrial Hygiene
Conference, June 1-6, 1975, Minneapolis, Minnesota.

"NIOSH Manual of Analytical Methods" HEW (NIOSH) Publication
No. 77-157. Second Edition. U.S. Government Printing Office,
Washington, D.C. 1977. P&CAM No. 153, 154.

. Methods of Air Sampling and Analysis

Katz, Morris, Editor
Intersociety Committee
R.R. Donnely & Sons Co.
Crawfordsville, IN

L ce No. 77-6826 pp. 16-26

“NIOSH Manual of Analytical Methods" HEW (NIOSH) Publication
No. 77-157A. Second Edition. U.S. Government Printing Office,
Washington, D.C. 1977. P&CAM No 209,

Nelson, Gary 0., "Controlled Test Atmospheres - Principles and
Techniques", Ann Arbor Science Publishers, Ann Arbor, MI 48106,
1971, LCC No. 73-141231.

=


Rotameters are frequently used to measure and monitor the flow rate
of air. They are relatively inexpensive and are available to cover
an almost infinite range of flow rates. For flow rates of less than
200 mL/min however, rotameters have been found to provide only
marginal performance even when calibrated frequently. For greater
flow rates, performance is satisfactory, provided that the incoming
air is free of particles or oil mist which could become attached to
the float or walls of the tube, thus changing its calibration. For
applications requiring absolute measurement of the air flow,
rotameters should be used only as indicators of flow and not primary
flow measurement devices unless the air is clean and dry, the
pressure carefully controlled, the temperature constant and the
calibration checked each time the flow rate is changed.

Electronic mass flow meters are available in a wide range of flow
rates and have been found to be especially useful in the low flow
rate range (500 mL/min and less) where the performance of rotameters
is marginal.

Mass flow meters are expensive, but may prove to be a good
investment since they are capable of accurately measuring extremely
low flow rates and are not affected by normal variations in ambient
temperature and pressure. The flow sensor may be located in a
remote area, while the readout can be conveniently placed near the
operator. Most mass flow meters are available with recorder output
so that a permanent record of the flow rate may be provided. Mass
flow meters should be individually calibrated with the gas (air,
N2, etc.) to be used and under the same conditions (temperature,
pressure) as they will be used (Table 14).

Another very useful device for measuring gas flow is the calibrated
orifice. There are two basic conditions under which the orifice can
be used: critical and limiting (see Figure 10). The critical
orifice is used primarily when a single flow rate is needed.
Critical flow is obtained by applying critical pressure to one side;
that is, the pressure drop across the orifice is 0.5 atmospheres or
greater. Under this condition, the flow rate through the orifice
varies directly with any pressure drop ahead of the orifice (i.e,
sampling devices). Critical orifices are often used for laboratory
sampling when several samples are collected simultaneously. It is
possible to obtain orifices from manufacturers in a variety of sizes
or they may be made in the laboratory if the material and equipment
is available (Table 15).

Limiting orifices are the same as critical orifices in appearance,
but are used at lower pressures as flow restrictors. Under a given
set of conditions, i.e., VY P ahead of the orifice and pressure to or
from the orifice, reproducible flow is obtained (Figure 10).

-17-


TABLE 4 (cont'd.)

Calculations for Permeation Tube Generation Systems

Rp-L
Cp =
Fa
where Cp = concentration, permeation tube (ng/L)
Ro = permeation rate (ng/cm/min)
L = length of permeation tube (cm)
Fq = flow rate of dilution gas (L/min)

Obviously, the concentration is convertible to any desired units by
employing the appropriate conversion factors.

Beside using the above calculation to determine the concentration
generated by the permeation tube at a given temperature and volume
of dilution gas, it is often wise, always recommended and sometimes
necessary to determine the concentration using some independent,
accurate and reliable analytical technique.

If further dilution of the contaminant stream is needed, three
pieces of information must be recorded:

the the concentration of the first-stage of the contaminant
stream, Cp

Ze the rate, Rep at which the contaminant stream with
concentration Cp is being pumped into the second dilution
chamber and

3. the flow rate volume of the additional dilution air. Fy2
Then
Cp xX Rep
Cy x
Rep + Faz

Calculations for Permeation Tube Generation Systems

where Cp = concentration ng/L
Rep = contaminant stream flow rate L/m
Fa2 = flow rate of dilution gas L/m
Ch = (new) desired concentration ng/L

=30-


SYSTEMS USED IN CONJUNCTION WITH BASIC GENERATION SYSTEMS

Air Purification Systems

When a dynamic generation system is used, large quantities of clean
air are required. It is usually impractical to supply this air by
purchasing it in cylinders, but most facility compressed air
supplies are contaminated with oil and water vapor and are subject
to day-to-day variation depending upon ambient conditions. It is
therefore recommended that air for the dynamic generation system be
supplied by an air purification system, or a clean air generator.

The air purification system most often used by our laboratory is one
which cleans up the in-house compressed air by removing most of the
oil and water vapor. It consists of a series of components,
beginning with an in-line cartridge containing a desiccant and
molecular sieve which removes gross amounts of oil and water vapor,
a small tube furnace operated at 1000 - 1200°F to burn the
hydrocarbons to C02, a condenser coil to cool the air to ambient
temperature, a charcoal filter to absorb any remaining organics, and
a micron pore size pleated membrane filter to collect any
particulates in the stream (Figure 8).

The pressure on the air line is maintained at about 15 psig by a
pressure regulator at the inlet and this will provide about 25 Lpm
at 6 - 8 psig at the outlet. A variable voltage controller is used
to maintain the tube furnace at the desired temperature which is
monitored by a thermocoupler readout meter (pyrometer) having an
ANSI Type K thermocouple located near the outlet of the tube. The
condenser coil is a jacketed tube through which a controlled amount
(2-5 Lpm) of tap water is passed.

This system was assembled in our laboratory using available
components, and requires periodic replacement of the various filters
and regeneration of the charcoal bed. This system may also be used
to clean ambient air supplied by a diaphragm, piston, or vane type
laboratory air compressor.

Another system for cleaning facility compressed air has been used,
and is similar to the system described above except that the tube
inside the furnace is packed with a pelletized catalyst which will
convert the hydrocarbons to COj. The temperature required to
Maintain the quality of the air is 300 - 400°C depending upon the
quantity of air needed and the size of the catalyst bed. This
temperature may be determined experimentally by monitoring the
output with a hydrocarbon analyzer.

E12=


DILUTION AIR STREAM
PURIFIED AIR FROM

GENERATOR
VENT
MIXING SAMPLING
CHAMBER BULB OR
MANIFOLD
MIXING
BULB
INJECTION
BULB
CONTAMINANT STREAM
PURIFIED AIR
MOTOR- DRIVEN FROM GENERATOR
> ISYRINGE PUMP |

%

MASS ————_=
FLOWMETER

ROTAMETER

SYRINGE PUMP GENERATION SYSTEM
FIGURE 5


Dynamic Systems

Dynamic generation systems provide a continuous supply of the
contaminant gas or vapor at a known concentration over a period of
time. Normally, dynamic generation systems consist of two flowing
streams of gas: the contaminant stream and the dilution air. By
varying the relative amount of gas flowing in each stream, it is
possible to generate a range of concentrations. These systems can
produce the desired concentration continuously for up to several
months if necessary, with only minor maintenance required. The
total gas flow through a dynamic system will depend upon many
factors, but the normal flow is between 1 - 12 Lpm. It is possible
to produce mixtures of two or more contaminants with a dynamic
generation system, and it is relatively easy to change from one
contaminant to another. It is also possible, by installing other
dilution air systems and mixing chambers, to produce proportionally
lower concentrations of the generated substance simultaneously from
the first concentration.

Dynamic Generation Systems - Components

The initial contaminant stream can be produced in several different
ways, but the major components of a dynamic generation system are
the same for each. These common components will be discussed first,
followed by a discussion of the individual contaminant producing
systems.

Two pieces of glassware, the mixing chamber and the sampling
manifold, are necessary for all dynamic generation systems. Two
other pieces of glassware, the liquid reservoir and the condenser,
are necessary for some dynamic systems. Our laboratory uses some
commercially available components for these as well as some custom
fabricated parts (Figure 1).

The mixing chamber can vary in size and shape yet still perform its
function, which is to receive the concentrated contaminant stream
and the dilution air stream and mix them to provide a homogenous
output to the sampling manifold. The mixing chamber our laboratory
uses was fabricated from a long-neck, 1 L round bottom flask. An
outlet port with an 18/9 ball and socket joint was installed in the
bottom of the flask and a side-arm port was installed in the neck
section to accommodate the second incoming gas stream (Figure 3).
Two or more side arm ports may be installed to accommodate other
contaminant streams if the production of a gas or vapor mixture is
anticipated. All ports should have ball and socket joints in order
to facilitate assembly and disassembly and to provide some
flexibility to the system.

The sampling manifold's function is to provide a convenient place to
sample the generated concentration, and its size and shape may vary

24>


TABLE 1 (cont'd.)

Static Concentration Calculations

If further dilution of the static concentration is required, three
pieces of information are required.

a The concentration of the contaminant gas as determined by
one of the above methods, Cy

20 The volume of the contaminant gas that is drawn from its
container, Vo

3H The volume of additional dilution air, Vg
The new concentration is handled in a straightforward manner:

Ve x Co
= Cy

Ve + Va

Where Cy is the new concentration of contaminant. Cp has the
same units as Co.

ae


TABLE 1 (cont'd.)

Static Concentration Calculations

Convert the volume of the contaminant gas to mg:
e.g. convert ul to mg
Vo x MW, x 103
= We
Vx 106
where: We = Weight of contaminant in mg.

Ve = Volume of contaminant in uL

V = Molar volume in liter/mole

MW,= Molecular weight of the contaminant
in grams/mole

103= Conversion factor mg/gm
106= Conversion factor uL/L
This equation can be reduced to

Ve x MW,
= We

V x 103

A direct conversion of ppm to mg/M3 :

Cm = Cp x MW, x Pg x Ty

Vx Po x Ts

Cm = conc. in mg/m3

Rho


INTRODUCTION

The Occupational Safety and Health Act of 1970 authorizes the
Secretary of the Department of Health and Human Services to
prescribe regulations requiring employers to measure, record and
make reports on the exposure of employees to substances which may
endanger their safety or health. The individuals responsible for
making these measurements of exposure will often find it necessary
to have available a reliable method for generating known
concentrations of the substances they are required to measure, in
order to calibrate the sampling instruments or verify the sampling
methods used in the workplace.

The Monitoring Research Section of the Monitoring and Control
Research Branch has been generating known concentrations of gases
and vapors for many years (1,2), in order to develop or improve
sampling and analytical methods and to evaluate the performance of
portable pollution monitors used in the workplace. Because of the
great variety of gases and vapors found in the workplace, no one
method or system for generation could be applicable to all, yet all
have some common components and some universal techniques. For the
most part, equipment needed to generate known concentrations of
gases and vapors is not commercially available as a complete unit,
but must be assembled in the user's laboratory with components from
various sources.

Several publications (3,4,5,6) have described one or more of the
generation systems used in the Monitoring Research Section's
laboratory, but these were brief and lacking in detail. The purpose
of this report, therefore, is to describe, with as much detail as is
practical, all the gas and vapor generation systems that have been
used in this laboratory, so that the methods, equipment, and
techniques which have evolved over the years will be available in
summary form.


STMaTMCS HO BisA

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If the ambient air in the laboratory is relatively clean and not
subject to outside contamination or day-to-day fluctuations, a
simple system consisting of an oil-less compressor followed by a
charcoal trap and a particulate filter may be all that is needed to
produce air of satisfactory quality for most experiments.

Commercial pure air generators are also available and have been used
in our laboratory (see Table 11). These devices are compact and
require little maintenance. Various sizes are available to provide
the quantity of air needed.

The air purification system that one chooses will depend upon the
quantity of air expected to be used, the availability and quality of
house air, space availability, the amount of money available to
invest, and the type and concentration of gas or vapor to be
generated.

The air produced by any of the above methods may contain some
impurities, but it will be within acceptable limits for the majority
of applications and it will be consistent over a long period of time.

Humidity Generation, Control, and Measurement

Air from most air purification systems is normally dry, (i.e.

10% RH) so it is necessary to add water vapor to the dilution air
stream in order to produce concentrations of gases and vapors with a
realistic relative humidity range, and to test methods over a range
of humidities. Since realistic range may vary between 10% and 80%
or more relative humidity, it is necessary to be able to control and
measure the amount of water vapor added.

A very simple system for generating humidity in the air has been
used by our laboratory for several years and has proven to be
reliable and consistent. This system consists of a container of
water over which varying amounts of the dilution air stream may be
passed (see Figure 8). The air stream is split and 2 control valves
are installed, making it possible to pass as much or as little of
the air across the water as is necessary to produce the desired
relative humidity.

Depending upon the air flow rate and the humidity level desired,
additional containers may be added and/or the water may be heated in
order to increase the amount of water vapor available. Because of
condensation, heating makes it more difficult to control the level
of humidity, and should be used as a last resort. This system
eliminates the problems caused by varying pressure drop and
entrainment of water droplets associated with those systems which
require bubbling the air through water.

=13=.


with cross contamination, wall reaction, and leakage than with
flexible containers. The disadvantage is that only a small portion
of the total contents can be removed without changing the pressure
and thus changing the concentrations. This method is recommended
when only a small amount of a generated atmosphere is needed, (e.g.,
for injecting a few mL into a gas chromatograph for calibration).

Some advantages of static generation systems are their ease and
simplicity of use and the fact that the equipment needed is not
expensive to purchase. It is possible to make mixtures of two or
more contaminants in a single bag. Known concentrations may be
generated in the laboratory and then transported to a remote
location for calibrating an instrument.

The major disadvantage of statically generated concentrations is
that the volume of sample one can generate is limited by the size of
the bag available. Many substances with a TLV of 25 ppm or less
require sample volumes of 50 - 100 L in order to satisfy the
analytical method for that substance. It would, therefore, be
impossible to produce a statistically valid set of test data from a
single bag.

Also, with some substances, there can be interaction with the walls
of the bag. This will result in loss of the concentration with time
or, in some cases, a “memory” for the substance so that each time
the bag is filled with air the contaminant is generated anew.
Whenever these situations occur, the bag is no longer useful and
some other method of generation should be employed. Information on
the inertness of some of the bag materials is available (Table 3),
but for new substances not previously tested, some trial and error
may be required to determine whether or not it will be suitable for
a particular bag material.

For static generation systems, the primary limitation is the volume
of the container used, so the total volume needed for a particular
experiment or within a given time must be considered. Bags which
are purchased from stock sizes available from fabricators usually
range in size up to 90 - 100 L capacity. Larger sizes may be
fabricated on order or made in-house. Most rigid containers of
glass or stainless steel are of only a few liter capacity, yielding
only 10 - 20 mL useful volume per liter. Rigid plastic containers
of 20 - 30 L capacity have been used, but, for the more reactive
compounds, there could be a reaction with or loss upon the walls.
If large chambers made of stainless steel or glass are available,
these can be treated the same as smaller ones.

S35


bath-circulators equipped with heaters and refrigeration units are
available and will provide a controlled environment in the range of
-20° to +70°C (Table 18). While these devices represent a
substantial investment, they are almost indispensable for generating
very low concentrations of substances which have a high vapor
pressure. The reservoir on these baths may be used directly or the
liquid may be circulated through jacketed vessels to provide the
controlled temperature in a remote area.

Temperature measurement may be done in several different ways,
depending upon the temperature range measured, the degree of
accuracy and readability required, and whether or not controllers
and/or recorders are needed. Three basic types of temperature
measurement devices are generally available: liquid in glass,
bimetallic dial type, and electronic.

Glass thermometers may be used for most applications up to about
400°C. Accuracy is easy to determine by ice bath or by boiling
water and readability may be as good as +0.1°C or better,
depending upon the scale selected. Most laboratory supply houses
offer a variety of glass thermometers in ranges to cover most
applications.

For situations where there is a possibility of the glass thermometer
being broken or where readability is a problem, metal thermometers
using a bimetallic sensor and a dial readout are a good choice.
Their range is up to 400°C and their accuracy is + 1% full scale
for most models although some are accurate to + 0.5% full scale.
Bimetallic dial type thermometers are readable to within + 0.05°C
and, depending upon the size dial selected, may be readable from
several feet away. These thermometers are also available from most
laboratory supply houses.

For higher temperatures, for remote monitoring, or for controlling
and/or recording temperature, electronic devices such as
thermocouples or thermistors with the appropriate readout device are
available. The cost of electronic temperature devices range from
modest to expensive. For example, a single thermocouple readout
meter may be obtained for about $50, whereas a digital meter capable
of reading several thermocouples and with recording capability may
cost several hundred dollars. The unit chosen will depend upon the
user's needs and resources. Thermocouples capable of measuring
temperatures up to 800°C or higher are available from laboratory
supply houses or directly from the manufacturer.

Laboratory Ventilation and Safety Considerations

Before any gas or vapor generation system is installed in the
laboratory, provisions must be made for venting the unused gases out

=19=


GAS AND VAPOR GENERATING SYSTEMS
FOR LABORATORIES

by
William J. Woodfin

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Centers for Disease Control
National Institute for Occupational Safety and Health
Division of Physical Sciences and Engineering
Cincinnati, Ohio 45226

August 1984


alg)

TABLE 13

Manufacturers of Various Detectors

Manufacturers of FID'’s:

Beckman Instruments Inc.
Process Instruments Division
2500 Harbor Boulevard
Fullerton, California 92634

Century Systems Corporation
P.O. Box 133
Arkansas City, Kansas 67005

Mine Safety Appliances Company
600 Penn Center Boulevard
Pittsburgh, Pennsylvania 15235

Melay Labs, Inc.
6715 Electronic Drive
Springfield, Virginia 22151

Health Consultants, Inc.
100 Tascor Drive
Stranghton, MA 02071

Antek Instruments,Inc.
6005 N. Freeway
Houston, Texas 77076

Analytical Instruments Development Inc.

Route 41 & Newark Road
Avondale, Pennsylvania 19311

==


In addition to these four basic parts, it is necessary to provide
glass elbows and adaptors to facilitate convenient arrangement of
the components, and a metal rod frame with clamps to support the
assembly.

Using a dynamic generation system offers several advantages over
static systems. Most obvious is the fact that the contaminant is
continuously produced in the quantity needed, so that long term
tests may be completed without interruption. Dynamic generation
systems also provide the flexibility necessary to meet almost any
requirements of concentration range from low ppb to high ppm. The
same component may be used for vapor pressure generation, permeation
and diffusion tube generation, syringe pump generation and gas
mixture generation, with only small modifications or additions
required to change from one system to another.

The major disadvantage of the dynamic generation system is the cost
of the components. Also, a certain amount of laboratory bench space
must be dedicated to this equipment. It is also necessary to
provide a supply of clean air continuously, and this may require an
additional investment for air cleaning equipment. The type of air
cleaning equipment and the situations under which it is needed will
be discussed later in this report.

Cylinder Dilution

For the generation of most gaseous contaminants, a relatively simple
dynamic dilution system may be used. Cylinders of pure gases or
mixtures are generally available, and these can be diluted to the
desired concentrations so that with a single cylinder of a gas a
range of concentrations may be generated (Figure 2). Dynamic
generation systems for cylinder dilutions require some means to
measure and control the contaminant and dilution air flow, plus a
mixing chamber and sampling chamber. Static concentration may also
be generated from gas cylinders by injecting or metering the
required amount into a certain volume of air. See calculations
(Table 1).

Permeation Tubes

For some gases, permeation tubes are available, and they offer some
advantages over cylinders. This is especially true for corrosive or
reactive gases which require special cylinders and regulators, and
which may remain stable for relatively short periods of time. For
these gases, e.g., Cla. NH3, HF, NOz, permeation tubes provide

a source which is relatively safe and easy to handle and which will
provide a stable output over a range of concentrations for up to
several months. (8)

-6-


depending upon the user's needs. The sampling manifold our
laboratory uses was fabricated from a 60 mm o.d. glass tube about
300 mm long. It has an inlet and outlet equipped with 18/9 ball and
socket joints and seven sampling ports 4 mm i.d. along a plane. The
sampling ports are about 30 mm long and 30 mm apart (Figure 2).

This design facilitates the attachment of sampling devices to the
manifold. Other designs could work just as well, and each user's
needs will determine which design is best.

Seven sampling ports are suggested because it is often necessary to
collect six samples simultaneously in order to validate a sampling
method, and the seventh port is used to connect the sample line of
the continuous monitor or other monitor or independent sampling
method being used for verification of the concentration being
produced.

Commercially available condensers (Allihn ball type 600 mm long with
35/25 ball and socket joints) are used, in conjunction with a
constant temperature bath-circulator, for dynamic vapor pressure
generation systems. These condensers can also be used with
permeation tube generation systems if a commercially available
permeation tube generation device is not available. These
condensers may be installed either vertically or horizontally and
maintain the outgoing contaminant gas stream of up to 100 mL at the
set temperature of the circulated liquid. A glass thermometer or
thermocouple may be installed near the exit of the condenser in
order to monitor the temperature of the outgoing contaminant gas
stream.

For vapor pressure generation systems, the condenser temperature is
always maintained at least 2°C below ambient in order to prevent
any further condensation of the vapors produced. For permeation
tube generation systems the temperature may be above or below
ambient, depending upon the permeation rates desired.

For vapor pressure generation systems, a reservoir is necessary to
contain the liquid contaminant from which the vapor is produced.
This reservoir is commercially available as a round bottom flask
(250 - 1000 mL size) with a 35/25 ball and socket top fitting and a
side arm port for introduction of the metered contaminant gas
stream. A 12/5 ball and socket joint may be added to this side arm
to facilitate disassembly, filling and cleaning. Impingers of
bubblers may also be used in this capacity, singularly or in series
(Figure 4a and 4b).

In many cases it is necessary to heat the liquid reservoir in order
to assure saturation of the outgoing stream with the vapor being
produced. Adding a heating mantle and power controller will allow
the operator to maintain the reservoir at 35°C or whatever other
elevated temperature is necessary to assure saturation of the stream.

-5-


CONTAMINANT

STREAM
DILUTION AIR STREAM PURIFIED an
FROM
|
MOLECULAR
SEIVE

SAMPLING BULB

OR MANIFOLD
H20 OUT *~|( )IcoNDENSOR
i
—
H,0 IN
EVAPORATORS ROTAMETER' |
CONSTANT OR}
TEMP, BATH FLOWMETER li
CIRCULATOR 4 NEEDLE
HoH VALVE
TEMP.

WG CONTAMINANT
BATH i———ef4Quid

VAPOR PRESSURE GENERATION SYSTEM
FIGURE 4b


The volume of 5.23 pL hexane when vaporized is:

Vp - 5-23 ML x 107? yL x 0.66 g/mL x 1 mole x 25.0 L = .001 L/min
Min uL 86.2 g mole

Therefore, the final concentration of hexane produced is:

1.0 L/min
1000 ppm x (1.0 + 0.001) L/min = 999 ppm

Bose


TABLE 1 (cont'd.)

Static Concentration Calculations

103 = Conversion factor, mL/L

MWe = Molecular weight of contaminant, gms/mole
P = Pressure of contaminant/system, mm Hg

Ty = Standard temperature, °K

Once the gaseous volume of the vapor is known, it can be treated the
same as a gas.

As an example, in order to make a bag of 500 ppm methylene chloride
(CHgCl2) the following facts are found: density of methylene
chloride, 1.3255 g/mL; molecular weight, 84.94; ambient temperature
24°C, ambient pressure, 745 mm Hg. In addition, some volume of
liquid to use must be assumed, normally 1 - 100 ug (0.001 - 0.1 mL)
depending upon the size of the static container and the concentration
desired. Then apply these numbers to the above formula.

0.05 mL x 1.3255 g/mL x 24.45 L/mole x 769 Torr x 298° x 103 mL
Ve =

84.94 g/mole x 745 Torr x 298°K
Vg = 19.46 mL gas

Thus, 0.05 mL (50 uL) methylene chloride liquid will produce 19.46 mL
methylene chloride vapor at room temperature. Next, determine how much air
will be needed in the bag to produce a concentration of 500 ppm.

Ve

y, = —

a c
Va = 19460 uL
500 ppm
Va = 38.92 L

If the other concentrations or different volumes are needed, it is
necessary to change the volume of liquid used, or to make an
additional dilution of the original concentration.

=96=


13_b.

TABLE 13 (cont'd.)

Manufacturers of Various Detectors

Manufacturers of Infrared Analyzers:

Foxboro Analytical
Box 5449
South Norwalk, Connecticut 06856

Sensors, Inc.
3908 Varsity Drive
Ann Arbor, Michigan 48104

Perkin-Elmer Corporation Industrial Division
Main Avenue
Norwalk, Connecticut 06856

Infrared Industries Inc.
P.O. Box 989
Santa Barbara, California 93102

CEA Instruments, Inc.
15 Charles Street
Westwood, New Jersey 07675

Bendix-Environmental and Process Division
1400 Taylor Avenue
Baltimore, Maryland 21204

=435


TABLE 1 (cont'd.)

Static Concentration Calculations

Cp = Conc. in ppm (uL/L)

MWe = Molecular weight of the contaminant
Ps = Pressure of the air sampled in mm Hg
T = Standard Temperature in °K

v = Molar Volume in liters/mole

Po = Standard pressure in mm Hg

Ts = Temperature of sample air in °K

For a Vapor the volume of gaseous sample generated from a given
liquid volume must be calculated.

This can be done using:

Vg = Vy xdxVxPyxTx 103

MW, x P x To

where:
Vg = Gaseous volume of contaminant in mL
v1 = Liquid volume of contaminant in mL
d = Density of contaminant in gm/mL
v = Molar volume at temperature, T in
liter/mole
Po = Standard pressured in mm Hg.
T = Temperature of contaminant/system, °K

W235 =


LIMITING FLOW {CRITICAL FLOW
REGION tREGION

FLOW
RATE

0.5 ATMOSPHERE
ZSP ACROSS ORIFICE WH,

ORIFICE FLOW DIAGRAM
FIGURE 10


Basic Systems Used - Static and Dynamic

Two different systems are used to generate known concentrations of
gases and vapors. They are static systems, or those that work by
having an appropriate volume of contaminant gas or volatile liquid
added to a known volume of air in a container; and dynamic systems,
or those that work by blending a concentrated stream of contaminant
gas or vapor with a dilution air stream to continuously produce a
desired concentration.

Static Systems. Static generation systems used in the laboratory
are usually containers with flexible walls (e.g. plastic bags) which
can hold a volume of air. A sufficient quantity of the contaminant
gas or volatile liquid is added to this air to make the desired
concentration (Table 1). After allowing some time for the contents
to come to equilibrium, samples may be taken from the bag or
analytical instruments may be attached to it.

In addition to bags equipped with an air inlet port with an on-off
valve and a septum, one needs a source of clean air, some means of
metering it into the bag, and syringes for injecting the contaminant
gas or volatile liquid.

For the most accurate statically generated concentrations, bags made
of relatively inert materials (e.g. Tedlar, Teflon, Mylar) are
commercially available or they may be fabricated by the user if the
stock materials and sealing equipment is obtained (Table 2). It is
necessary to carefully measure the volume of air and the quantity of
volatile liquid or gas contaminant put into the bag if accurate and
reproducible concentrations are needed. Tedlar or Mylar bags are
most often used by this laboratory because they are heat-sealable,
durable, and relatively inert materials. A dry test meter is used
to measure the volume of air metered into the bag, and gas-tight
syringes are used to add contaminant gases while microliter syringes
are used to add contaminant liquids (Figure 1).

Occasionally, situations arise when an approximate concentration
would be sufficient. For generating an approximate concentration,
nothing more than a plastic bag with a tube secured in the opening
is required. Air may be metered into it or the volume may be
estimated and the required amount of contaminant added. This method
cannot produce very accurate concentrations, but can be useful to
help you determine rather quickly whether or not an instrument is
responding to a particular substance and the approximate magnitude
of response.

In addition to bags, rigid containers of glass, stainless steel, or
plastic are useful for making known concentrations of gases and
vapors. The advantages of rigid containers are that the volume does
not have to be determined each time and there are fewer problems

he


Several methods and devices for measuring air flow are currently in
use. Each has some advantages and some shortcomings, but by careful
selection and application of these various methods, it is possible
to maintain the exact amount of air that is needed.

Any flow measurement device must be calibrated before use in order
to assure its accuracy. The most convenient method for calibrating
flow measuring devices is the soap film flow meter, or bubble meter
as it is sometimes known. These operate by permitting the air
stream to force a soap (film) bubble through a calibrated glass
tube. The time required for the soap film to travel through a
certain volume in the tube is an accurate measure of the air flow
rate. This is a primary standard by which other measurement devices
may be calibrated (Figure 9).

Soap film flow meters may be assembled in the laboratory, using
Class A burets which are usually available in any laboratory, or
they may be obtained in kits which include a stand and other
necessary components. For convenience, at least 2 sizes should be
obtained: small (100 mL) size for flow rates of 500 mL/min or less,
and a large (1 - 2 L) size for flow rates up to 10,000 mL/min.

Other sizes are available but these will be adequate for most
situations. Soap film flow meters may be used to measure a stream
of air under vacuum or with positive pressure. For measuring
streams with positive pressure, the soap solution is placed in a
rubber bulb and the air enters by a side port whereas for streams
under vacuum the tube is inverted and the soap solution is held in a
beaker (see Figure 9). Soap film flow meters are also useful for
calibrating sampling devices so that the total volume sampled may be
determined very accurately. In addition to the buret and soap
solution, a stopwatch is necessary and a small calculator is very
useful and time saving, especially in situations where the flow rate
is being adjusted to a given level.

Secondary standards, such as wet test meters or dry test meters, are
very useful devices for accurately measuring the flow rate of an air
stream or for measuring the total amount of air going into a static
system. Wet test meters must be leveled and the water level checked
frequently. For those meters equipped with a manometer for
measuring the pressure drop across the meter, the manometer must be
checked periodically for fluid level. For both wet and dry meters,
it is important never to exceed the flow rate or pressure drop
specified by the manufacturer. If a wet test meter is used to
measure the air going into a system where dry air is desired, a
desiccant cartridge should be placed after the meter because the air
passing through the meter will collect some moisture, the amount
depending upon the temperature, humidity, and flow rate of the
incoming stream. Both wet and dry test meters should be calibrated
periodically with a soap-film flow meter.

-16-


DILUTION AIR STREAM
PURIFIED AIR FROM

CONTAMINANT STREAM

GENERATOR
MIXING
CHAMBER
MOLECULAR
SAMPLING SEIVE
BULB OR
I MANIFOLD
PERMEATION
TUBES
GNEEDLE
CONDENSER YERYE
| oon
ee ae sesso. 7h
CONSTANT
TEMP. BATH MASS
FLOWMETER

~ROTAMETER

PERMEATION TUBE GENERATION SYSTEM

FIGURE 3


WATER OUT

ON OFF
\\yaLvE |
=
AIR FROM
COMPRESSOR
6.
| = 9. |
r 3
z VALVE 3
ON-OFF FLOW--&
13. CONTROL VALVE

TAP WATER int

12. i.

AIR TO GENERATION
SYSTEM

AIR PURIFICATION & RH GENERATION
FIGURE 8


TABLE 7 cont'd.)

Calculations for Vapor Pressure Generation

The Fe flow rate is the stream of air or the other matrix gas that
is passed over the round-bottom flask, through the condenser, and
into the mixing chamber. The value of F, is set at some nominal
value (e.g., 50 mL/min) before calculation of the flow rate of the
dilution air, Fg. Also, determine the ambient barometric pressure
for these calculations, 745 mm Hg is used because it is a typical
barometric pressure for this area.

Sample Calculations: Assume a concentration of 100 ppm styrene is
needed. The vapor pressure of styrene is 5.6 mm Hg at 20°C, and

for the initial calculation a contaminant stream flow of 50 mL/min
is assumed. First, the concentration in mole fraction is calculated.

Py
ct 2 =

Pa
Cme . 3-6

745
Cpe =  -0075

Then, the amount of dilution air needed to achieve the 100 ppm
concentration is calculated.

Fe X Cpe x 1000

Fa ake
Ca

Fg = 50_x _ .0075 x 1000 - 50
100

Fq = 3.70 L/min

Thus, with a 50 mL/min contaminant stream flow through the condenser
held at 20°C, the addition of 3.70 L/min dilution air to the

mixing chamber will produce a concentration of 100 ppm at the
sampling manifold.

If other concentrations are needed, then it is necessary to change
only the dilution air flow or the contaminant stream flow.

=35—)


TABLE 7 (cont.d)

Calculations for Vapor Pressure Generation

For substances which have a high vapor pressure, a second dilution
of the first concentration may be necessary in order to produce a
low concentration. Acetone, for example, has a vapor pressure of
175 mm Hg at 20° and 112 mm Hg at 10°C. It is immediately
apparent that the amount of dilution air necessary to produce a
concentration of 100 ppm acetone will be great, and perhaps more
than the system can handle, at ambient pressures, even if the
condenser temperature is reduced at 10°C.

Coe = 12 = .150
745
Fq = 50 x .150 x 1000 - 50 = 74.95 L/min
100

At this point, two approaches may be taken. First, the contaminant
stream flow, F,, may be reduced to 5 mL/min. This would reduce
the amount of dilution air needed to 7.5 L/min which is reasonable.
The second approach is to increase the concentration produced to
1000 ppm and then take a stream from this concentration into a
second mixing chamber where additional dilution air is added to
produce the desired new concentration.

Ca xX Reg

Ch
where
Cy is the new concentration
Cq is the first concentration
Req is the rate at which the contaminant Cy is
being pumped into the second dilution chamber.

L/min (assumed)

Fg2 is the flow rate of the second dilution air
stream. L/min.

3


TABLE 19

Some Contaminants Generated by Various Methods

Vapor Pressure
Trichloroethylene
Mercury
Methylene Chloride
Toluene
Acetone
Hexane
Styrene
Carbon Disulfide
MEK
TDI
Benzene
Epichlorohydrin
Methyl Alcohol
PCB's
Diethylmercury
Dimethylmercury
2-Nitropropane
Acrylonitrile

DBCP

Permeation Tubes
NO2
S02
H2S
NH3
Cl2
Vinyl Chloride
Acrylonitrile

HCN

Trichloronitromethane

Dimethylethylamine

Vinylidene Chloride

Syringe Pump
Methylene Chloride
Epichlorohydrin
Acrolein
CS2
Hexane

MEK

24 9=

Cylinder Dilution
co
C02
Methane
NH3
Vinyl Chloride
Arsine
Nickel Carbonyl
Hydrogen Fluoride
Ethylene Oxide
N20

Methyl Chloride


10.

wh

alan

3r

14.

BB

16.

17.

age

ale):

TABLES

Static Concentration Calculations

Sources of Gas Sample Bags

Sources of Information About Bag Material

Permeation Tube Calculations

Sources of Permeation Tubes

Sources of Vapor Pressure Information

Vapor Pressure Calculations

Substances Generated by Vapor Pressure and Range of

Concentration

Syringe Pump Calculations

Manufacturers

Some Sources of Air Cleaning Equipment

Manufacturers

Manufacturers

Manufacturers

Manufacturers

Manufacturers

of Ozone Generators

of RH Sensors and Readout Meters

of Various Detectors
of Mass Flow Meters
of Orifices

of Manometers

Some Laboratory Supply Houses

Some Sources of Constant Temperature Bath

List of Contaminants Generated By Various Methods

22,

Circulators


j._DILUTION AIR STREAM
PURIFIED AIR FROM GENERATOR

MEASUREMENT

[ FLOW CONTROL AND VENT<

MIXING CHAMBER MANIFOLD OR

SAMPLING BALL

¢ LIMITING
A ROTAMETER ORIIGEY
S =
c MASS FLOW
Y METER =
L s+ | cS
1 O00
N
D
E “ipP— = Soe
R Xa
FINES | \—— CONTAMINANT
METERING STREAM

VALVE

CYLINDER DILUTION SYSTEM
FIGURE 2


PLUG

AIR our
7 AIR OUT
=! BURET OR =
=| CALIBRATED = \.
=| GLASS TUBE =
= = SAMPLING
= = DEVICE OR
= = ORIFICE
= AIR IN [=
AIR IN [= =
- () TEE OR ZB (6p, —
SIDE ARM PUMP OR
CONTROLED
RUBBER BULB
VACUUM
WITH SOAP fey
SOLUTION

SOAP
SOLUTION

SOAP FILM FLOW METERS
FIGURE 9


TOAST ERA

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svat dotdy weppindoed smon dae bealigue ae aaeteyr aeltasenwg af?
bedasesrq six begeiacah aeed



TABLE 12

Manufacturers of RH Sensors and Readout Meters

Phys-Chem Research Corporation
36 West 20th Street
New York, New York 10011

Weathertronics, Ine
2777 Del Monte Street
West Sacramento, California 95491

American Instrument Company
8030 Georgia Avenue
Silver Spring, Maryland 20910

Delmhorst Instrument Company
117 Cedar Street
Boonton, New Jersey

Bacharach Instrument Company
2300 Leghorn Street
Mountain View, California 94043

General Eastern Instrument Corporation
36 Maple Street
Watertown, MA 02172

aihil=


TABLE 10

Manufacturers of Ozone Generators

Cole Parmer Instrument Company
7425 North Oak Park Avenue
Chicago, Illinois 60648

Columbia Scientific Industries
P.O. Box 9908
Austin, Texas 78766

Ultra-Violet Products, Inc.
5100 North Walnut Grove Avenue
San Gabriel, California 91778

Atlantic Vetraviolet Corporation
250 North Fehu Way
Bay Shore, New York 11706

Dasibi Environment Corporation
616 East Colorado Street
Glendale, California 91205

Horizon Ecology Company
9435 North Oak Park Avenue
Chicago, Illinois 606438

eo9=


(i.e., given for only one temperature) or in some cases incorrect or
conflicting. Even so, this method is useful if an independent
method is available to verify the generated concentration level,
(i.e., gas chromatographic analysis based on external standards).

Syringe Pump Systems

Another useful method for generating known concentrations of vapors
is by continuously injecting a volatile liquid into a flowing air
stream using a variable speed syringe pump, or infusion pump

(Figure 5). The injected liquid vaporizes in the flowing air
stream, and by knowing the rate of injection and the amount of
dilution air added, the contaminant concentration may be calculated
(Table 9). It is necessary in some cases to heat the injection area
in order to achieve complete vaporization of the liquid, while
others will vaporize satisfactorily at room temperature. However,
if a lot of heat is used to evaporate a relatively nonvolatile
liquid, there may be a problem with condensation on the glassware if
the room is cool or if the flow rate of air is low.

Since the output from the syringe pump and the volume of air added
are the only variables, no constant temperature device is needed for
this generation system. Otherwise, the same components used for a
vapor pressure system may be used for this system.

Variable speed syringe pumps are available in different models, and
those which offer the greatest range of pumping rates will provide
the user with the versatility needed to cover a wide range of
concentrations. In addition, the syringe size used can be selected
from those of a few microliter capacity up to those of 20 mL
capacity or greater. It is possible to inject as little as 1 uL per
hour using a 10 uL syringe with the pump near its minimum setting.

The problem most often encountered with syringe injection systems is
cyclic output, especially whenever the pump drive is operated in the
lowest speed range with a microliter syringe. This problem is
sometimes corrected by providing a larger than normal mixing chamber
before the sampling chamber. For example, if the observed cycles
occur every 4 minutes and the flow rate of the contaminant stream
is 50 cc/min into a 500 mL mixing chamber located before the
dilution air stream is added, the cycles can be eliminated by
replacing the 500 mL mixing chamber with a 2 L mixing chamber or
additional 500 mL chambers in series. This will provide sufficient
residence time for the cycles to "smooth out" before the dilution
air is introduced. Obviously, as the mixing chamber volume is
increased, more time will be required for the stream to reach
equilibrium.


MICROLITER

s SYRINGE
L
N —
D
E
R |<———CONTAMINANT GAS OR
CONTAMINANT LIQUID——>
CLEAN
AIR

SERTUM

ria oy SEPTUM
VALVE tH

ES

GAS
VALVES

METER
SOLID CONTAINER OF
KNOWN VOLUME

STATIC GENERATION SYSTEM COMPONENTS
FIGURE 1


DILUTION AIR STREAM

PURIFIED AIR
FROM VENT
GENERATOR {
MIXING
CHAMBER
SAMPLING BULB
OR MANIFOLD
DIFFUSION
TUBE
~) 6cm length
4mm 1.D.
= CONTAMINANT STREAM
50mm Flask = PURIFIED AIR OR No
ae | =
CONSTANT -
TEMP. BATH a
NEEDLE
LIQUID VALVE
LEVEL

ROTAMETER

DIFFUSION TUBE GENERATION SYSTEM
FIGURE 6


produce ozone by passing a controlled flow of air through a quartz
tube which is irradiated by an ultraviolet light. The concentration
of ozone produced can be controlled by varying either the intensity
of the UV radiation reaching the quartz tube and/or by varying the
flow of air through the quartz tube (see Figure 7, Ozone Generator
Schematic).

Most commercial units will be supplied with a calibration curve from
the manufacturer. The calibration should be checked periodically by
an independent analytical method‘7) in order to determine the

actual ozone concentration for a given air flow rate and/or lamp
shutter setting.

The U-Tube Generation System

The U-tube generation method can be used to load a sorbent tube with
a known quantity of a specific contaminant in the vapor phase. This
method is especially useful when an exact loading, but not an exact
concentration, of some contaminant is needed to test a sorbent tube
or other active sampling device.

In using this method, the sampling device is connected to a closed
U-tube which may be modified by the addition of a septum port. The
exact amount of liquid to be evaporated is injected into the tube
and the sampling device is activated as the U-tube ports are opened.
Sampling is complete when all the liquid is evaporated from the tube.

Obviously, this method must be used with caution because most
hydrocarbons and halogenated compounds will evaporate rather
quickly, often in one minute or less. This rapid evaporation could
momentarily overwhelm the sorbent, causing loss of the sample and an
erroneous determination of the true capacity of the sorbent for that
particular substance.

Some control over the evaporation rate may be maintained by
controlling the temperature of the U-tube either below or above
ambient, although this method is normally used at ambient
temperature and with substances that have relatively low vapor
pressure.

Variations of this method may be used in individual situations. For
example, a glass T with a septum will accomplish the same results as
the U-tube when only a few microliters are to be evaporated at
ambient temperatures. No calculations are necessary since the
amount of substance evaporated is the amount absorbed by or loaded
on the sorbent, except as noted above.

==


ABSTRACT

Systems used in the Monitoring Research Section'’s laboratory to
generate both static and dynamic concentrations of gases and vapors
for industrial hygiene sampling and analytical method development or
instrument evaluation are presented. The advantages and
disadvantages of each system are discussed and illustrations and
examples are included.

The equipment, apparatus, and calculations used in conjunction with
the generation systems are outlined and some techniques which have
been developed are presented.


TABLE 7

Calculations for Vapor Pressure Generation

Dynamic concentrations of vapors may be calculated if the vapor
pressure at a given temperature of the species of interest is known.

Initially, the concentration of the contaminant stream must be
determined. The liquid, from which the vapor is generated, is
warmed (usually in a round—bottom flask in a heating mantle) to some
above ambient temperature (32° - 36°C). A small stream (known

flow rate) of air or other matrix gas is passed over the liquid,
resulting in the air stream becoming saturated with the vapor. This
stream then passes through a condenser held at a constant, known
subambient temperature at which the vapor pressure of the
contaminant liquid is accurately known, and at which its
concentration can be determined in mole fraction, since -

Py
Coe = —
Pa
where Cme = concentration of the vapor in mole fraction
Py = vapor pressure of the contaminant at the
experimental temperature, mm Hg
Pa = ambient barometric pressure, mm Hg

The necessary flow of dilution air, Fg, needed to give the final
desired concentration, can be calculated from the mole fraction
(above calculation) and the desired concentration of vapor in the
final contaminant stream.

F, x Cpe x 1000

Fa a -F,
Ca
where
Fg = flow rate of dilution air (L/min)
Fe = flow rate of contaminant stream (mL/min)
Cme = concentration in mole fraction (unitless)
Ca = desired final concentration, ppm (,L/L)
1000 = conversion factor (yL/mL)

=32—


of the room. In some situations, it is best to have the entire
generation system installed in a laboratory fume hood, while in
other situations it may be necessary to vent only the exit port of
the generation system after making sure there are no leaks in the
generation system. It will be necessary to make a determination of
the potential hazards of each substance generated, based on its
known or assumed properties.

As with any work area, it is desirable to have good air circulation
in the laboratory and a constant, comfortable temperature, both from
the standpoint of worker comfort and for the operation of precision
laboratory instruments, some of which may produce erroneous readings
if there are wide fluctuations in the ambient temperature.

SUMMARY

Gas and vapor generation systems are valuable tools for many
applications, including the evaluation of the performance of
sampling methods and sampling instruments.

With the exception of some permeation tube and diffusion tube
devices, pre-assembled generation systems are not commercially
available, and must therefore be assembled by the individual user.
The installation may range from very simple, inexpensive, and rigid
systems up to complicated, expensive and versatile systems. The
system installed will depend upon many factors, and each should be
considered before any investment is made (TABLE 19).

Experience has shown that the greatest single source of error in any
generation system is in the measurement and control of the air flow
rate. Therefore, emphasis should be placed on this aspect by
obtaining the best in air flow measurement and control devices that
can be afforded and by becoming proficient in their operation and
calibration.

The basic components of dynamic generation systems are air supply,
air control and measurement, mixing chamber and sampling chamber or
manifold. Each is used with vapor pressure, permeation tube, or
syringe pump generation systems. By designing these with some
flexibility, it will be easy to install a different system if the
first one no longer meets the needs of the laboratory.

=20=


DISCLAIMER

Mention of company names or products does not constitute endorsement

by the National Institute for Occupational Safety and Health.

DHHS (NIOSH) Publication No. 84-113


TABLE 13 (cont'd.)

Manufacturers of Various Detectors

i3c. Manufacturers of Photoionization Detectors
HNU Systems, Inc.
30 Ossipee Road
Newton, MA 02164
Aid Inc
Route 41 & Newark Road
Avondale, Pennsylvania 19311
13d Manufacturers of Chemiluminescence Detectors

Thermo Electron
100 First Avenue
Waltham, MA 02154

Scott Environmental Systems
Division of ETC

County Line Industrial Park
Southampton, Pennsylvania 18966

Nutech Corporation
2806 Check Road
Durham, North Carolina 27704

Monitor Labs, Inc.
4202 Sorrento Valley Boulevard
San Diego, California 92121

Columbia Scientific Industries
Box 9908
Austin, Texas 78766

-44-


TARLE 2

Sources of Gas Sample Bags

Pollution Measurement Corp.
P.O. Box 6182
Chicago, Illinois 60680

Fluorodynamics Inc.
Diamond State Industrial Park
Newark, Delaware 14711

The Anspec Company Inc.
P.O. Box 7044
Ann Arbor, Michigan 48107

Calibrated Instruments Inc.
731 Saw Mill River Road
Ardsley, New York 10502

Analabs, Inc
80 Republic Drive
New Haven, Connecticut 06473

Cole-Parmer Instrument Company
7425 North Oark Park Avenue
Chicago, Illinois 60648

TABLE 3

Sources of Information about

Environmental Measurements Inc.
215 Leidesdorff Street
San Francisco, California 94111

Lapine Science Corporation
6001 South Knox Avenue
Chicago, Illinois 60629

NASCO
901 Janesville Avenue
Ft. Atkinson, Wisconsin 53538

Polyfoam Packers Corporation
6415 North California Avenue
Chicago, Illinois 60645

Scientific Gases & Instruments
P.O. Box 6
Houston, Texas 77001

Varian Instrument Service Center
670 East Arques
Sunnyvale, California 94806

Bag Material

Teflon - Tedlar - Mylar:

E.I. DuPont De Nemours and Company (Inc.)
Film Department
Wilmington, Delaware 19898

Scotch — Pac:
3M Company

3M Center
St. Paul, Minnesota 55101

=20=


TABLE 9

Syringe Pump Generation Calculations

In order to determine the liquid flow (from the syringe) necessary
to generate a given gaseous concentration use

Cq x Fq x MW,

a
Vxdx 103
where
Fy = the liquid flow required (uL/min)
Cg = desired concentration in ppm (uL/L)
Fq = flow rate of the dilution air (L/min)
MWe = molecular weight of the contaminant (g/mole)
v = molar volume (25 L/mole at 745 mm Hg and
25°C)
d = liquid density of the contaminant (g/mL)

It is important to point out that Fy includes the small stream of

air or other matrix gas that carries the contaminant vapor into the
main stream in addition to the bulk dilution air or other matrix gas.

Once the necessary liquid flow has been determined, it is necessary
to select an appropriate syringe and the proper operating settings
for a given syringe pump. The choice of syringe and pump settings
is a function of the required liquid flow and the sampling time
desired. Always bear in mind that instruments generally function
most accurately when they are operated in the 30 - 70% full scale
range.

The volume of the vapor produced by the evaporation of the liquid is
small compared to the total air volume of the contaminant and
dilution air streams. Example: To generate 1000 ppm Hexane at 1
L/m dilution air.

Fi = 1000 ppm x 1 LPM x 86.2 g/mole

25.0 L/mole x 0.66 g/mL x 103

= 5.23 ul hexane liquid/min

37


TABLE 1

Static Concentration Calculations

Static concentrations of contaminant gases and vapors are generated
by introducing a known volume or weight of the contaminant into a
known volume of air or other dilution gas.

A. For a gas the concentration is calculated in a straightforward
manner

Ve =C
Va

where

V,. is the quantity of the contaminant

Vg is the volume of the dilution gas

C is the concentration of the contaminant in the dilution gas
The units of V., Vq and C may vary.

Some common examples are:

Ve =uLl, Vg = L; C = pL/L or ppm

If C_ is to be given in units of mg/m3, the Vg must be given

in M3 and Ve must be given in mg. This can be accomplished
in either of two calculations:

=23=


Complete permeation tube generation systems are available from
manufacturers, or they may be assembled by individual laboratories.
An example of a dynamic permeation tube generation system may be
seen in Figure 3.

The rate of permeation of gas from a tube depends upon the length of
the tube, the wall thickness, the temperature, and the permeability
of the polymer tube (Table 4). For any given gas with a given tube
dimension, the permeation rate, normally expressed as ng/cn/min,
will remain constant after initial equilibrium as long as the
temperature remains constant. Permeation tube manufacturers supply
information necessary to determine the size and number of tubes
needed for a particular application. A list of some permeation tube
manufacturers may be seen in Table 5.

Permeation tubes may be calibrated by the manufacturer or they may
be calibrated by the user if a microanalytical balance is

available. The normal calibration procedure is to weigh the tube
once per day over a period of several days while maintaining the
tube at a constant temperature. When the recorded rate of weight
loss becomes stable, the tube is ready to use, e.g., at equilibrium,
and the weighing may be repeated thereafter on a weekly basis for as
long as the temperature remains constant. The calibration procedure
is necessary only if the exact permeation rate is needed. For a
procedure requiring only an approximate concentration or where an
independent analytical method is available to determine the exact
concentration, the batch calibration provided by the manufacturer is
satisfactory. (9)

Moisture in the air stream that is used to purge the permeated gas
into the mixing chamber can adhere to the permeation tube and cause
the permeation rate to change, so only dry air should be used. If
dry air is not available or difficult to obtain, dry nitrogen is
recommended since the total amount used is normally small in
relation to the dilution air flow.

One disadvantage in using permeation tubes is the relatively large
initial expenditure required to purchase the constant temperature
device used; and this is especially true for systems which require
maintaining the tubes at subambient temperatures and for user
calibration if a microanalytical balance must be purchased.

Another is that the permeation rate is small for some compounds, so
many tubes are required or the temperature must be elevated in order
to obtain the desired contaminant level at the needed dilution air
flow rate.


Figure 8--LEGENDS

Components

1.

10.

iin

12

130

Air Purifier - Shell with replaceable cartridge for removal of oil
and water.

Line Pressure Regulator.

Tube furnace with 1-1/4" x 16" long SS tube with reducing fittings to
1/4" tube on each end.

Variable voltage control 0-120 volt, 10 amperes.

Thermocouple readout meter with Type K thermocouple.

Cooling tube (may be bent or coiled to save space) 1/2" M copper tube
4' long with 1/4" reducing T on each end and 1/4" copper or SS tube
inside.

Water flow indicator 0-10 Lpm water rotameter.

Canister containing charcoal (500-1000 g) or may be combined with
ascarite, silica gel or molecular sieve, or other canisters containing
these may be added in series with the charcoal.

Pleated membrane filter cartridge (2-3 range).

Rotameter — capacity as needed.

Mass flow meter - may be used in place of or in addition to rotame. ..
Humidifier (if needed) 2 in series —- 1 L capacity glass bottles of
flask with 24/40 € to accept stem part of midget impinger (stem is cut
off before taper.) Valves for control of percent of total air passing
over water. Additional flask may be added or flask may be heated if

more humidity is needed.

Electronic Humidity Sensor-Readout Device.


TABLE 14

Manufacturers of Mass Flow Meters

Manufacturers of Mass ower

Hasting-Raydist
Hampton,
Virginia 23361

Sierra Instruments, Inc.
P.O. Box 909
Carmel Valley, California 93924

Kurz Instruments, Inc.
P.O. Box 844
Carmel Valley, California 93924

Tylan Corporation
19220 Normandie Avenue
Torrence, California 90502

TABLE 15

Manufacturers of Orifices

Millipore Corporation
Bedford,
Massachusetts 01730

Richard H. Bird and Company, Inc.
1 Spruce Street
Wotham, Massachusetts 02154

-45-


TABLE 17

Some Laboratory Supply Houses

Curtin Matheson Scientific, Inc.
4220 Jefferson Avenue

P.O. Box 1546

Houston, Texas 77001

Sargent-Welch Scientific Company
7300 North Linder Avenue
Skokie, Illinois 60076

Arthur H. Thomas Company

P.O. Box 779

Vine Street at 3rd
Philadelphia, Pennsylvania 19105

Fisher Scientific Company
711 Forbes Avenue
Pittsburg, Pennsylvania 15219

Ace Glass Incorporated
P.O. Box 688

1430 Northwest Boulevard
Vineland, New Jersey 08360

GCA/Precision Scientific
3737 West Courtland Street
Chicago, Illinois 60647

Markson Science, Inc.
522 Oak Street
Del Mar, California 92014

Cole-Parmer Instrument Company
7425 North Oak Park Avenue
Chicago, Illinois 60648

Bel-Art Products
Pequiannock,
New Jersey 07440

Lab Safety Supply Company
P.O. Box 1363
Janesville, Wisconsin 53545

==


Another means to reduce cyclic output from a syringe pump generation
is to impale a small piece of felt fabric on the tip 6f the needle
so that the liquid will flow out of the needle, spread out over the
felt and evaporate more evenly. This may be used alone or in
combination with larger mixing or surge chambers in difficult cases.

In addition to cyclic output, the other disadvantage to syringe pump
systems is that the syringe must be refilled periodically, and this
refilling operation can cause a disruption to the constant output
level. It will, therefore, be to the user's advantage to carefully
select the syringe size and pump speed so that refilling will come
at the end of the predetermined sampling period rather than during
the sampling period.

Diffusion Tube System

Diffusion tubes are a modification of the vapor pressure generation
method, and may be used for many vapors, especially in the lower
concentration ranges. The physical appearance of diffusion tubes
may take many forms, but basically they consist of a reservoir
containing the liquid and a capillary tube extending from above the
liquid into a flowing stream of air (see Figure 6 — Diffusion Tube
Generation System). At a constant temperature and a fixed tube
length and diameter, the output of the vapor will be constant, and
may be varied by changing the temperature of the reservoir or the
length or diameter of the capillary tube. A constant temperature
bath is ideal to control the temperature of the diffusion device.
Room air or circulating tap water are satisfactory if their
temperature is constant, or experiences only minor temperature
fluctuations throughout the day.

As with other generation systems outlined here, it is better to pass
a small stream of air across the diffusion device and then combine
it with a dilution air stream in a mixing chamber. This will permit
easy adjustment of the final concentration without disturbing the
temperature or capillary length or diameter, and will minimize the
cyclic output observed if the flow of air across the diffusion tube
becomes large enough to cause uneven air currents or back-pressure.

Permeation tubes, previously discussed under Systems Used For Gases
may also be used for vapor generation. The procedures to use and
the equipment required are the same for both gases and vapors.

OTHER GENERATION SYSTEMS

Ozone is of concern to many in the industrial hygiene field and it
is not covered by any of the previously described systems. Several
ozone generators are commercially available which will produce
stable concentrations of ozone in the range of interest to
industrial hygiene (0.05 - 0.5 ppm) (Table 10). These systems

=