NBSIR 73-292

Gas Generation Systems for the

Evaluation of Gas Detecting Devices

E. E. Hughes, W. D. Dorko, E. P. Scheide, L. C. Hall, A. L. Beilby, and J. K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

October 1973

Final Report

Prepared for National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati. Ohio 45202

NBSIR 73-292

GAS GENERATION SYSTEMS FOR THE

EVALUATION OF GAS DETECTING DEVICES

E. E. Hughes, W. D. Dorko, E. P. Scheide, L. C. Hall, A. L. Beilby, and J. K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

October 1973

Final Report

Prepared for National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director
r
r.AS GENERATING SYSTEMS

FOR THE

EVALUATION OF GAS DETECTING DEVICES

A Report to

National Institute for Occupational Safety and Health

by

E. E. Hughes, W. D. Dorko, E. P. Scheide, L. C. Hall

A. L. Beilby and J. K. Tavlor

Air Pollution Analysis Section

Analytical Chemistry Division

National Bureau of Standards

Washington, D. C. 2 0234

GAS GENERATING SYSTEMS FOR THE EVALUATION

OF GAS DETECTING DEVICES

ABSTRACT

This report summarizes a project concerned with the

design, testing and calibration of systems for producing known

low concentrations of a number of substances in air at concentrations in the vicinity of the TLV (Threshold Limit Value).

The substances include carbon dioxide, carbon monoxide, ozone,

sulfur dioxide, hydrogen sulfide, nitrogen dioxide, benzene,

carbon tetrachloride, tr ichloroethylene , tetrachloroethylene

ammonia and chlorine. The systems are of four basic types:

direct delivery from cylinders of known concentration, dynamic

blending of high concentrations with air, permeation tubes and

generation of the substance directly in an air stream. Monitoring systems are suggested to allow confirmation of the quantities

generated. Details of construction are given.

Key words: Gas mixtures, gas detectors, trace gas analysis.

TABLE OF CONTENTS

PAGE

1. INTRODUCTION . 1

2. GAS CYLINDER SAJvlPLING APPARATUS (CO, CO^) 3

A. Introduction 3

B. Preparation for Use. 5

C. Use o£ the Apparatus 7

D. Troubleshooting 8

1 . Flow meter 8

2. Leaking Valves 8

E. Components 8

1. Valves 8

2. Tubing and Connections 8

3. Flow Meter 9

4. Filter 9

F. Summary of Instructions 9

3. GAS BLENDING SYSTEM (C.H. , CCl ,

, C ^HCl , C^Cl,) 11 00 42 3,2 4

A. Introduction 11

B. Description of Gas Blending System 12

1. Components 12

2. Diluent Air 17

3. High Concentration Mixture 17

C. Operation 19

1. Preparation of Binary Mixtures 19

2. Analysis 19

3. Addition of Other Substances 21

4. Calculations 21

D. Materials of Construction 22

4. OZONE SYSTEM 2 3

A. Introduction 23

B. Principle of Operation 23

1. Gas Flow Unit 23

2. Ozone Generator and Power Supply 27

3. Oxidant Analyzer 28

C. Operation 29

1. Gas Flow Unit 29

a. Dry Air 29

b . Humid Air 29

2. Ozone Generator 30

3. Oxidant Analyzer 31

4. Shut-Down 33

D. Maintenance 33

1. Gas Flow Unit 33

2. Ozone Generator 34

3. Oxidant Analyzer 34

E. Calibration of the Ozone Generator 34

F. Materials of Construction 35

i
PAGE

5. PERMEATION TUBE SYSTEM (SO^ .H^S , NO^ ) 36

A. Introduction 36

B. Description of System 36

1. General Description 36

2. Temperature Control 40

3. Air Flow Control 41

4. Tube Storage Device 42

C. Use o£ Apparatus 45

1. General Instructions 45

2. Sulfur Dioxide Generation 46

a. Permeation Tubes 46

b. Analytical System 47

3. Hydrogen Sulfide Generation 49

a. Permeation Tube 49

b. Analytical System 49

/ ~ 4. Nitrogen Dioxide Generation 50

a. Permeation Tubes 50

b. Analytical System 51

D. Materials of Construction 52

6. AMMONIA SYSTEM 53

A. Introduction 53

B. Principle of Operation 53

1. Gas Mixing Unit 57

2. Analytical Unit. ....... 57

3. Gases; Gas Mixtures 61

4 . Reagents 61

5. Procedure 61

C. Operation 63

1. Precautions 63

2. Initial Set-Up 63

3. Analysis of Bulk Mixtures by Direct Sampling ... 64

4. Preparation of Dilute Air-Ammonia Mixtures .... 66

5. Shut-Down 66

D. Maintenance 67

1. Gas Mixing Unit 67

2. Analytical Unit 68

E. Materials of Construction 68

F. Performance Evaluation 68

1. Analysis of Bulk Mixtures 68

2. Stability of Bulk Mixtures 70

3. Analysis of Ammonia-Air Dilutions 70

4. Effect of Sampling Rate 70

G. Conclusions 73

ii
PAGE

7. CHLORINE SYSTEM ,74

A. Introduction 74

B. Principle of Operation 74

1. Gas Blending Unit 77

2. Analytical Unit. 77

3. Gases, Gas Mixtures . 79

4. Procedure 79

C. Operation 82

1. Precautions 82

2. Initial Set-Up 82

3. Analysis of Bulk Chlorine Mixtures 83

4. Analysis of Bulk Mixtures 84

5. Preparation of Dilute Air-Chlorine Mixtures. ... 86

6. Shut-Down 86

D. Maintenance 86

1. Gas Mixing Unit 86

2. Chlorine Analysis Unit 87

E. Materials of Construction 88

F. Performance Evaluation 88

1. Conditioning of Gas Cylinders 88

2. Tank Stability 88

3. Analysis of Bulk Mixtures 89

G. Conclusions 91

8. GAS HUMIDIFICATION SYSTEM 92

A. Principle of Operation 92

B. Description of System 93

C. Operation 93

D. Maintenance . , 97

9. PARTS LIST 99

111
LIST OF FIGURES

FIGURE NO. . PAGE

2.1 Schematic of Cylinder Sampling Apparatus 6

3.1 Photograph of Gas Blending Unit 13

3.2 Arrangement of Components Inside Cabinet 14

3.3 View Under Chassis of Gas Blending Unit 15

3.4 Schematic Diagram of Gas Blending System 16

4.1 Photograph of Ozone System Showing Location of .... 24

Principal Parts

4.2 Schematic Diagram of Ozone System 25

4.3 Detail of the Ozone Generator 26

4.4 Calibration of Ozone Generator Number 7 32

5.1 Photograph of Permeation Tube Apparatus and Tube ... 37

Conditioner

5.2 Interior View of Permeation Tube Apparatus 38

5.3 Schematic Diagram of Permeation Tube System 39

5.4 Tube Storage Device 43

6.1 Photograph of Ammonia System 54

6.2 Interior View of Ammonia System 55

6.3 View Under Chassis of Ammonia System ......... 56

6.4 Schematic Diagram of Ammonia System 58

6.5 Wiring Diagram for Ammonia System 60

6.6 Effect of Sampling Rate 72

7.1 Photograph of Chlorine System 75

7.2 Interior View of Chlorine System 76

7.3 Schematic Diagram of Chlorine System 78

7.4 Wiring Diagram - Chlorine System 80

7.5 Comparison of Analytical Methods for Chlorine 90

Analysis

8.1 Schematic Diagram of Humidifier 94

8.2 Front View of Humidifier 95

8.3 Side View of Humidifier 96

iv
LIST OF TABLES

TABLE NO. PAGE

1.1 Substances and Concentrations Generated 2

2.1 Concentration of CO and CO^ 4

3.1 Value of K for Calibration with Either Propane or . . . 21

Methane

5.1 Flow Rate, Tube Length, and Concentration Produced, . . 47

for Sulfur Dioxide Permeation Tubes at 30°C

5.2 Flow Rate, Tube Length, and Concentration Produced. . . 49

for Hydrogen Sulfide Permeation Tubes

5.3 Flow Rate, Tube Length, and Concentration Produced for. 51

Nitrogen Dioxide Permeation Tubes at 25°C

5.4 Value of K for Sulfur Dioxide, Hydrogen Sulfide and . . 52

Nitrogen Dioxide at Various Temperatures

6.1 Direct Analysis of Bulk Mixture 69

6.2 Analysis of Ammonia-Air Dilutions 71

9.1 Key to Parts Shown on Schematic Diagrams 100

9.2 List of Commercially Available Parts . . . . 102

V

1. INTRODUCTION

This report summarizes work concerned with tlie development

of gas generation systems for use in verifying the performance

of gas detection devices or for the calibration of gas analysis

instrumentation. The systems were designed to nrovide mixtures

of a number of different gases and vapors in air at concentrations

representing fixed proportions of the Threshold Limit Value (TLV)

Furthermore, they furnish a sufficient volume of gas to allow

continuous testing of devices while maintaining a predetermined

concentration level.

Several methods have been employed to achieve those

purposes, the simplest of which is the utilization of stable gas

mixtures. Those mixtures can be standardized and then used

directly as calibrants. Alternatively, they can be incorporated

into a reliable gas blending system and quantitatively diluted

to produce dynamic calibrant mixtures of lower concentration.

Gas mixtures of less than desirable long term stability can also

be utilized if they are periodically re-analyzed. Another

approach is the application of calibrated gas generators such

as the ultra-violet ozone generator or permeation devices in a

gas dilution system. In each instance an analytical system has been

described or included as an integral part of the apparatus

to provide a means of verifying the composition of the pas

mixtures produced. The object, whenever possible, has been to

provide traceability to the National Bureau of Standards

by the use of NBS Standard Reference Materials. V^'here

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these are not presently available, NBS will furnish primary

standards or perform calibrations as required, on a costreimbursable basis to insure traceability

.

Table 1.1 lists the substances of initial interest to NIOSH,

for which gas generation systems ivere developed. The systems

are described in detail in the following sections of the report.

The ranges of concentrations generated by the systems for these

substances and the Threshold Limit Value (TLV) are also shown

in Table 1.1.

2. GAS CYLINDER SAMPLING APPARATUS

(CO, CO^)

A. Introduction

A number of gas mixtures have sufficient long term

stability to allow direct use from cylinders without

monitoring the concentration in the sampling manifold.

Carbon monoxide and carbon dioxide are examples of gases

that may be used in this way. The mixtures can be

analyzed with an absolute accuracy of better than ± 2 percent. Storage of carbon dioxide mixturesfor periods of one

year will not result in reduction of the concentration through

reaction or adsorption. Mixtures of carbon monoxide, however,

should be reanalyzed at least every three months. Gas

mixtures of carbon monoxide and carbon dioxide of the following

composition have been prepared and analyzed, (Table 2.1):

3
Table 2.1

Carbon Monoxide

Cylinder

Number

A-11191

A-11193

A-11167

A-11173

Carbon Dioxide

Cylinder

Number

A-11159

A-11145

A-11146

A-11150

Concentration of CO and CO

Nominal

Concentration

25

50

100

250

Analyzed

Concentration

PPT" . 25.0

54.5

99.2

249

Nominal

Concentration

Mole Percent

.0.25

; 0.50

1.0

2.5

Analyzed

Concentration

Mole Percent

0.253

0.493

0.972

2.43

4
B . Preparation for Use

The apparatus used with stahle gas mixtures is shown

schematically in Figure 2.1. Four gas mixtures in cylinders

may be attached simultaneously and used successively.

Each mixture is controlled by a valve on the front panel.

The valve number corresponds to the inlet fitting at

the rear of the cabinet. The flow into the manifold

is measured by the flow meter. The only reason for this

measurement is to insure that the flow of gas into the

sampling manifold is greater than the sampling rate.

After attaching the appropriate number of cylinders

to the inlets at the rear of the cabinet it is necessary

to flush each line from the cylinder to the control valve

on the panel. The cylinder must be equipped with a control

valve of some kind. This may be in the form of a regulator

with a sliut-off valve or more simply, a control valve

without regulator. The system is designed conservatively

for a maximum pressure of 1500 psi. Gas mixtures will be

supplied as needed at an initial pressure below ISOO psi

so tliat a pressure regulator is not necessary. In any

case, the control device on the cylinder should be flushed

before attaching the line to the cabinet. After attaching

this line, it should be flushed by bleeding gas from the

cylinder through it with the valve on the panel open. After a few hundred cubic centimeters iQf gas have passed, the control

5
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valve on the panel should be closed. With this valve

closed, there should be no indication of flow by the flow

meter. Repeat this procedure with each cylinder. The

system is now ready for use.

C . Use of the Apparatus

Select the gas mixture to be introduced into the

manifold. Turn the appropriate control valve on the

panel until a flow of gas is registered on the flow meter.

Restrict one of the ports in the manifold with a cork.

Flush for a minimum of 2 minutes at approximately 200 cm^/min.

Adjust the flow rate to some value at least 25 percent

greater than the sampling rate of the pump which will

be used to draw a sample from the manifold. The gas

may now be sampled.

If samples are to be taken continuously it is not necessary

to reduce the flow between samples. If, however, some time may

elapse between sampling, it is advisable to reduce, or shut off,

the flow to conserve the contents of the cylinder.

After completion of sampling with a mixture, the valve on

the panel is closed. After the indicated flow has dropped to

zero, or some insignificant value, the next sample may be introduced. The manifold should be flushed for one or two minutes at

200 cm"^/minwith each succeeding gas mixture before sampling.

7
D. Troubleshoo t ing

L, Flow meter

Tlie manual for the flow meter installed in this unit

is included with the unit and includes troubleshooting

information.

2 . Leaking Valves

If flow is observed with a valve apparently closed,

it is an indication of a leak across the valve seat.

A judgment must be made as to whether the leak

is of sufficient volume to require drastic remedies.

The leak may either be ignored, eliminated, or rendered

harmless to subsequent samples by shutting off the line

containing the leaking valve at the cylinder. The line

should then be briefly opened to the manifold to reduce

pressure in the line.

E . Components

1 . Valves

The valves are coarse regulating valves with good

shut-off features.

2 . Tubing and Connections

Tubing is 1/8 in or 1/4 in o.d. by 0.032 in

wall refrigeration grade copper tubing. This tubing

8
should not be used for pressures above 1500 psi. All

connections are 1/4 in or 1/8 in "Swagelok."

3 . Flow meter

The flow meter is a commercial instrument which

senses flow by measuring the rate of heat loss from an

element exposed to the flowing gas. The flow measurement

for this apparatus need not be precise but must only assure

the operator that the flow into the manifold exceeds

the flow out through any sampling device.

The flow meter can be removed easily for recal ibrat ion

or use elsewhere. The calibration is valid for air or nitrogen

but would require recal ibrat ion for gases of differing

thermal conductivity.

Recal ibrat ion , if this becomes necessary, is described

in the instruction manual for the flow meter.

4. Filter

The filter is a sintered metal disc capable of retaining

particles greater than 50 micrometers in diameter. Particles

smaller than this will not affect the flow meter.

F . Summary of Instructions

1. Attach regulator or valve to cylinder and flush

with gas mixture.

2. Attach cylinder to apparatus with clean 1/8 in

copper tubing.

9
Flush each line separately.

Check for leakage across each valve.

Flush manifold.

Adjust flow of desired mixture to appropriate

Begin sampling.

10
3. GAS blending; system

(C^II^, CCI4, C2HCI3, C2CI4)

A. Introduction

A number of substances can be prepared as relatively concentrated mixtures in air at elevated pressure but v'hich may not

have sufficient stability to permit long term storage or use

without intermittent analyses. However, if the short term

stability of such mixtures is adequate, they may be used for

preparing other mixtures by quantitative dilution, if some

means is available for measuring tlie concentration relative to

a reliable standard. Hydrocarbons and halogen substituted

hydrocarbons, such as tr ichloroethylene, tc tracli loroethylene , and carbon tetrachloride, give adequate and reproducible

response with a flame ionization detector (FID). The flame

ionization detector can be calibrated with mixtures of stable

hydrocarbon mixtures such as methane or propane in air.

The system described herein produces mixtures of a gas or vapor in air at predictable and adjustable concentrations. While the system was designed primarily for the

preparation of less stable gas mixtures it may be used for the

preparation of a variety of binary or even multi-component gas

mixtures. In such cases, the num.ber of calibration standards

required in a given situation may be minimized.

11
The apparatus consists of a sub-system for controlling

and measuring the flow of a stream of clean and dry air and of

a sub-system for introducing a gas or vapor, or a mixture of

a gas or vapor in air, at a controlled rate into the air stream.

The combined stream is mixed and transported to a manifold from

which samples may be taken for calibration. In addition, a third sub-system is included which may be used to admit a third

substance to the combined stream. The third substance may be

either water vapor or a substance suspected of being an interference in the analytical system to be calibrated. It may

either be admitted from a container under pressure or may be

pumped in from a system at atmospheric pressure. Photographs

of the gas blending unit are shown in Figures 3.1, 3.2, and

3.3. Figure 3.1 is a view of the assembled apparatus. Figure

3.2 sliows details of the arrangement of components within the

cabinet while Figure 3.3 shows the arrangement beneath the

chassis

.

B. Description of Gas Blending System

1. Components

The system is shown schematically in Figure 3.4, and consist

of the following sections:

a. A sub-system (Air) for introducing air at a controlled and measured rate.

b. Two sub-systems (A, B) for introducing other

gases at controlled and measured rates into the diluent air.

c. A sub-system (C) for introducing a gas from a pressure source at a measured rate.

12
Figure 3.1 Photograph of gas blending unit

13
Figure 3.2 Arrangement of components inside cabinet

A. Mixing chamber B. Manifold C. Pump

14
15
16
d. A pump (D) for introducing a gas from a source at

atmospheric pressure, at a measured rate.

e. A mixing chamber for combining two or more of the

above gas streams.

f. A sampling manifold from which the mixed gases may

be withdrawn for analysis.

2 . Diluent Air

Air at a pressure greater than 10 psi and less than 100 psi

is admitted to the system at the connection marked "AIR" located

at the rear of the chassis. A valve, (V-1), is provided to shut

off the supply of air when the system is not in use. The air

passes through two small cylinders located beneath the chassis which

are filled with silica gel and charcoal. These are included to

protect the system from minor quantities of water vapor and

organic materials in the supply air. The supply air should be

moderately clean and dry to minimize the necessity for the reconditioning of the dryer and purifier in the system. (If this is not

the case, an external trapping system should be inserted between

the air supply and the system.) The air then passes through a sintered metal filter into a flow control system and through a flow meter to the mixing chamber. The flow control system consists

of a differential flow controller (DFC-l) and a flow control valve

(V-2).

3 . High Concentration Mixture

The substance to be diluted with air to produce the mixture

may be introduced from a gas cylinder at the connection marked "A"

or "B" located on the left side of the chassis and accessible

17
through the slot cut into the side of the cabinet. The pressure at

this inlet should be greater than 10 psi and less than 100 psi.

Shut-off valves, (V-3) and (V-5), are located between the inlet

connections and the rest of the system. Control of the flow

is accomplished with differential flow controllers (DFC-2) and

(DFC-3) and control valves (V-4) and (V-6). It should be noted

that V-4 is a fine control valve and not a shut-off valve. It

should never be closed more than finger tight. The flow from

inlet "A" passes directly to a flowmeter and then to the mixing

chamber. The flow from inlet "B" passes first through a 3-way

valve, (V-9), then through a floi\nTieter (FM-3) to the mixing cham.ber

Inlet C, passes directly through a flow control valve (V-7)

to the 3-way valve (V-9). A cylinder of gas at elevated pressure

may be connected to "C" and may be bled into the system but not

with the same degree of control offered by A or B.

Inlet D is provided with a pump which allov/s gases at

atmospheric pressure, for example, those generated in another

system, to be introduced into the mixing chamber. The valve (V-8)

and flowmeter (FM-3) are used to control and measure the quantity

of gas introduced at this inlet.

The mixing chamber is designed to rapidly mix any of the

streams with the air prior to passage into the manifold.

The manifold is provided with sampling ports from which

samples may be withdraim for testing or analysis. An additional

outlet is provided for direct connection to an analytical

instrument.

18
C . Operation

1. Preparation of Binary Mixtures . Connect a supply

of clean and dry air to the air inlet at the rear of the chassis.

Connect to inlet A a cylinder containing the desired component,

either as the pure substance, or diluted to some known value

considerably above the final desired concentration. Calculate

the ratio of flow rates necessary to produce the desired final

concentration. Consult the calibration curves accompanying

FM-1 and FM-2 to determine the proper settings. Close V-2 and

V-4. Open V-1 and V-3. Adjust V-2 until the proper flow is

indicated by FM-1. Adjust V-4 until the proper flow is indicated

by FM-2. The gas delivered to the manifold should now be of the

calculated composition.

2. Analys is . The agreement between a calculated composition

based on a measurement of flow rates and the actual composition

produced by the unit may differ somewhat depending on a number of

factors. Very high flow rates, for example, may result in back

pressure in parts of the system which may then alter the composition from the predicted value. Consequently, a means of monitoring the concentration produced is desirable for producing mixtures

of a moderate degree of accuracy. The mixture produced by the

generating system is essentially a binary gas mixture composed

of a substance in air. A flame ionization detector (FID) is an

ideal analytical tool for the analysis of such systems because

of the high sensitivity to compounds containing C-H bonds and

because of the extreme insensit ivity to the components of air. A

FID is supplied with this unit and is connected to sample directly

out of the manifold,

19
Different organic substances give different responses to

equal molar quantities. Therefore, it is necessary to calibrate

the FID for each substance which is to be analyzed. Four bulk

gas mixtures are supplied with this unit which consist of high

concentrations of benzene, carbon tetrachloride, perchloroethylene

and tr ichloroethlene , all contained in air. The response of each

of these substances relative to either methane or propane has

been determined. Calibration of the FID for each substance is

accomplished by determining the response of the instrument to

known concentrations of either methane or propane and relating

the signal observed from the other substances to the actual concentration by use of the predetermined response factors.

Attach a short length of Teflon tubing between a sampling

port of the manifold, or the extra connection on the upper end

of the manifold, and the FID. Consult the instruction manual

accompanying the FID and prepare it for analyses. The diluent

"air" is used to establish the "zero" and either propane or

methane is used to establish the "span". Attach either the

cylinder of methane or propane in air mixtures at C and rotate

V-9 to the position marked "C or D". Adjust the span gas flow

with V-7 and measure the flow with FM-3. Alternate the air and

the methane or propane standard until a reproducible signal is

obtained. The instrument is now direct reading for whichever

hydrocarbon is used to establish the span.

Attach the bulk mixture to C and observe the signal. The

signal is either the methane or propane equivalent of the

concentration of the particular substance in the bulk mixture.

20
This signal must be multiplied by an appropriate factor to

convert the observed signal, C^^^, to the concentration of the

substance in the bulk mixture, C . The concentration is

calculated as follows:

C = C , X K

X obs

where K is chosen for either methane or propane. The

appropriate values for K are given in Table 3.1.

Table 3.1 Value of K for Calibration with Either Propane

or Methane

Bulk Mixture 4 ^3^8

Carbon Tetrachloride 11.1 33.3

Trichloroethylene 0.562 1.69

Tetrachloroethylene 0.440 1.32

Benzene 0.204 0.611

3. Addition of Other Substances . Addition of water

vapor or other substances to the binary mixture requires that

the bulk mixture be attached at A. A cylinder containing the

substance to be added should be prepared at an appropriate

concentration determined by the nature of the experiment.

Water vapor is introduced as saturated air at C from the

Saturator. (Section 8.)

4, Calculations . The concentration of a substance

prepared by dilution of a bulk mixture with this apparatus

can be calculated as follows:

*^x " F +P . * ^b X air

21
where = concentration produced in the manifold

F - measured flow rate of bulk mixture

X

F . = measured flow rate of air air

= concentration of bulk mixture in ppm

The concentration of the same substance in a mixture diluted

further with an interfering substance or with water vapor is

calculated as follows:

F

C = - • C ^x F +F +F . b

X y air

where F = measured flow rate of interfering substance or

y

^ moist air.

The relative humidity of the mixture resulting from addition of moist

air is calculated with the following equation:

F

Rh.

2 = Rh.1

• ^

F +F +FJ . X y air

where Rh2 = relative humidity of the mixture

Rh = relative humidity of the moist air

1 F^ = measured flow rate of the moist air.

The relative humidity of the moist air, Rh-j,, is calculated as

described in Section 8, if the same or similar apparatus is used

to produce the moist air.

D. Materials of Construction

The system is constructed of copper, brass, stainless steel,

glass and Teflon. The "Air" subsystem is constructed primarily

of brass and copper while the subsystems "A", "B", and "C" are

constructed primarily of stainless steel wherever practical. The

pump has an aluminum body and a Neoprene diaphragm.

22
4. OZONE SYSTEM

A. Introduction

This system produces known concentrations of ozone in

a flowing air stream. The system is composed of two parts;

(1) the main cabinet which houses a gas flow unit to provide

a constant, known flow of air, an ozone generator and a gas

manifold to sample the air stream, and (2) a separate power

supply for the ozone generator. In addition, a small oxidant

analyzer, a commercial microcoulomb oxidant sensor, is also

supplied to continuously monitor the ozone concentration if this

is desired.

B. Principle of Operation

A controlled flow of air is passed through a quartz tube

which is irradiated by an ultraviolet source. The concentrations of ozone can be controlled by varying either the

intensity of uv radiation reaching the quartz tube and/or

by varying the flow of air through the quartz tube. A

photograph of the interior of the apparatus showing the

location of the principal parts is shown in Figure 4.1.

1 , Gas Flow Unit

A diagram of the system is shown in Figure 4.2. Clean and

dry air at a maximum pressure of 100 psi enters the system

through valve V-1. The air passes successively through a column of charcoal and a column of silica gel. These serve

only as a precaution to remove residual moisture and other

impurities from the air supplied to the unit.

23
Figure 4.1 Photograph o£ ozone system showing location of

principal parts

A. Shutter B. Lamp C. Sampling manifold

24

26
The flow of air through the ozone generator is controlled

by V-2 . The flow is maintained at a constant value at anysetting of V-2 by a differential flow control DFC . The

flow is measured with a rotameter. Valve V-3 is used in

conjunction with an external supply of air of known humidity

(See Section 8) . Valve V-3 is normally closed except when

it is necessary to operate the generator with air of known

humidity. In this case, V-2 is fully closed and the flow is

controlled by V-5 on the humidifier.

2 . Ozone Generator and Power Supply

The ozone generator is located in the main cabinet and is

operated by the separate power supply. Construction details

of the generator are shown in Figure 4.3. The air stream

flows through the quartz tube which extends through the length

of the box. An ultraviolet mercury vapor lamp is located in

a metal tube parallel to the quartz tube. A narrow slot is

cut into the length of the metal tube to restrict the light

impinging on the quartz tube. Ozone is produced in the air

stream as it passes through the quartz tube by the reaction

of the uv light with oxygen in the air. The concentration

of ozone produced is varied by adjustment of a movable sleeve

which slides inside the first metal tube over the lamp and which

determines the amount of light that reaches the quartz tube.

The sleeve is marked with a scale of one hundred equal intervals

27
so that its position can be reproduced. The actual ozone

concentrations for a given flow rate at various shutter settings

are determined by chemical analysis during the initial

calibration of the generator.

The separate power supply contains a constant-voltage

transformer to insure a constant light intensity and a highvoltage transformer for the operation of the lamp. Three

convenience outlets controlled by the main switch of the power

supply are located on the back of the chassis along with two

fuses. A ISA fuse protects the entire system and a 3A fuse

protects the power to the transformer primary. The outlets

may be used to supply power for auxiliary units such as the

oxidant analyzer and a recorder. A timer is included in the

primary circuit of the lamp transformer which indicates the

number of operational hours of the lamp.

3 . Oxidant Analyzer

The oxidant analyzer, a separate commercial microcoulometric oxidant sensor, is included in the system to allow

the continuous monitoring of the ozone produced in the air

stream while the system is in operation. It is not intended

that the analyzer be used to verify the actual concentration

of ozone produced as this is determined by the shutter setting.

The oxidant analyzer contains a continuous solution system

to allow unattended operation for two weeks or more.

28
C . Opera t ion

1 . Gas Flow Unit

a . Dry Air .

1. Close all waives (V-1, V-2 and V-3)

2. Attach the air supply to the unit with

stainless steel or copper tubing at the connection at the rear

of the cabinet marked "Dry Air".

3. Adjust air supply pressure to a value

less than 100 psi but greater than 10 psi.

4. Open V-1 (Dry Air Off) and adjust flow with

V-2 (Dry Air), to the normal value of 5 1/min.

b. Humid Air . (Refer to Section 8 before proceeding.)

1. Close V-4 (On-Off) and V-5 (Control) on

humidifier

2. Turn regulator handle fully counterclockwise.

3. With 1/4 ii^ copper tubing, attach humidifier

to air supply at a pressure between 10 and 100 psi.

4. Attach humidifier to ozone generator with

1/4 in copper tubing.

5. Open V-4 slowly v/ith V-5 closed.

6. Select desired relative humidity and adjust

regulator to obtain pressure shown below:

29
Relative Humidity

in Percent

Pressure

100 0

75 5

50 15

33 45

25 60

7. With V-3 fully opened and V-2 fully closed

flow is adjusted with V-5 on the humidifier to the normal

value of 5 1/min as indicated by the rotameter on the ozone

generator cabinet. The rotameter on the humidifier should not

be used to measure flow for the ozone generating system.

2 . Ozone Generator

The ozone generator is operated in the following manner:

a. Attach lamp cord to the power supply.

b. Turn on the main power supply switch, which is

the upper switch on the power supply cabinet.

c. Turn on the switch for the uv lamp which is

the lower switch on the power supply cabinet.

d. Line up the mark on the shutter tube which

corresponds to the desired concentration of ozone with the end

of the outer brass tube.

30
The shutter setting for the desired concentration of

ozone is obtained from the calibration curve of shutter setting

vs ppm of ozone, Figure 4.4. This calibration curve

is valid only for a flow rate of 5 l/min corresponding

to a rotameter reading of 68.0 for the top of the stainless

steel float. If other flow rates are used a recalculation of

the ozone produced must be made based on the new flow rate.

The lamp of the ozone generator should be turned on several

hours before use to obtain a stable ozone output. To avoid

accumulation of ozone in the atmosphere in the vicinity

of the unit it is recommended that the shutter tube be in the

zero position during warm-up periods and at any other time

when the system is left on but is not in use. The ozone

concentration produced by the generator stabilizes within

seconds after the shutter setting is changed. The oxidant

analyzer, however, may require a considerably longer

time to re-establish equilibrium.

3 . Oxidant Analyzer

Detailed instructions for the operation of the

analyzer are given in the manufacturer's instruction

manual, which should be consulted before use. It should be noted

that the analyzer is turned on 24 hours before use. The analyzer

may be plugged into one of the convenience outlets on the back

of the power supply which are activated by the main power switch.

If the system is to be used over an extended period of time

31
32
the analyser should be left on even while the ozone generator

is turned off.

The air stream is sampled by the analyzer using a length

of Teflon tubing inserted into the top of the manifold inside

the cabinet.

4 . Shut -Down

Complete shut-down requires that V-1, V-5 and the main

power switch on the electrical cabinet must be shut off. Because

the system requires a period of several hours to reach equilibrium

it may be advisable at times to shut off only the air supply

leaving the generator and oxidant analyzer turned on. The generator shutter should be placed in the fully closed position to

prevent production of ozone in the area.

D . Maintenance

1 . Gas Flow Unit

a. The drying cylinders, located underneath the

main cabinet chassis, occasionally require reactivation

depending upon the purity and/or dryness of the air supply.

The panel with the chassis attached can be removed from the

cabinet by removing the eight screws on the two edges of the

panel. Tubing is removed from one end of the driers leaving

them attached with the short loop of copper tubing at the other

end. Heating for about one hour at 110°C while continuously

flushing with dry air is sufficient to regenerate the adsorbants.

After cooling, replace the cylinders in the chassis and the

chassis and panel in the cabinet.

33
b. The rest of the gas flow unit should require

no maintenance, other than replacement of components which

fail in service.

2 . Ozone Generator

The uv lamp ages with use and the ozone output may

decline. Therefore, the ozone generator should be recalibrated

after 1000 hours of use as indicated by the elapsed time meter

on the power supply cabinet, or at least once a year.

3 . Oxidant Analyzer

Instructions for the maintenance of the analyzer are

included in the manufacturer's instruction manual. Since

the analyzer supplied with this system includes a continuous

solution system, the reagent solution needs to be changed only

every sixty days. Distilled water should be added to the

solution reservoir every two weeks to compensate for evaporation .

E . Calibration of the Ozone Generator

The primary calibration of the ozone generator is given

by Figure 4.4. The calibration curve for shutter setting vs

ppm of ozone has been determined by the neutral potassium

iodide method. The generator may be returned to the National

Bureau of Standards whenever recal ibrat ion is required or the

recal ibrat ion may be performed by a competent chemist using the

procedure outlined in "Tentative Method for the Manual Analysis

of Oxidizing Substances in the Atmosphere" which appeared in

Health Laboratory Sciences, Volume 7, page 152 (1970).

34
F . Materials of Construction

The airflow system upstream of the generator is constructed

of copper and brass. The short line between the irradiation

tube and the sampling manifold is glass. The irradiation tube

is quartz.

35
5. PE^RflEATION TUBE SYSTEM

(SO^, H^S, NO^)

A. Introduc t ion

Permeation tubes arc devices which are partially filled

with a volatile liquid, often at elevated pressures, which

escapes through the walls of the tube at a predictable rate.

The tubes are generally constructed of Teflon, although other

porous plastics may be used. At constant temperature, the

material in the tube will effuse through the walls at a constant rate. The rate may be determined by weighing the

tube periodically while it is held at constant temperature.

The weight loss per unit time can then be calculated.

If a permeation tube is placed in a stream of air flowing

at a constant rate, then a constant concentration of the

material effusing from the tube will be produced in the air

stream. The concentration in the air stream may be easily

changed by varying either the rate of flow of the air or by

changing the temperature of the permeation tube. The apparatus

described herein maintains permeation tubes at a known

temperature and provides a variable and measured floiv of air.

B . Description of System

1 . General Description

A photograph of the unit is shown in Figure 5.1. Figure

5.2 is the interior of the unit showing the location of the

principal parts including the tube conditioner. A diagram of

36
Figure 5.1 Photograph of permeation tube apparatus and

tube conditioner

A. Tube conditioner B. Charcoal trap on tube

condi t ioner

37
Figure 5.2 Interior view of permeation tube apparatus

A. Saturator B. Flow meter sensor head

C. Manifold D. Enclosure for exposure chamber

and precondi tioner F. Tube conditioner

F. Charcoal trap

38
3 9
the system is shown in Figure 5.3. The system includes a trap

packed with charcoal and silica gel (Dryer), a flow regulating

device (DFC-1), a flow meter, a water jacketed precondit ioner

for the supply air, a water jacketed exposure chamber for the

permeation tube and a sampling manifold. An external circulating

water bath controls the temperature of the exposure chamber.

An auxiliary system allows a fixed portion of the main

air stream to pass through a saturator, bypassing the tube

exposure chamber and recombining with the main stream prior

to passage into the mixing manifold. This allows humidification of the gas in the manifold without passing humid

air over the permeation tube.

A second auxiliary system allows a sample to be drawn

from the manifold and to be mixed in a fixed proportion with

air drawn through a charcoal trap. This reduces the concentration of any substance drawn from the manifold by a fixed

amount, allowing use of analytical instruments whose range

would be exceeded by the undiluted sample.

The flow restrictions are short lengths of 1/8 in stainless

steel tubing.

2 . Temperature Control

The permeation rate of the tube is determined by the

temperature of the air passing over the tube. The temperature

of the air is determined by the temperature of the water

circulating through the exposure chamber and the precondit ioner

.

Tubes are best used at or near the calibration temperature.

40
The temperature of the room and that of the tube should not differ

by more than 5°C. The circulation rate of the water is

such that the exposure chamber should be within ±0.1°C of the

temperature indicated at the circulating water bath if room and

bath do not differ by more than S°C. Temperature in the exposure

chamber may be measured by inserting a thermometer into the

chamber and allowing it to reach equilibrium with the air stream.

The top of the exposure chamber should be plugged during this

operation to prevent circulation of room air into the chamber

which may result in erroneous temperature readings. This is

easily accomplished by using a cork drilled to fit the thermometer.

During normal use of the unit a glass cap serves to close the

upper end of the chamber and there is no danger of room air

entering. The circulating bath should be set at the calibration

temperature and after equilibrium has been reached, the temperature

of the chamber is measured. If there is a difference of greater

than ±0.1°C the lines between the bath and the unit should be

shortened or insulated. The bath provided with the apparatus

is described in detail in a manual accompanying the bath.

3 . Air Flow Control

The air flow system is shown in detail in Figure 5.3.

Air which has been previously dried and freed of organic

vapors is brought into the system through the bulkhead connection marked "Air" at the rear of the chassis. A minimum

pressure of 10 psi is required. Maximum pressures should not

exceed 100 psi. Valve V-1 controls the flow of air into

the system and serves to isolate the system when it is not in

41
use. The air then passes through a cylinder packed with layers

o£ silica gel and activated charcoal. The purpose of these

IS to provide assurance that the air passing through the

instrument is dry and free of hydrocarbons. Valve (V-2)

controls the flow through the system. The system incorporates

a constant flow device so that once a flow is set, it will

remain constant until the valve is again adjusted. Tlie flow

rate is indicated on a mass flow meter. The scale is marked

in standard cubic centimeters per minute (cmV^iiri). "Standard"

conditions are defined here as 20 °C and a pressure of 760 mm.

Variations in pressure and temperature of the air in the system

do not affect the indicated reading. In all calculation the

volume of air will be that volume indicated by the meter

at the defined standard condition. Details of operation

and adjustment can be found in the manual provided with the

flow meter.

4 . Tube Storage Device

A period of time is required for permeation tubes stored

at one temperature to reach equilibrium at a different

temperature. Furtlier, tubes should never be exposed to the

atmosphere other than during the brief time required to

transfer a tube from storage to exposure chamber and back.

A device is included in the system which allows tubes to be

stored at approximately the same temperature as the exposure

chamber. This is accomplished by circulating the water from

the exposure chamber through the tube conditioning device before

42
o

Pi

H o Eh Cm <

3:; H < CO

1-1

uw

H o O

H f<

P-. CO . uU 'SCDU>Q

0)oMH«

•H

43
returning it to the water bath. A slow flow of clean air, from

the same source of supply as the exposure chamber, is flushed

through the tube conditioning device to remove effused material

from the device. The device shown in Figure 5.4 has three

separate chambers in which either three different kinds of tubes

can be stored or where tubes of the same type can be stored

separately. Valve V-4 is mounted external to both systems and

is used to close off air flow to the storage device when it is

not in use. Valve V-5 is used to control the flow through the

chambers of the storage device. The tubes carrying the air and

the return water are thermally connected to assure that the tube

are subjected to temperature condit ions almost identical to those

in the exposure chamber.

The storage device will be most often employed when it

is necessary to use several tubes of different lengths and

in various combinations to produce a range of concentrations.

When tubes are not in the exposure chamber they are kept in

the storage device ready for immediate use. Each chamber is

equipped with a flow-through cap in which a small amount of

activated charcoal is placed to adsorb material effused from

the tube.

This device should never be used to store tubes for long

periods of time. When tubes will not be needed within a day or two they should be placed in a sealed container

together with a small amount of silica gel and activated

charcoal and stored in a refrigerator or a freezer.

44
C . Use of Apparatus

1 . General Instructions

A source of dry, clean air at a pressure between 10 and

100 psig is connected to the single bulkhead connector at rear

of chassis, marked "Air", A short length of Tygon tubing is

connected between outlet of circulating pump on temperature

control bath and upper connection of exposure chamber. This

connection is accessible through the holes in the door at the

rear of the cabinet. The tube storage device is positioned

close to the main cabinet and a piece of Tygon tubing is connected

between the lower hose connection on the exposure chamber

and one of the hose nipples on the storage device. Another

length of tubing is used to connect between the other hose nipple

and the inlet of the circulating pump. The bath is filled, turned

on, and adjusted to the desired temperature.

A supply of air from the same source as is used to feed

the main instrument is connected to the bulkhead connection of

the storage device using an external shut off valve V-4. A

flow meter is connected to the outlet of one chamber while the

other two chambers are plugged. The valve V-4 is opened until

a flow of 100-500 cm /min is observed.

The power switch to the flow meter on the front panel is

turned on. Twenty minutes is needed for complete warmup and, if

necessary, adjust the zero according to manufacturer's instructions

at this time.

45
Valve V-1 is opened fully, and then V-2 is slowly opened

until a deflection of the flow meter is noted. Allow f-^Q systems

to flush for several minutes. Permeation tubes of proper length

are placed either in the exposure chamber or in the storage

device. Tubes which have been stored at low temperature should

be placed in the system at least 16 hours before use. Equilibration for shorter periods of time may result in a concentration

as much as 10 percent lower than calculated.

2 . Sulfur Dioxide Generation

a. Permeation Tubes . Sulfur dioxide permeation

tubes furnished with this apparatus are National Bureau of

Standards, Standard Reference Materials, and have been individually calibrated over a temperature range from 20 to 30 °C.

The nominal rate for this type of tube is 0.4 yg/min per cm

of tube length at a temperature of 30 °C. Table 5,1 is a guideline to the number and length of tubes needed to produce

the indicated concentration at the indicated flow rate. Minor

adjustment of the flow rate will be needed to compensate for

actual permeation rates which are given in the SRM certificate.

46
Table 5.1 Flow Rate, Tube Length, and Concentration Produced,

for Sulfur Dioxide Permeation Tubes at .30 °C

Concentration, Tube Length, Flow Rate,

ppm cm £/inin

2.5 10 0,625

5.0 20 0.625

10.0 40 0.625

25.0 40 0.250

The permeation rate in micrograms per minute, G, the rate

of air flow in liters per minute, F, and the concentrations

of sulfur dioxide in the air stream, C, are related as follows

C = p-. K

The values of K at various temperatures may be found

in Table 5.4. The equation can be used to calculate the

required volume flow to produce a fixed composition for any

sulfur dioxide permeation tube or combination of tubes.

When the required tubes and the flow rates have been

selected, the tubes are hung in the chamber and the flow is

carefully adjusted to the required value.

b. Analytical System . A flame photometric sulfur

analyzer is provided to monitor the sulfur dioxide in the

generated atmosphere. This instrument will indicate the

satisfactory functioning of the total system, but is not

intended to provide the absolute measure of the sulfur dioxide

concentration. This value is determined by the permeation

tubes, which are the primary standards.

47
The instrument responds only to sulfur atoms and does

not distinguish betv\reen different sulfur containing compounds.

This, however, is not a disadvantage since equimolar amounts

of hydrogen sulfide and sulfur dioxide give equal response.

Consequently, a calibrated sulfur dioxide permeation tube

may be used to calibrate the system either for sulfur dioxide

or hydrogen sulfide. An instruction manual accompanies the

sulfur analyzer and should be consulted for operational instructions.

The upper concentration range of the sulfur analyzer is

not adequate for the high concentration produced by the

generator and it is necessary to dilute the sample. Gas from

the manifold is drawn at a slow rate through a short length of

small diameter tubing. A larger amount of room air is then

drawn into the system through a short length of activated charcoal. The moving force for the two gas streams in this part

of the system is the pump attached to the sulfur analyzer. The

two streams are joined and the sulfur dioxide is diluted by the

ratio of the flows of the two gas streams. It is not necessary

to know or measure the ratio of the two air streams as long as

they remain constant relative to each other.

48
3 . Hydrogen Sulfide Generation

a. Permeation Tube . Atmospheres cont-'iinin.d hydrogen

sulfide are produced v^^ith permeation tubes which, however, are

not primary standards. However, the aoproximate permeation rate for

these tubes is known and is used to set up the systems. The

actual concentration is determined by measurement of the signal

due to sulfur with the flame photometric sulfur analyzer which in

turn will have been calibrated with an NBS Standard Reference Sulfur

Dioxide Permeation Tube.

The approximate concentrations produced' by hydrogen sulfide

tubes of different lengths and nominal rate at 30°C of 0.4 yg/min

per cm of length belov\f are shown in Table 5.2.

Table 5.2 Flow Rate, Tube Length, and Concentration Produced

for Hydrogen Sulfide Permeation Tubes at 30°C

Concentration, Tube Length, Flow Rate

ppm cm l/min

5 10 0.625

10 20 0.625

20 40 0.625

50 40 0.250

The same relationship described for sulfur dioxide exists

between flow, permeation rate and concentration except that the

constant is different as shown in Table 5.4.

All other operational features are the same for sulfur dioxide,

b. Analytical System . A calibration curve is furnished

with the sulfur analyzer whicli will serve as a guide to the calibration of the instrument. A check of this calibration curve or

preparation of a revised calibration should be made before

beginning the generation of mixtures of hydrogen sulfide.

49
Sulfur dioxide tubes are selected to give maximum output

and the flow rate is reduced to give a calculated concentration

between 40 and 50 ppm. The flow from the generator manifold

through the dilution apparatus is adjusted until a signal is

obtained with the sulfur analyzer which is less than 10 ^ A

This value, taken as an upper limit, places the calibration

points on a more linear portion of the log-log calibration

curve of the instrument. After a stable signal is obtained,

the flow over the tube is increased. The indicated concentration

will be reduced by a factor dependent on the ratio of the

original flow to this succeeding flow rate. This procedure

is repeated up to maximum flow through the generator. A

calibration curve can now be prepared from these points.

If no adjustment is made to the dilution apparatus, and

if the sulfur analyzer is adjusted in the same manner each

time, then it will not be necessary to redraw the calibration

curve, but merely to confirm the previous curve.

Hydrogen sulfide tubes may now be substituted for the

sulfur dioxide tubes and the flow rates adjusted to give the

desired concentration. Since the instrument measures total sulfur

a part per million of sulfur dioxide is equal to a part per

million of hydrogen sulfide when expressed on a volume or molar

basis.

4 . Nitrogen Dioxide Generation

a. Permeation Tubes. Nitrogen dioxide permeation

tubes serve both as standards and as the means of generating

50
atmospheres of known nitrogen dioxide. While nitrogen dioxide

permeation tubes will not be NBS Standard Reference Materials,

they will have been calibrated by NBS and can be considered

absolute within the time limits shown on the data sheet

accompanying each tube or group of tubes. Nitrogen dioxide

permeation tubes are available commercially with rates at 25°

of about 1.0 Mg/ min per centimeter of length. The combination

of tube lengths and the flow rates necessary for the production

of concentrations of nitrogen dioxide are given in the table

be low

Table 5.3 Flow Rate, Tube Length, and Concentration Produced

for Nitrogen Dioxide Permeation Tubes at 25 °C

Concentration, Tube Length, Flowf Rate,

ppm cm £/min

2.5 5 1.06

5.0 5 0.53

10.0 10 0.5 3

25.0 15 0.32

The relationship between concentration, permeation rate

and air flow is the same as for sulfur dioxide and hydrogen

sulfide with the appropriate constant given in Table 5.4.

b . Analytical System

The oxidant analyzer used to monitor the ozone output

of the ozone generator may be used to monitor the nitrogen

dioxide produced with the permeation tube apparatus. The inlet

line of the oxidant analyzer is placed into the manifold

through one of the sampling ports. The signal from the analyzer

should be used to verify changes in concentration which result

51
when the flow rate or number of tubes is changed but is not

used to determine the absolute concentration of nitrogen

dioxide. This value is determined from the measured flow

rates and permeation rate of the tube.

Table 5.4 Value of K for Sulfur Dioxide, Hydrogen Sulfide

and Nitrogen Dioxide at Various Temperatures

Substance 20 °C

Value

25

of K

°C 30 °C

Sulfur Dioxide 0.375 0. 382 0. 389

Hydrogen Sulfide 0.708 0. 720 0.732

Nitrogen Dioxide 0.523 0 . 532 0 .541

D . Materials of Cons tructdion

Parts of the system that are exposed to the gas mixture

beyond the permeation tube chamber are constructed of glass,

Teflon, and stainless steel. The system upstream of the

permeation tube chamber is constructed primarily of copper

and brass.

52
6. AMMONIA SYSTEM

A. Introduction

This system produces mixtures of ammonia in air over a

wide concentration range. The system consists of two units;

a gas mixing unit and an analytical unit. The gas mixing

unit produces low concentrations of ammonia by combining

a relatively high concentration of ammonia- in-air (i.e.

the bulk mixture) with a stream of clean, ammonia-free air,

(i.e. the diluent). The analytical unit consists of a system

which directs the bulk mixture through an impinger where the

ammonia concentration is determined by titration of an

acid solution of known strength. Analysis of the bulk mixture

prior to use serves to confirm that the mixture has not

deteriorated between analyses or serves to assess the extent

of deterioration.

B. Principle of Operation

The gas mixing unit and the analytical unit are combined

in one system. A photograph of the system is shown in Figure

6.1. Figures 6.2 and 6.3 show the interior of the unit and

the arrangement of components beneath the chassis. The gas

cylinder containing the bulk mixture is connected to the

bulkhead connector on the frontpanel marked "Ammonia Inlet."

The diluent air is connected at the rear of the chassis at the

connector marked "Air In." The bulk mixture and the diluent

air are mixed in known proportions and are directed to the

53
i

I

54
Interior view of ammonia system

A Mixing chamber B. Solenoid valves

c! Manifold D. Timer

55
View under chassis of ammonia system

A. Differential flow controller B. Drier

56
sampling manifold from which a sample, or samples, may be

withdrawn for analysis.

1 . Gas Mixing Unit

A diagram of the gas flow system is shown in Figure 6.4.

The flow of diluent air is controlled by V-1 and the flow

of the bulk mixture of ammonia in air is controlled by V-2 .

The flow rates are measured by means of FM-1 and FM-2,

respectively. The differential f 1 ow control lers , (DFC-1) and

(DFC-2), maintain a constant flow in each leg of the system

after the initial setting of the flow control valves and will

maintain this constant flow until these valves are readjusted.

Valves 4 and 5, are shut-off valves and isolate the unit when

it is not in use.

The "dryer" shown in Figure 6.4, contains both activated

charcoal and silica gel and is intended as a final dryer for

the diluent air.

The flow of diluent and bulk mixture combine in the

mixing chamber and pass to the sampling manifold. The concentration of ammonia in the air in the manifold is calculated

from the observed flow rates and from the measured concentration

of ammonia in the bulk mixture. The concentration of ammonia

in the bulk mixture is determined with the analytical unit.

2 . Analytical Unit

A 3-way valve, (V-9), is located downstream from FM-2 in

the bulk-mixture stream. In the "mixing" position the valve

directs the flow to the mixing chamber. In the "analysis"

position, the sample is directed to a 3-way solenoid valve

57
58
(V-7). In the normal position, V-7 vents the sample to

atmosphere. When the solenoid is energized, the sample is

directed to the glass socket joint marked (A). A midget

impinger containing a measured volume of acid and an acidimetric

indicator is connected to this soclcet. A timer is connected in

parallel with the circuit which energizes V-7 as shown in the

wiring diagram, Figure 6.5. When the timer switch, S-2, which

also controls V-7 and V-8 is in the "on" position, the timer

operates and gas flows through the impinger at the flow rate

indicated by FM-2. When the titration of the acid by the

ammonia in the bulk mixture is complete, as indicated by a

change in color of the indicator, the solenoid is de-energized

by manually turning off the timer switch so that both the flow of

bulk mixture through the impinger and the timer are stopped. The

length of time of the titration and the observed flow rate are

used to calculate the total volume of gas which passed through the

impinger. The normality and volume of the acid allow a calculation of the amount of ammonia in the measured volume of bulk mixture.

A further refinement to the system enables a sample to

be drawn at a controlled flow rate from the sampling manifold.

The midget impinger is connected at B and C of Figure 6.4 and

the vacuum system draws the sample from the manifold as indicated.

The flow is controlled with V-3 and measured with FM-3. With

solenoid valve, (V-8), in the normally open position, air is

drawn in through the vent at a rate controlled by V-3. When the

59
60
solenoid is energized, flow is directed from the manifold at the

preset rate, through tlie impinger and then through the control

system to the vacuum line.

This portion of tlie system is not normally utilized but

has been included to give the system more versatility should

this be necessary or desirable. Titration of the diluted bulk

mixture will require acid of lesser strengtli than is used for

the bulk mixture and would require standardization of the end

point at the lower concentration. N'o attempt should be made

to dilute the acid provided with the unit for determining the

concentration of dilute mixtures.

3 . Gases; Gas Mixtures

A pressurized cylinder containing a bulk mixture of

ammonia in nitrogen was prepared with an ammonia concentration

of 1000 ppm. The cylinder was pressurized to a total pressure

of 1200 psi to provide approximately 3600 liters of the

1000 ppm mixture. The source of diluent air can be either a pressurized cylinder of "breathing" air or "house" air from

the bench. In either case the air is passed through a silica

gel-charcoal auxilary-dryer to remove major contaminants.

4 . Reagents

The solution contained in the impinger consists of 10.0 ml

of 0.005 N H2S0^ which has been standardized and to which 2 drops of methyl purple indicator has been added.

5 . Procedure

The concentration of ammonia in the bulk mixture is

analyzed by determining the volume of gas required to neutralize

61
an accurately measured volume of standardized sulfuric acid,

based on the color change of the indicator. The concentration

of ammonia, C^^j^ , in parts per million is calculated from the

following equation:

N • V • E • 10^

Cmu = — — (1)

'3 • ^

where,

- ammonia concentration (ppm)

- N = normality of H^SO^ in impinger

' V = volume of H^SO^ (ml)

E = Molar volume (24.5 at 25°C and 760 mm pressure.

If conditions differ significantly from this the

RT

, value of h is given by 1: = — where the appropriate

values of P and T are substituted.)

3

^NH ~ ammonia flow rate (from FM-2) (cmVmin)

t = titration time (min)

The concentration of ammonia in the dilute mixture produced

when the bulk mixture of ammonia is blended with air is calculated

using tlie following equation:

= F . F ^2)

^NH^ AIR

where

,

C = dilute ammonia concentration (ppm)

^NH ^ bulk ammonia flow (cm /min)

62
C.,L, = bulk mixture concentration (ppm) Nn ^ ^AIR " ^-^^^^ ^^"^ /min)

C . Operation

1 . Pr ecaut ions

The maximum pressure to which the components of the

system upstream of V-1 and V-2 should be subjected is 100

psi. The flow controllers (DFC-1) and (DFC-2) will not

operate satisfactorily at pressures below 10 psi. Therefore,

both the diluent air and the bulk mixture should be controlled

at a pressure between 10 and 100 psi. A stainless steel gas

cylinder regulator and a short length of flexible stainless

steel tubing are provided with the unit for connecting the

bulk mixture. The diluent air supply may be any source of clean

and dry air at a pressure greater than 10 and less than 100 psi.

The internal dryer is not intended to clean large volumes of

grossly contaminated air but rather is intended to reduce

residual moisture and organic vapors to a reasonably low level.

Valves V-2 and V-3 are fine control valves and should never be

closed more than finger tight.

2 . Initial Set-Up

a. The bulk mixture, air and vacuum shut-off valves

and regulator valves are closed and that the four electrical

switches are in the off position.

b. Connect the air line to the air inlet at the

back of the chassis.

63
c. Connect a vacuum line to the vacuum inlet

of the chassis.

d. Connect the instrument to a 115 volt AC source.

e. Connect the bulk mixture of ammonia - in-air to

the inlet on the left side of the front panel.

3 . Analysis of Bulk Mixtures by Direct Sampling

The concentration of ammonia in the bulk mixture is

analyzed by determining the volume of gas required to neutralize

an accurately measured volume of standardized sulfuric acid.

The following procedure should be used.

a. Turn the sampling valve (V-9) to "analysis".

b. Adjust the bulk mixture regulator to a pressure

less tlian 100 psi.

c. Open the valve on the bulk mixture and adjust

the ammonia flow to the desired value with (V-2) "Ammonia",

3

on the instrument front panel. A flow rate of 365 cm /min

(50.0 scale divisions on FM-2 using the stainless steel float)

is recommended when using a bulk mixture containing approximately

1200 ppm of ammonia in air. The bulk mixture flow should be

shut off at the tank when actual measurements are not being

made in order to conserve the supply of this gas.

d. Turn on "AC Power" switch (S-1) .

e. Turn on the "Flush" switch (S-3) for several

minutes which will effectively flush air or the previous gas

mixture from the sampling lines to the sample outlet on the

64
analysis unit. The gas mixing unit is now ready for sampling

directly from the bulk mixture.

£. Carefully break the neck of an ampoule containing

10 ml of the 0. 005N ^^2^^A supplied with the unit. Add the

entire contents to a clean impinger tube. Add 2 drops of methyl

purple indicator. Rinse the ampoule with a small amount of

distilled water and add to the impinger. Add distilled water

until the total volume of liquid in the impinger tube is 20 ml.

g. Make sure that the sampling line has been flushed

as described in Step e, and that the "Flush" switch, CS-3) has

been turned off. Attach the middle tube of the impinger to

the direct sampling outlet (A) (the socket joint on the left

in the sampling compartment) . The side arm of the impinger

tube is not connected.

h. Turn on "Lamp" switch. (The lamp should be turned

on only when performing a titration to avoid heating the sample

compartment .

)

i. Make sure that the timer is set at 00.00 minutes

by pushing the reset button on the timer.

j. Turn on the "Timer" switch (S-2). Gas will now

bubble through the impinger and the timer will operate. As

soon as the indicator changes from blue to the final shade of

green , turn off S-2.

A preliminary titration should be performed, bubbling past

the end-point, in order to become familiar with the color change

of the indicator at the end-point. The concentration of ammonia,

^NH ' parts per million is calculated from Equation (1).

3

65
The results of the three titrations should agree to within 1

to 2 percent. The bulk mixture should be analyzed at the

beginning of each day of use and whenever a new bulk mixture is

introduced. Because the concentration of ammonia in the bulk

mixture may change with time, the concentration as determined

by titration should be used for all calculations.

4. Preparation of Dilute Air-Ammonia Mixtures

a. Turn the sampling valve (V-9) to "Mixing".

b. Determine the flow rates of air and ammonia

which will yield the desired concentration using the equation

in instruction 5.

c. Open the air shut off valve (V-4) and adjust

the air flow with V-1, "Air", to the desired value.

d. Open the ammonia shut off valve (V-5) and adjust

V-2, "Ammonia", for the desired flow of the bulk mixture.

(These rotameter settings should be made before sampling in

the impinger .

)

e. Calculate the concentration of ammonia in the

manifold, C , by Equation (2).

5 . Shut -Down

The bulk mixture should be shut-off at the cylinder at

anytime when immediate use of the system is not anticipated.

This is a precaution taken to avoid loss of the bulk mixture

through leakage. If the system is to be left unattended,

the air, the bulk mixture, and the vacuum should be shut off

by closing V-4, V-5 and V-6. If there is any possibility of

66
external pressure from these sources entering the system and

exceeding the maximum pressure of 100 psi then both the

"Ammonia" and 'y\.ir" valves, (V-1) and (V-2) should be left open.

The power switch, (S-1), should be in the off position whenever

the system is not in use.

D . Maintenance

1 . Gas Mixing Unit

The silica gel and the activated charcoal in the drying

cylinder may occasionally need to be replaced depending on the

purity of the house air supply and the amount of use of this

unit. The drying cylinder can be removed after taking the

instrument out of the cabinet by taking off the supporting straps

and disconnecting the tubing at each end of the cylinder.

The cylinder may be reactivated by placing in an oven at

about 110°C while passing a slow flow of clean air through

the cylinder. Alternatively, the cylinder may be refilled

with freshly activated silica gel and activated charcoal in

the appropriate amount and order shown on the cylinder. Add

sufficient glass wool so that the silica gel and charcoal are

firmly packed in the cylinder. Replace the fittings and

reinstall the cylinder in the chassis.

Both the gas mixing chamber and the gas sampling manifold

can be easily removed by loosening the appropriate connections

when repair or replacement is required.

The procedures for cleaning the rotameters and flow

controllers are given in the manufacturer's instructions.

67
2 . Analytical Unit

The lamp may be replaced after removing the four screws

in the corners of the sampling compartment which hold the

translucent plastic sheet in place at the back of the sampling

compartment

.

The electrical system is protected with a 1 ampere fuse

located inside the cabinet on the top surface of the chassis.

The glass connectors (A, B, and C of Figure 6.2) may be removed

and replaced by loosening the Swagelolc connectors and removing

the retaining strap.

E . Materials of Construction

The materials of construction are brass, copper, Teflon,

stainless steel, and glass. The portion of the system which

is exposed to ammonia is constructed entirely of stainless steel.

Teflon and glass. The diluent air system is composed primarily

of copper and brass.

F. PERFORMANCE EVALUATION

1 . Analysis of Bulk Mixtures

Table 6.1 shows the results of replicate analyses of samples

taken directly from the bulk mixture cylinder. The sample

flowed directly into the impinger without going through the

sampling manifold. The precision of these analyses is very

good indicating that the analytical unit is reliable. The

accuracy of the analyses was confirmed by titration of several

acid samples with NaOH. These analyses agree to ±1 percent

of those obtained by titration with gaseous ammonia.

68

I
TABLE 6.1

Direct Analysis of Bulk Mixture

Sample Titration Time

1 2.74 1232

2 2.73 1236

3 2.73 1236

4 2.72 1241

5 2.72 1241

6 2.73 1236

7 2.73 1236

8 2.73 1236

9 2.72 1241

10 2.73 1236

average 2.728 1237.1

standard deviation .006 3.0

rel. std. dev. 0.23"^ 0.24^

conditions o£ analysis:

3 , . 375 cm /mm

N--^ .005165

V 10.0 ml

E 24.5

69
2 . Stability of Bulk Mixtures

The bulk mixtures of ammonia in nitrogen were re-analyzed

periodically over a period of 5 months with less than 1 percent

change in concentration indicating that high concentration ammonia

mixtures have good long-term stability and can be stored in

pressurized cylinders. Even if the mixtures should deteriorate,

this would pose no problem for re-analysis of the bulk mixtures

can be done quite easily with the analytical unit of the

instrument ...

3. Analysis of Ammonia-Air Dilutions

Various mixtures of ammonia in air were prepared by dilution

and analyzed. Table 6.2 shows a comparison of the concentration

calculated from air and ammonia flows with that obtained by

analysis. The average difference is 1 percent with no bias

in either direction.

4 . Effect of Sampling Rate

Figure 6.6 shows the effect of the rate of withdrawal of

sample from the sampling manifold on the analysis. The

apparent decrease in ammonia concentration at higher flow rates

is probably due to incomplete absorpt ion of the ammonis in the

impinger solution. For this reason, sampling should be done at

a rate less than 700 cm^/min.

70
TABLE 6.2

Analysis of Ammonia-Air Dilutions

Concentration (ppm)

Calculated Analyzed

35.6 35. 2 36.1 36.3

52.1 52.5

52.1 51.8

52.1 53.0

93. 9 95. 7 100.1 99.2

100.6 104.0

243. 3 242 .0

Ammonia Bulk Mixture = 1272 ppm

Difference Diff. in Percent

-0.4 1.1

+ 0.2 0.6

+ 0.4 0.8

-0.3 0.6

+ 0.9 1.7

+ 1.8 1.9

-0.9 0.9

+ 3.4 3.3

-1.3 0.5

71
o a

u o C C7> O Cl

1rt •H

.—

1

o o oo

rt •H E

/

r—

\

p: bO

/

•H

(-<

»-i O O

•H r

iH 1 IS :2 o o

P< o o o o

1-H iH 00

TO o

CO

te o

ion o

o

«o o p: o •H

iH

<4-( w > Co o am CO ^ vp < VO

ire

< oo

W) •H

1 1 1

1 1 1 1

1 o

o o o

o ^ cs

(ludd) uoiq.i?j.auoDU03 Biuomuiv

72
G. CONCLUSIONS

The system described in this report is capable of producing

test atmospheres of varying concentrat ionsof ammonia in air

by blending a bulk mixture of ammonia in nitrogen from a pressurized cylinder with a stream of diluent air. Analysis can

be easily performed by titrating a standard acid sample with

the ammonia in the gas stream.

It should be possible to adapt this system to the generation and analysis of other gas mixtures as long as the bulk

mixture possesses adequate tank stability and a titration

reaction can be performed in the analysis.

73
7. CHLORINE SYSTEM

A . Introduction

This system produces mixtures of chlorine in air over

a wide concentration range. The system combines t\<o units;

a gas blending unit and an analytical unit. The gas blending

unit produces low concentrations of chlorine by diluting a relatively high concentration of chlorine in nitrogen (t.e. the

bulk mixture) with a stream of clean, chlor ine - f ree air,

(i.e. the diluent). The analytical unit consists of a system

which directs the bulk mixture through an impinger where the

chlorine is collected after which it is determined colorimetrically. Analysis of the bulk mixture prior to use

serves to confirm that the mixture has not deteriorated between

analysis or serves to assess the extent of deterioration.

B . Principle of Operation

A photograph of the system is sliown in Figure 7.1.

The location of principal parts is shown in Figure 7.2. The

gas cylinder containing the bulk mixture is connected to the

bulkhead connector on the front panel marked "chlorine inlet."

The diluent air is connected at the rear of the chassis at

the connector marked "air in." The bulk mixture and the

diluent air are mixed in known proportions and are directed to

the sampling manifold from ;vhich a sample, or samples, may

be withdrawn.

74
Figure 7.1 Photograph of chlorine system

75
Figure 7.2 Interior view of chlorine system

76
1 . Gas Blending Unit

The gas flow system is shown in Figure 7.3. The

flow of diluent air is controlled by adjustment of V-1 and

the flow of the bulk mixture of chlorine in air is controlled

by V-2. The flow rates are measured at FM-1 and PM-Z,

respectively. The differential flow regulator (DFC-l) maintains

a constant flow in the diluent side of the system depending on

the setting of the flow control valve (V-1). Valves (V-4}

and (V-5) are shut-off valves and isolate the unit when it

is not in use.

The "dryer" shoun in Figure 7.3, contains both activated

charcoal and silica gel and is intended as a final dryer for

the diluent air.

The diluent and bulk mixtures combine in the mixing

chamber and pass to the sampling manifold. The concentration

of chlorine in the air in the manifold is calculated from the

observed flo\v' rates and from the measured concentration of

chlorine in the bulk mixture as determined with the analytical

unit

2 . Analytical Unit

A three-way valve (V-3) is located downstream from FM-2

in the bulk mixture stream. In the "mixing" position the valve

directs the flow to the mixing chamber. In the "analysis"

position, the sample is directed to a three-way solenoid valve

(V-6) . In the normal position ¥-6 vents the sample to the

77

exhaust port. When the solenoid is energized, the sample is

directed to the glass socket joint. A midget impinger containing

an absorbing solution is connected to this socket. A timer

is connected in parallel with the circuit which energizes

V-6 as shown in the wiring diagram. Figure 7.4. When S-2 , which controls V-6, is in the "on" position, gas flows through

the impinger at the flow rate indicated by FM-2 and the timer

operates. The solenoid is de-energized by opening S-2 so that

both the flow of bulk mixture through the impinger and the timer are

stopped. The time m.easurement and the observed flow rate

allow a calculation of the total volume of gas which passed

through the impinger.

3. Gases, Gas Mixtures

A pressurized cylinder containing a bulk mixture of

chlorine (99+ percent) in nitrogen was prepared with a chlorine

concentration of 50 ppm. The cylinder was pressurized to a total pressure of 1200 psi which means that approximately

3600 liters of the 50 ppm mixture are available. The source

of diluent air can be either a pressurized cylinder of "breathing"

air or "house" air from the bench. In either case, the air is

passed through a silica gel-charcoal dryer to remove major

contaminants. An external pre-drier may be required if "house"

air is used.

4 . Procedure

The concentration of chlorine in the bulk mixture may be

determined by passing a known volume of the gas through a solution contained in the impinger producing a color change

79
8(|
which is determined spec trophotome tr ical ly and is related

to the chlorine content of the sample. An alternate method

of analysis consists of titrating a standardized thiosulfate

solution, contained in neutral -buffered potassium iodide,

with the gaseous chlorine mixture to a starch- iodine end

point.

In the spec trophotometr ic method, the chlorine concentration is determined by comparison to a calibration curve,

whereas in the thiosulfate titration, the chlorine concentration is determined by use of the following equation:

r = 1/2 N ' V - E ' 10^ . . ^l^- F,^^ . At

where

,

^Cl ^ chlorine concentration (ppm)

N = normality of thiosulfate solution.

V = volume of thiosulfate (ml).

E = molar volume (24. 5£ at 25°C and 760 mm pressure.

If conditions differ significantly from this, the

value of E is given by E = RT/P, where the appropriate

values of P and T are substituted.

F^^ = chlorine flow rate (from FM-2) (cm /min)

At = titration time - blank time (min).

The concentration of chlorine in the dilute mixtures

produced when the bulk mixture of chlorine is blended with air

is calculated using the following equation:

81
c X

(2)

C

F

CI

X

dilute chlorine concentration (ppm)

.

bulk chlorine flow (cm /min)

.

2 C

F AIR

bulk mixture concentration (ppm)

.

diluent air flow (cm'^/min) .

C . Operation

1 . Precaut ions

The maximum pressure to which the components of the system

upstream of V-1 should be subjected is 100 psi. The flow

controller (DFC-1) will not operate satisfactorily at pressures

below 10 psi. Therefore, the diluent air should be controlled

at a pressure between 10 and 100 psi. A gas cylinder regulator

with a monel diaphragm and a short length of flexible Teflon

tubing are provided with the unit for connecting the bulk mixture.

The diluent air supply may be any source of clean, dry air at

a pressure greater than 10 and less than 100 psi. The internal

dryer is not intended to clean large volumes of grossly contaminated air but rather is intended to reduce residual moisture

and organic vapors to a reasonably low level. Valve V-2 is a fine control valve and should never be closed more than finger

tight.

2 . Initial Set-Up

valves and regulator valves are closed and that the four electrical

switches are in the off position.

a

.

Be sure that the bulk mixture and air shut-off

82
b. Connect the air line to the air inlet at tlic hack

of the cliassis.

c. Connect an exhaust line to the exhaust port

at the back of the chassis.

d. Connect the instrument to a 115 volt AC source.

e. Connect the bulk mixture of chl or ino - in -n i tro gen to

the inlet on the left side of the front panel.

3 . Analysis of Bulk Chlorine Mixtures

a. Turn the sampling valve (V-3) to the "analysis"

position.

b. Adjust tlie bulk mixture regulator to a pressure

less than 10 psi.

c. Open valve V-5 and the chlorine flow valve (V-2).

d. Coarsely adjust the flow of chlorine to the

desired value using the chlorine flov; valve (V-2).

e. Fine adjustment of the chlorine flov/ is made

using the pressure control on the chlorine bulk mixture regulator.

(A flow rate of 100 cm"^/min (11.1 scale divisions on the chlorine

rotameter using the stainless steel float, is recommended). The

bulk mixture flov/ should be shut off at the tank when actual

measurements are not being made in order to conserve the supply

of this gas.

f. Turn on the AC power switch (S-1).

g. Turn on the flushing switch (S-4) for at least

15 minutes which will effectively flush air or the previous

gas mixture from the system. The gas mixing unit is now ready for

sampling directly from the bulk mixture.

83
4 . Analysis of Bulk Mixtures

This method is essentially the titration of a standard

thiosulfate solution with a chlorine in nitrogen gas mixture

using starch as tlie indicator.

Steps a. thru h. establish a blank needed for the chlorine

analysis which is given in steps i. thru r.

a. Place 5.0 ml of the neutral buffered KI

solution in the impinger.

b. Add 2 ml starch solution.

c. Dilute to 20 ml with distilled water.

d. Make sure the sampling line has been flushed

as described in Step g, Section 3, and that the flush

switch (S-4) has been turned off. Attach the impinger to

the sample outlet.

e. Make sure that tlie timer is set at 00.00

minutes by pushing the reset button on the timer.

f. Turn on the timer switch, (S-2). Gas will

now bubble through the impinger and the timer will operate.

g. As soon as the first indication of a faint

purple color appears in the solution, turn the timer off.

h. Perform the titration three times. An

agreement of ±3 percent in the time required for three

titrations is an adequate blank. The average of these three

times is later subtracted from the time required for titration

of a sample in the procedure described belov/ in steps i.

through r.

84
i. Place 5.0 ml of the neutral buffered KI

solution in an impinger.

j. Add 2 ml starch solution.

k. Add contents of the standard thiosulfate

- 4 ampoule (1.035 x 10 N, volume = 10.0 ml).

1. Rinse with distilled water.

m. Dilute to 20 ml with distilled water.

n. Make sure the sampling line has been flushed

as described in Step g, Section 3, and that (S-4) has been

turned off. Attach the impinger to the sampling outlet.

o. Make sure that the timer is set at 00.00

minutes by pushing the reset button on the timer.

p. Turn on the timer s\vitch, (S-2). Gas will

now bubble through the impinger and the timer will operate.

q. As soon as the first indication of a faint

purple color appears in the solution, turn the timer off.

r. Perform the titration three times. The times

of the three titrations should agree to within 3 percent.

The titration blank obtained earlier is subtracted from

this value and the concentration of chlorine in nitrogen in the

bulk mixture is determined by the use of Equation (1). The

bulk mixture should be analyzed at the beginning of each day of

use and whenever a new bulk mixture is introduced. Because the

concentration of chlorine in the bulk mixture may change with

time, the concentration as determined by titration should be

used for all calculations.

85
5 . Preparation of Dilute Air-Clilor ine Mixtures

a. Turn the sampling valve, (V-3), to the mixing

position.

b. Determine the flow rates of air and chlorine

whicli will yield the desired concentration.

c. Open the air-off valve (V-5) and adjust the

chlorine flow valve (V-2) and the control valve on the chlorine

bulk mixture regulator (V-7) for the desired flow of the bulk

mixtures.

6 . Shut-Down

The bulk mixture should be shut off at the cylinder at

all times when immediate use of the system is not anticipated.

This is a precaution taken to avoid loss of the bulk mixture

through leakage. If the system is to be left unattended and

it is not feasible to disconnect the diluent air and the

bulk mixture, then the system should be isolated by closing

the chlorine-off valve, the air-off valve and the control

valve on the chlorine stock tank regulator. If there is any

possibility of external pressure from these sources entering

the system and exceeding the maximum pressure of 100 psi, then

both the air flow valve and the chlorine flow valve should be

left open.

The AC power switch, (S-1), should be in the off position

whenever the system is not in use.

D . Maintenance

1 . Gas Mixing Unit

The silica gel and activated charcoal in the drying

86
cylinder may occasionally need to be replaced depending on

the purity of the house air supply and the amount of use of

this unit. After removing the system from the cabinet the

drying cylinder can be dissembled by taking off the supporting

straps and disconnecting the fittings on each end of the

cyl inder

The cylinder may be reactivated by placing it in an oven

at 110°C while passing a slow flow of clean air through it.

Alternatively, the cylinder may be refilled with freshly

activated silica gel and activated charcoal in the appropriate

amount and order shown on the cylinder. When refilling the

cylinder, be certain that sufficient glass wool is used so

that the silica gel and charcoal are firmly packed in the

cyl inder

Both the gas mixing chamber and the gas sampling manifold

can be removed easily by loosening the appropriate connections

in case repair or replacement is required.

2 . Chlorine Analysis Unit

The lamp may be replaced after removing the four screws

in the corners of the sampling compartment which hold the

translucent plastic sheet in place at the back of the sampling

compartment

The electrical system is protected with a 1 ampere fuse located

inside the cabinet on the top surface of the chassis. The

glass connectors may be removed and replaced by loosening the

connectors and removing the retaining strap.

8 7
E . Materials of Construction

The materials of construction are brass, copper, Teflon,

stainless steel, and glass. The portion of the system which

is exposed to chlorine is constructed entirely of stainless steel,

Teflon and glass. The diluent air system is composed primarily

of copper and brass.

F. PERFORMANCE EVALUATION

1 . Conditioning of Gas Cylinders

/ Mixtures of chlorine in nitrogen placed in pressurized

steel cylinders will show a decrease in chlorine concentration

with time. This decrease can be slowed down considerably by

conditioning the cylinder by prior exposure to a chlorine

mixture of slightly greater concentration than the final

mixture. If the cylinder is conditioned with a chlorine

mixture of greater concentration than the final mixture, the

concentration will increase with time, whereas, if the

cylinder is conditioned with a mixture of lower concentration,

the concentration will decrease with time. This behavior

indicates that an adsorpt ion-desorption process of chlorine onto

the cylinder walls is taking place.

2. Tank Stability

Once the cylinder is conditioned properly and allowed

to reach equilibrium, the change in concentration with time

is fairly slow. But, because a change is continually occuring,

analysis of the bulk mixture is advised before each day's

operation.

.

88
3 . Analysis of Bulk Mixtures

Several different analytical methods were investigated for

the analysis of chlorine in air. These were as follows:

(1) o-tolidine

(2) methyl orange

(3) neutral buffered KI spectrophotometr ic

(4) Kl-starch indicator

In methods 1, 2 and 3 an absorbing solution is placed in an

impinger and the chlorine-air mixture is bubbled through this

solution for a pre-determined time period. A portion of this

solution is then placed in a cuvette in a spectrophotometer

and the absorbance read at the appropriate wave length. The

chlorine concentration is then determined by the use of a calibration curve made using known chlorine mixtures. The

sensitivities of these three methods are compared in Figure 7.5.

Figure 7.5 shows the o-tolidine method to be the most

sensitive, A problem arises, however, in the use of this

method. Complete absorption of the chlorine in the impinger

solution does not occur which produces consistently low results.

This can be corrected by using two impingers connected in

series. Typical results are: 80-90 percent absorption in the

first impinger.

The methyl orange method requires some care in the

preparation of the impinger solution. Good reproducibility in

the analyses is not obtained unless the methyl orange concentration is exactly the same each time a new solution is made.

89
ppm Clo

Figure 7.5 Comparison o£ Analytical Methods for

Chlorine Analysis

90
The preparation of the KI solutions are less critical

and complete absorption occurs in the first impinger. Since

the bulk mixture concentration of chlorine should be 50 ppm

±50 percent, the neutral buffered KI method is recommended

for analysis of dilute mixtures since it possesses adequate

sensitivity and is easy to perform.

The Kl-starch indicator method provides a way of titrating

the chlorine gaseous bulk mixture to a color change at the

end point. This method is also recommended and is described

earlier in this report. The Kl-starch indicator is not

recommended for the analysis of chlorine mixtures below 10 ppm

chlorine.

G. CONCLUSIONS

The system described in this report is capable of producing

well-defined test atmospheres of varying concentration of

chlorine in air by blending a bulk mixture of chlorine in

nitrogen from a pressurized cylinder with a stream of diluent

air. Analysis can be easily performed by titrating a standardized

thiosulfate solution to a starch end point with the chlorine in

the gas stream, or by measuring the color produced spec trophoto

metrically.

91
8. GAS HUMIDIFICATION SYSTEM

A. Principle of Operation

The gas humidif icat ion system or "saturator" is

designed to deliver a stream of air with water content

ranging from 100 to 15 percent relative humidity (RH)

.

The saturator was developed to supply the ozone generator

with air of known humidity or to supply the gas mixing system

with humid air, wliich is added to the binary gas mixture produced

with that unit. It may be used with any other system where

air of known humidity is required.

; Varying humidities are produced in the stream of delivered

air by saturation at elevated pressure followed by expansion

to atmospheric pressure. The relationship between the saturation pressure, P^, the delivery pressure, P^, and the RH is:

P.

RH = ^

X 100.

s This equation accurately describes the relative humidity

of the delivered air providing that the temperature of the

saturator and the temperature of the delivered air are

the same and that the pressures are expressed in the same units.

In practice, this will be the case since both saturator, and the

unit to which the humid air is delivered will be at the same

temperature.

92
B . Description of System

The saturator is shown schematically in Figure 8.1 while

Figure 8.2 and 8.3 are photographs of the front and interior

of the unit. Clean air enters the system at A, and passes

through the regulator to V-4. iVhen \'-4 is opened, the pressure

in the saturators is determined by the setting of the

regulator and is indicated by the gauge. Air saturated at

tlie indicated pressure is expanded to atmospheric pressure

across \'-5 and the flov; is indicated by the flow meter FM-1.

Connection to the ozone generating system, or other system,

is made from "B" on humidifier to the appropriate position

on the other system.

C . Operation

1. Close valves marked "Air On" (\'-41 and "Air Control"

(V-5)

2. Unscrew handle of regulator to full counter clockwise

position. Handle will fall off if unscrewed too far. If

this happens merely screw back in a couple of turns.

3. Connect 1/4 in copper tubing between supply of clean

dry compressed air at a pressure greater than 10 psi and less

than 150 psi at connection marked "Air In".

4. Connect 1/4 in copper tubing between "Air Out" on

humidifier and the system to which the humid air is to be added.

5. Turn on the air supply.

6. Open fully valve marked "Air On" (A'-4) on the humidifier.

Caution: Never open (V-4) valve unless the pressure gauge

reading is "zero".

93
94
FicTure 8.2 Front view of humidifier

95
Figure 8.3 Side view o£ humidifier

A. Saturator B. Differential flow controller

C. Regulator D. Pressure gage E. Flowmeter

96
7. Turn handle of regulator until desired saturation

pressure is attained. Relative humidity and pressure relationship are shown below:

Saturation Pressure,

ps ig Percent Relative Humidity

5 75

15 50

30 33

45 25

8. Slowly open "Air Control" (V-5) to produce desired

flow rate.

D . Maintenance

Both the trap and the saturator are filled with fine

gravel and the saturator should contain about 10 to 20 ml of

liquid water at the beginning of any period of use. A liter

of air saturated at one atmosphere contains about 25 mg of

water and an initial filling of 10 ml in the saturators

should be sufficient to humidify about 400 liters of air to

a relative humidity of 100 percent. Larger volumes can be

humidified at lower relative humidities. In any case,

water should be added to the saturators periodically during

use. The connection between the tops of the two saturators

is removed and water is added until the level in each

saturator is about 1/3 the height of the saturator body.

Water can be added conveniently with a large syringe to which
is attached a length of 1/16 in flexible tubing. The water

level can be adjusted after addition of excess water by

positioning the flexible tube in the center tube of each

saturator so that the end of the flexible tube is 1/3 of the

height of the saturator from the bottom of the saturator.

The syringe is held in this position and water is withdrawn

to the bottom of the tube.

98
9. PARTS LIST

Whenever possible, commercially available parts have

been used in the systems described in this report. These

parts were chosen primarily for availability and function.

Any similar parts from manufacturers other than those

shown which have the proper operating characteristics

can be substituted for those listed in Table 9.2. This

table lists the part number, a description, the manufacturer,

and either a catalog number or manufacturers designation.

Table 9.1 identifies the individual components on the

schematic diagrams.

Replacement of parts which are not commercially

available and which were made especially for these systems

may be obtained from the National Bureau of Standards

at the cost of fabrication. Alternatively, they may be

fabricated as needed by the user. None of these parts

are dimensi onal ly critical and an understanding of their

function and approximate dimensions should be sufficient

to allow their duplication. Sketches and dimensions are

available on request from the National Bureau of Standards.

99
Table 9.1 Key to Parts Shown on Schematic Djagrams

Figure Ji'umber Component Part Numb er-Ta ble 9 .2

2_a V-1,2,3,4, 103-1 or 100-2

Filter "'.ilter 400-1

Flowmeter 200-1

lii V-1

V-2

V-4

V-6,7,8,

V-9

. ' •- DFC-1

-

•

' DFC-2,5

FM-1

' FM-2,3

Pump

' Filter

_4,_2 V-1,3 100-2

V-2 100-1

DFC-1 300-1

FM-1 200-3

5^3 v-1,3,4 100-2

V-2, 5 100-1

DFC-1 300-5

Flowmeter 200-2

6.4 v-1 100-3

V-2, 3 100-4

V-4,5,6 100-5

V-7,8 100-6

V-9 100-7

DFC-1 300-1

DFC-2 300-2

DFC-3 300-3

FM-1 200-3

FM-2,3 200-4

6.5 S-1,2 500-2

S-3,4 500-3

Timer 500-4

V-7,8 100-6

100-10

100-3

100-9

100-11

100-12

100-7

300-1

300-2

200-3

200-4

500-1

400-1

100
Table 9.1 continued.

Figure Number Component Part Number-Table 9.2

7.5 V-1 100-1

V-2 100-14

V-3 100-16

V-4 100-2

V-5 100-15

V-6 100-6

DFC-1 300-2

FM-1 200-3

FM-2 2 00-6

Filter 400-1

7.4 S-1,2 500-2

S-3,4 500-3

Timer 500-4

V-6 100-6

8.1 V-4 100-1

V-5 100-8

DFC-1 300-4

FM-1 200-5

Regulator 500-5

101
Table 9.2 List of Commercially Available Parts.

Designation

P^art Number Description Manufacture r* or Cat. No.

lUU-1 Va Ive iW ni t ey

*1 T> CI Jl 1RS4

luU-d. V a ive i/v ni cey IV o4

lUU-p Va Ive Whltey 3I1 i.o4

100-4 '/a Ive -micro meter Ing vv ni tey 22KS4

"1 c; lLUl'-O Va Ive w ni uey 3Nj3o4

100-D Va Ive-solenold Skinner v-* iV J L V ' 100" f Valve-3 ''^ay Whltey 42x54

100-8 Valve Whltey 4rs4

100-9 Va Ive Whltey 3N3S4A316

100-10 Valve Whltey 3'7S4

100-11 Va Ive Whltey 22RS4316

100-1-^ Valve-3 way Whltey 3IR043ID

100-13 Valve-3 way Whltey 42aS431o

100-14 Va Ive -Nylon Nupro Z, - 4 V 100-1^ v a ive - J. ei ion wupro 1 - 4 V U

Ii00-l6 Va Ive-monel Hoke 7165G4M

200-1 Flowmeter-? lotron 1 C i^'iaLheson OII2-OII3

200-2 Flowmeter Ha stings

-Raydlst All'-5KX

200-3 Rotameter Brooks R-2-15-^

200-4 Rotameter Brooks R-2-I5-D

200-5 Rotameter Brooks R-2-I5-B

200-6 Rotamete c Ma the son Tube no. 300

300-1 Flow 'controller lioore 633D

300-2 Flow controller Moore 633DL

300-3 Plow controller Moore 63BUL

300-4 Flow controller Moore 63 3U

300-5 Flow controller Moore 63BDL

400-1 Line filter Nupro 4F-60

5OO-I Diaphragm pump Neptane Model 2

500-2 Toggle swltch-DPST

500-3 Toggle switch-SPST

500-4 Timer Precision Timer SR-240-:

500-5 Regulator Ma the son 40H

^Mention of manufacturer does not constitute a recoinmendat ion

or endorsement by the NBS nor does it imply that the material

or equipment is necessarily the best available for the purpose.

102
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1 . PU B LIGATION OR RE PORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA NPSIR 73-292 ***** SHEET

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Gas Generating Systems for the Evaluation of Gas

Detecting Devices

5. Publication Date October 1973

6. Performing Organization Code

7 aiithorK) E . E - Huffhes W D Dorlcn F P Srhpirlp

L.C. Hall, A.L. Beilby, and J.K. Taylor

8. Performing Organization

NBSIR 73-297

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON D C 202^4

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. sponsoring Organization Name and Address

National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

13. Type of Report & Period

Covered

Final

14. Sponsoting Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This report summarizes a project concerned with the design, testing

and calibration of systems for producing known low concentrations of

a number of substances in air at concentrations in the vicinity of

the TLV (Threshold Limit Value). The substances include carbon dioxide,

carbon monoxide, ozone, sulfur dioxide, hydrogen sulfide, nitrogen

dioxide, benzene, carbon tetrachloride, tr ichloroethylene , tetrachloroethylene, ammonia and chlorine. The systems are of four basic

types: direct delivery from cylinders of known concentration, dynamic

blending of high concentrations with air, permeation tubes and generation

of the substance directly in an air stream. Monitoring systems are

suggested to allow confirmation of the quantities generated. Details of

construction are given.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Gas mixtures; gas detectors; trace gas analysis.

18. AVAILABILITY STATEMENT

I I UNLIMITED.

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

21. NO. OF PAGES

106

22. Price

UNCLASSIFIED

^^ia 1 USCOMM-SC 66244.P71