NBSIR 73-255

System for Preparation of Known

Concentrations of Methane in Air

Ernest E. Hughes, William D. Dorko, and John 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 73-255

SYSTEM FOR PREPARATION OF KNOWN

CONCENTRATIONS OF METHANE IN AIR

Ernest E. Hughes, William D. Dorko, and John 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

TABLE OF CONTENTS

Page

1. Introduction 1

2. Description of System 1

3. Operation of System 2

4. Chromatographic Analysis of Methane Mixtures 4

5. Response Time - Generating System 5

6. Response Time - External Exposure Chamber 5

7. Operation - Summary of Instructions 6

8. Safety Precautions 7

9. Parts and Maintenance 7

i
LIST OF FIGURES

P age

1. Schematic Diagram of Methane Generating System 9

2. Photograph of the Front of Methane System 10

Including Separate Chamber

3. Photograph of the Rear and Interior of Methane 11

System

4. Typical Calibration Curve 12

5. Calibration of FM-1 Air Flow Rotameter tube: 13

R-2-15C

6. Calibration of FM-2 Methane Flow Rotameter 14

tube: R-2-15AA

7. Plot of Reciprocal of Air Flow Versus Concen- 15

tration of Methane Produced at a Fixed Flow of

Methane ; ,

8. Schematic Diagram of Gas Chromatograph Sampling 16

and Analysis System

9. Response of Generating System to Changes in 17

Methane Concentration

10. Response Time of External Exposure Chamber 18

11. Sketch of Mixing Chamber and Sampling Manifold 19

12. Sketch of External Exposure Chamber 20

LIST OF TABLES

1. Typical Calibration Data 21

2. Data Obtained at Constant Flow of Methane and 2I

Various Flows of Air

3. Air Flow Necessary to Produce Specific Concen- 22

trations of Methane at Fixed Methane Flow

4. Reproducibility of the System When Used to Produce 22

Specific Concentrations of Methane

5. Response Time of Generating System 23

6. Response Time of External Chamber 24

7. Parts List 25

ii
SYSTEM FOR PREPARATION OF KNOWN CONCENTRATIONS

OF METHANE IN AIR

ABSTRACT

A gas generating system is described for

producing concentrations of methane between 0.5 and

5.0 mole percent in air or other gases. The system

has been evaluated and a calibration procedure is

described. The system is equipped with adequate

safety measures so that mixtures in the explosive

range can be generated safely.

' 1. INTRODUCTION

This system produces mixtures of methane in air at concentrations ranging from fractions of a percent to above

5 percent. Pure methane is metered into a mixing chamber

where it is mixed with a metered stream of air. The combined stream then flows into a manifold where samples can

be removed for calibration of instruments or for analysis.

The system is provided with flashback arresters to prevent

propagation of ignition within the system and check valves

to prevent diffusion of methane into the air system or of

air into the methane system. A connection is available for

direct attachment to an instrument to be evaluated or to an

instrument for analysis of the concentration of methane

produced. In addition, a chamber is provided in which methane

detectors which depend on diffusion of methane into the

detector can be mounted for evaluation.

2. DESCRIPTION OF SYSTEM

The system is shown schematically in Figure 1 and a photograph of the system and the separate chamber are shown

in Figure 2. Air at a pressure above 15 psi and below 150 psi

is attached at the bulkhead connector marked "Air-In" . . located at the rear of the chassis, Figure 3. Methane at a similar range of pressure as the air is connected to the

system at the bulkhead connector marked "Methane-In" located

next to the air connection. Flow of air and methane into the

system are controlled with on-off valves marked "Air-On" and

"Methane -On" , respectively. These valves are identified as V-1 and V-2 in Figure 1. The separate gases pass through

differential flow controllers, DFC-1 and DFC-2, which control

the air and methane, respectively. The flow of each gas is

controlled by valves marked "Air-Flow" and "Methane -Flow"

,

V-3 and V-4, respectively. The rate of flow of the air is
indicated by FM-1 and the flow of methane is indicated by

FM-2. Each gas then passes through a sintered metal filter

which serves as a flashback arrester, F-1 and F-2, and then

through a check valve, CV-1 or CV-2. The separate streams

then pass into a mixing chamber and the resulting mixture

passes to the sampling manifold. The sampling manifold has

an opening through the front panel into which a probe may be

inserted. This connection is threaded and may be capped. An

additional connection located at the lower end of the manifold

inside the case is attached to a bulkhead connector on the rear

of the chassis marked "Sample Out". When the front port of the

manifold is capped, the mixture exits from the manifold through

the connection at the rear. The auxiliary chamber or an

analytical instrument may be attached at this connector.

The entire system is constructed of metal with the

exception of the glass rotameters. The valves, filters, flow

controllers, and connectors are commercial items and can be

readily obtained and replaced. All tubing is 1/4" O.D. by

0.032" wall, refrigeration grade copper tubing. The mixing

chamber and manifold are fabricated from standard size copper

pipe fittings and large diameter copper pipe.

' . 3. OPERATION OF SYSTEM

The concentration of methane produced by measured flows

of air and methane can be calculated for concentrations up to

about 2 percent. Above this concentration the observed value

tends to decrease relative to the calculated value and the

deviation becomes larger as the concentration increases (Figure

4) . For this reason it is advisable to calibrate the system

periodically to determine the exact relationship between

observed flow rates and the concentration produced. Calibration

gases consisting of mixtures of methane in nitrogen at concentrations of approximately 0.5, 2.0, 4.0, and 5.0 percent are

provided. The exact concentration is indicated on the individual

cyl inder s

.

A calibration curve is constructed by analyzing each

standard using a gas chromatograph . The gas generating system

is then adjusted to produce a variety of concentrations each

of which is analyzed. Subsequent operations depend on whether

the system will be set to produce a specific concentration or

whether it will be used to produce random concentrations.

A typical calibration curve is shown in Figure 4. The

system was attached directly to a gas chromatograph through

the connection marked "sample out". Various concentrations of

methane were generated by varying the flow of methane. The

2
data from which the curve was constructed is given in Table 1.

The calibration curves for FM-1 and FM-2 are shown in Figures

5 and 6. The chroma tograph was calibrated with various known

concentrations of methane in nitrogen as discussed later. It

is obvious that above 2 percent methane the predicted concentration is higher than that observed. This type of calibration

is adequate for use when random concentrations are generated

and when it is not necessary to set the system at a specific

concentration. If a specific concentration or a series of

specific concentrations are needed then a somewhat more complicated calibration procedure is followed. This involves setting

a constant flow of methane and changing the air flow at which

specific concentrations of methane will be produced. A single

setting of methane flow, if of the proper magnitude, can be

used to produce concentrations of methane from 0.5 to greater

than 5.0 percent. The generator is adjusted for a methane flow

of about 0.04 1/min. and a series of mixtures are prepared at

measured air flow rates. The signal is measured for each

concentration produced and the actual concentration of methane

is calculated. A curve can then be constructed of air flow

versus methane concentration. The curve, however, is not

linear and a more convenient form in which to represent the

data is to plot the methane concentration against the reciprocal

of the air flow. It is then quite easy to determine the air

flow necessary to produce any concentration of methane at the

fixed flow of methane.

Table 2 presents data obtained at a methane flow of 0.0412

1/min. and at varying flows of air. The concentration is that

calculated from the observed signal and calibration data obtained

with standards of known methane content. Figure 7 is a plot of

methane concentration versus the reciprocal of the air flow.

Table 3 is constructed from Figure 7 and is simply the air flow

required to produce specific concentration of methane at the

indicated flow of methane. The flow of methane is in all cases

considerably less than that of air and control of the methane

flow, or more precisely the adjustment of methane flow to a particular value, is much more difficult than the adjustment of

air flow to a particular value. This is the only reason for

using a fixed flow of methane and varying the flow of air to

produce varying concentrations of methane.

Table 4 represents the reproducibility possible when the

system is operated to produce specific concentrations of methane.

The three sets of values, I, II and III, were obtained on three

different days by the procedure just described. The relative

standard deviation does not exceed 5 percent in the worst case

and the reproducibility is certainly adequate for the calibration of methanometers

.

3
The accuracy with which a mixture of methane can be prepared depends on how carefully the system is calibrated. The

standards provided with the system are compounded by a pressure technique with an inaccuracy of about 1 percent. If

the system is carefully calibrated using these standards it

should be possible to produce mixtures known with the same

degree of accuracy. However, such accuracy will seldom be

required for the calibration of conventional methanometer s or

similar devices. The system is operated with the least degree

of accuracy when mixtures are prepared using the measured flow

rates of the methane and air to calculate the methane concentration. The maximum error using this method of operation is

about +10 percent at concentrations of 5 percent methane. The

error becomes smaller at lower concentrations and at 2 percent

methane it is about ±3 percent. The error at higher concentrations is due to a system phenomenon while the error at the

lower level is random and is almost entirely due to imprecise

reading of the flowmeters.

4. CHROMATOGRAPHIC ANALYSIS OF METHANE MIXTURES

The gas mixtures produced by this system can be treated as

binary gas mixtures providing that the chromatographic detector

is sensitive only to one constituent, methane not air. A flame

ionization detector is ideally suited for this analysis because

of the low sensitivity to oxygen and nitrogen. The sensitivity

of this detector is so low for air that at the concentrations

of methane measured the contribution of air is not measurable.

Therefore, it is not necessary to use a column that will separate

methane from air but it is only necessary to use a column which

will retain methane slightly and which will yield a peak which

can be easily measured. In the studies described here peak

areas were measured electrically and the conditions were adjusted

so that the methane peak appeared in less than one minute. The

operating parameters are given below and a schematic diagram of

the sampling system is shown in Figure 8.

,: . Column packing 50 percent vinyl substituted

silicone rubber

Column length and diameter 6' x 1/8"

Column temperature 25°C

: Carrier flow rate 60 cc/min.

Sample volume 2 cc

4
A sample is withdrawn from the manifold by applying a slight vacuum at the outlet of the gas sampling valve. Sample

flow through the valve was adjusted to an approximate flow of

100 cubic centimeters per minute (cc/min.). Just before injecting the sample into the chromatograph , the vacuum control valve

was closed and as soon as the flow shown by the sample flow

meter had dropped to zero, the sample was injected. This

assured that the pressure in the gas sampling loop was the same

from sample to sample.

5. RESPONSE TIME - GENERATING SYSTEM

The generation system reaches equilibrium in a very short

time after a change in flow of either methane or air. The time

required is somewhat less than 2 minutes depending on the total

rate of flow. Figure 9 is a graphical representation of the

time required to reach equilibrium after an adjustment of flow

rates to produce a change in concentration. Prior to zero time

the system was adjusted to produce 2 percent methane. The flow

of air was changed and a sample was withdrawn from the manifold

for analysis as soon as possible. This first analysis is shown

as zero minutes. Samples were withdrawn periodically for analysis and the concentration was changed several times during

these analyses. The data from which Figure 9 was drawn is

shown in Table 5.

6. RESPONSE TIME - EXTERNAL EXPOSURE CHAMBER

The response time of the system in terms of the time

required to reach equilibrium in the manifold is quite short

but the time required to reach equilibrium in the external

exposure chamber is considerably longer. The sample is

conducted directly from the manifold into the external chamber

but the time required to flush the external chamber of the

previous sample is about 12 minutes. Figure 10 is a graphical

representation of the measured concentration of methane in the

chamber as the chamber was flushed with air and with samples

of different concentrations of methane. The chamber contained

a mixture of methane in air at somewhat less than 2 percent.

At time zero the first sample was analyzed after air only at a flow of 2 1/min. had been flowing for about 1 minute. The

concentration of methane was adjusted to 2.0 percent at 15

minutes, to 0.5 percent at 32 minutes and to 0 percent at 49

minutes. At 50 minutes, the flow of air was increased to 8

1/min. The data on which Figure 10 is based is shown in Table 6.

5
7. OPERATION - SUMMARY OF INSTRUCTIONS

A. Procedure

1. Connect air and methane as described in Section 2.

2. Calibrate analytical system using gas standards consisting of methane-nitrogen mixtures.

3. Connect analytical system to connection marked "Sample

Out" at rear of chassis.

4. Adjust flow of methane and air to predetermined values.

5. Analyze methane concentration produced by system using

the analytical system.

B . Calibration of Methanometer

s

1. Methanometer s equipped with sample probes and internal

pumps.

a. Insert probe into manifold. Cap the connection

marked "Sample Out" before activating methanometer.

b. Begin sampling.

c. Change methane concentration as described previously.

2. Methanometer s without pumps and which depend on diffusion.

a. Remove cap or plug from connection marked "Sample

Out".

b. Connect external exposure chamber to connection

marked "Sample Out: with 1/4" copper tubing.

c. Place methanometer in chamber with methanometer in

operating mode.

d. Adjust flow rate of air and methane.

e. Cap front connection on manifold with pipe cap

provided or with cork of proper size.

f. Adjust methane and air flow to produce concentrations needed for calibration, allowing at least 12 minutes

to reach equilibrium.

6
8. SAFETY PRECAUTIONS

The system is adequately protected from internal ignition

but at times mixtures of methane in air at or above the

explosive limit may be present in and around the manifold and

in the external exposure chamber. The flow through the system

is very low when high concentrations are being generated and the

concentration external to the system will quickly drop to safe

levels. The quantity of methane released by the system is

always small and in most operational modes would not exceed

about 0.04 1/min. This quantity of methane poses no threat to

safety. However, the volume of the external exposure chamber

is sufficient to pose a slight threat if an ignition occurs

when an explosive mixture is present. Consequently, the exposure

chamber should be positioned so that the removable side is positioned away from the operator. The removable side is lightly

held in place with springs which should allow almost instant

release if an ignition occurs. In any case, the danger is

slight and can be eliminated entirely by restricting the production of mixtures to those below the explosive limit. A

little common sense and care will prevent even minor difficulties

9. PARTS AND MAINTENANCE

Table 7 shows the commercially available parts that were

used in the construction of this system. These parts were

chosen because of their availability and parts from other manufacturers of similar operating characteristics may be substituted

The only other parts of the system which are not readily available and which were constructed specifically for this apparatus

are the mixing chamber, the manifold, and the external exposure

chamber. The exposure chamber is constructed from 1/4" clear

plastic with the approximate dimensions shown in Figure 12.

The joints may either be glued or fastened with machine screws.

The manifold. Figure 11, is constructed from two reducing

tees, a short length of 1 3/8" copper pipe, two reducing bushings

three brass tube connections, and a 1/2" pipe to tube adapter.

The dimensions are not critical but the overall internal dimensions of the manifold should be about 8" long by 1 1/4" in

diameter. The tube fittings are machined or bushed to fit a 1/2" i.d. opening while the copper pipe to tube adaptor is

chosen to fit. Connections are either soldered or brazed. The

interior should be thoroughly cleansed of flux after fabrication.

The mixing chamber is made from a single reducing tee, two

reducing couplings, and three 1/4" tube fittings. The mixing

7
chamber is soft soldered or brazed. The reducing bushings

used for both the mixing chamber and the manifold are one size

smaller than the reducing tees and fit within the tee.

The only parts of the system which may ever need maintenance are the flow controllers and the flow meters. The flow

controllers are protected from particulate contamination by the

filters which serve also as flashback arresters. If they

become inoperative because of particles lodging in the orifices

they may be removed and replaced or may be disassembled and

cleaned. The flowmeters will not require maintenance unless

they are grossly contaminated with grease or dirt. If this

should occur they should be removed and cleaned by flushing with

a solvent. They should be thoroughly dried after cleaning to

prevent contamination of the system with hydrocarbons other

than methane

.

Certain commercial equipment, or materials are identified

in this paper in order to adequately specify the construction.

In no case does such identification imply recommendation or

endorsement by the National Bureau of Standards, nor does it

imply that the material or equipment identified is necessarily

the best available for the purpose.

8
9
Figure 2. Photograph of the Front of Methane System Including

Separate Chamber.

10
Figure 3. Photograph of the Rear and Interior of Methane System

11
6.0

1.0 2.0 3.0 4,0 5.0 6.0

OBSERVED CONCENTRATION OF METHANE IN MOLE PERCENT

_

Figure U, Typical Calibration Curve,

12


1.6

1.4

w

COw

H

t—

I

—1

1.2

1.0

o

l-H

0.6

o

<

u

o

^0 4

0.2

/

I 1.0 2.0 3.0 4.0 5.0

CONCENTRATION OF METHANE IN MOLE PERCENT

6JD

Figure^. Plot of Reciprocal of Air Flow Versus Concentration

of Methane Produced at a Fixed Flow of Methane.

15
Flow

Control

Valve

Vacuum

Flow Meter

1

I

Methane

Generating

System

Gas Sampling Valve

of Chromatagraph

Gas

Chroma to graph

Figure 8. Schematic Diagram of Gas Chromat ograph Sampling

and Analysis System,

16
1=; o

•H

•P

nJU

+-> 0

o

O

u

CDca

+J

•H

(/) (DW)

03 X.

CJOP

a;

to

•H

-P

OJ

CD

o

CD

t/5 P!O

P.

•H

I,N30tf3cI 310W NI NOIiVHiNaOMOD QaAHSSgO

17
2.0 o o •

1.5

i

1.0

M

0% 2.0% 4- 0.5% 1 0%

O 0.5

o

o

CO

^ tt a ot l »o o

10 20 30

ELAPSED TIME IN MINUTES

40 50 6(

Figure 10. Response Time of External Exposure Chamber,

18
Mixing Chamber

Figure 11. Sketch of Mixing Chamber and Sampling Manifold

19
n

Vent

"3 J

Cover held loosely

with springs

Connection

to

"Sample -out"

CZ Zl

^

Dimensions

I

. ' Base- 6 1/2" x 7 3A*'

Height 8 3A"

Materiali 3/8" Lucite

Figure 12. Sketch of External Exposure Chamber

20
Table 1. Typical Calibration Data

Flow Rate

Liters per Minute

Air Methane

Concentration of Methane

in Mole Percent

Calculated* Observed**

98

98

98

98

98

98

0.099

. 0370

. 0375

. 0200

. 0525

.0715

76

83

86

00

58

53

20

83

86

06

2.48

3. 09

* Based on observed flow rates of air and methane

** Based on calibration of chromatograph with 5.0

mole percent methane in nitrogen standard.

Table 2. Data Obtained at Constant Flow of Methane and

Various Flows of Air

Observed Flow Rate Observed Concentration of

Liters per ^Unute Methane in Mole Percent

Air Methane"

0.93 0.0412 - 3.80

0.66 .0412 4.95

3.02 .0412 1.35

4.03 .0412 1.01

1.95 .0412 2.01

1.56 .0412 2.43

1.05 .0412 3.43

21
Table 3. Air Flow Necessary to Produce Specific

Concentrations of Methane at Fixed Methane Flow

Methane Concentration Flow of Air in

in Mole Percent Liters per Minute*

0.5 7.7

1.0 4.6

1.5 . . 2.7

2.0 2.0

2.5 1.6

3.0 ' 1.3

3.5 1.1

4.0

'

^ 1.0

4.5 0.88

5.0 .78

* Methane flow fixed at 0.0412 liters per minute.

Table 4. Reproducibility of the System When Used to

Produce Specific Concentrations of Methane

Methane Concentration Required Air Flow

Mole Percent Liters per Minute Average Relative s.d.

T n TTT

0.5 . : 7.4 0 7.40 7.70 7.53 2.0

1.0 4.08 4.16 4.00 4. 08 2.0

1.5 2.77 2.77 2.63 2.72 2.9

2.0 : 2.06 2. 06 1.91 2. 01 4.5

2.5 :. : 1.63 1.61 1.53 1.59 3.1

3.0 1.33 1.32 1 . 25 1.30 3.1

3.5 1.11 1.11 1 . 06 1.09 2.8

4.0 ; 0.93 0.95 0.91 0.90 2.2

4.5 :.79 .83 .79 .80 2.5

5.0 .71 .74 .70 .72 2.8

22
Table 5. Response Time of Generating System

Concentration of Methane

Calculated from Flow Rates Observed

Time in Minutes at Indicated Time Concentration

0 3.68 3.34

1 3.67

2 3.69

3 3.69

4 3.68

5 5.74

7 5.70

8 5.73

9 5.75

10 5.74

11 4.33

12 4.35

13 4.33

14 4.33

15 4.32

16 4.33

17 2.7 3

18 2.73

19 2.74

20 2.72

21 2.74

22 1.56

23 1.58

24 , 1.57

25 1.56

25
Table 6. Response Time of External Chamber

Time of Analysis Observed Concentration

in Minutes of Methane in Percent

0 0.03

1 . 02

2 01

....4.^. . 008

. 005

8 . 003

9 .002

11 . 002

tZ'-' . 001

14 . 001

15 Concentration ad j u s t ed to 2%

16 1 . 01

18 1 .46

20 1.72

22 1.86

23 1 . 92

25 1 96

27 2 .

0

29 2 0

31 2 . 0

3 3 r o n c e n t T a t i o n ad "i 11 9 1 f»d to n 5?; 34 1 7 36 0.69

38 . 56

40 .53

42 .51

45 .51

47 .51

48 Concentration adjusted to

50 0.09

. 009

53 . .003

.002

56

24
Table 7. Parts List

Part Designation

(Tig. 1) Description

Manufacturers Part or

Catalog Number

V-1

V-2

V-3

V-4

FM-1

FM-2

DFC-1

DFC-2

Flashback

Arresters

On-off valve, Whitey 4VS4

On-off valve, Whitey 1VS4

Metering Valve, Whitey 4RS4

Micro-metering Valve,

Nupro

Flowmeter, Brooks

Floivmeter , Brooks

Diff. Flow Control, Moore 63BD

Dif£. Flow Control, Moore 63BDL

Nupro 4F-60

Check Valve Nupro 4C

25

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NBSIR 73-255

i. Hi- n.i . A =• ..!.„•

•

1 I 1 r ) I '

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OVbLcl'l XUI rIL[JtlLcLLJ.U[l Ui- IvIlUvVll L,UUCL-IlLIiJ.LXUIlr>

of NIethane in Air

5. I^lllil. D.Kv October 1973

All MORES' E . E. Hughes, W. D. Uorko, and J. K. Taylor 3. l-'-.-rf ormi:; ^ Ori<un i cion MBSIR 7 3-255

Pb:i<l- OKMING ORC.AMZA nON NAMK AND ADORESS

NATIONAL Bb'RKAU OK STANDARDS

DEPARTMKNT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Spt'Osoring Organization Njme aaJ Ac!i.ires3 National Institute for Occupational Safety and

Health

Cincinnati, Ohio 45202

13. Type of Report & Periud Covered

Final

14, Spoasorixig Agency Code

15. SUPPLEMENTARY NOTES

^i>- ABSTRACT (A 200-word or less factual suinniary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) A gas generating system is described for producing concentrations

of methane between 0.5 and 5.0 mole percent in air or other gases.

The system has been evaluated and calibration procedure is described.

The system is equipped with adequate safety measures so that mixtures

in the explosive range can be generated safely.

17. KEY 'TORDS (Alphabetical order, separated by semicolons) ExplOSiVC mixtUTeS; GaS mixes;

Industrial hygiene; Methane; Mine safety.

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31

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