This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles

for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

Designation: D1945 − 25

Standard Test Method for

Analysis of Natural Gas by Gas Chromatography1
This standard is issued under the fixed designation D1945; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

1. Scope Dioxide by Gas Chromatography (Withdrawn 2016)3

1.1 This test method covers the determination of the chemi- D4150 Terminology Relating to Gaseous Fuels
cal composition of natural gases and similar gaseous mixtures E260 Practice for Packed Column Gas Chromatography
containing the following components: hydrocarbons, helium,
hydrogen, oxygen, nitrogen, carbon dioxide, and hydrogen 3. Terminology
sulfide by gas chromatography. Sensitivities of 0.01 % by mole 3.1 Definitions—For definitions of general terms used in
are achieved with the exception of hydrogen sulfide (0.3 % by D03 Gaseous Fuels standards, refer to Terminology D4150.
mole). Upper ranges are component dependent and range from 3.2 Definitions of Terms Specific to This Standard:
1 % by mole to 100 % by mole. 3.2.1 cricondentherm, n—the maximum temperature at
1.2 This test method may be abbreviated for the analysis of which liquids and gases (vapor) can coexist.
lean natural gases containing negligible amounts of hexanes 3.2.1.1 Discussion—Cricondentherm is the maximum tem-
and higher hydrocarbons, or for the determination of one or perature at which the components of any gas mixture begin to
more components, as required. condense out of the gaseous phase.
3.2.1.2 Discussion—At temperatures higher than criconden-
1.3 The values stated in SI units are to be regarded as
therm only a gas phase exists at any pressure.
standard. No other units of measurement are included in this
standard. 3.3 Acronyms:
1.4 This standard does not purport to address all of the 3.3.1 BMEE—bis-(2-(2-methoxyethoxy)ethyl)ether
safety concerns, if any, associated with its use. It is the 3.3.2 GC—gas chromatograph/gas chromatography
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter- 4. Summary of Test Method
mine the applicability of regulatory limitations prior to use. 4.1 This test method covers the determination of the chemi-
1.5 This international standard was developed in accor- cal composition of natural gases and similar gaseous mixtures
dance with internationally recognized principles on standard- within the range of composition shown in Table 1.
ization established in the Decision on Principles for the
4.2 Components in a representative sample are physically
Development of International Standards, Guides and Recom-
separated by gas chromatography (GC) and compared to
mendations issued by the World Trade Organization Technical
calibration data obtained under identical operating conditions
Barriers to Trade (TBT) Committee.
from a reference standard mixture of known composition. The
numerous heavy-end components of a sample can be grouped
2. Referenced Documents
into irregular peaks by reversing the direction of the carrier gas
2.1 ASTM Standards:2 through the column at such time as to group the heavy ends
D2597 Test Method for Analysis of Demethanized Hydro- either as C5 and heavier, C6 and heavier, or C7 and heavier. The
carbon Liquid Mixtures Containing Nitrogen and Carbon composition of the sample is calculated by comparing either
the peak heights, or the peak areas, or both, with the corre-
sponding values obtained with the reference standard.
1
This test method is under the jurisdiction of ASTM Committee D03 on Gaseous
Fuels and is the direct responsibility of Subcommittee D03.06.01 on Analysis of 5. Significance and Use
Major Constituents by Gas Chromatography.
Current edition approved Aug. 1, 2025. Published August 2025. Originally 5.1 This test method is of significance for providing data for
approved in 1962. Last previous edition approved in 2019 as D1945 – 14 (2019). calculating physical properties of the sample, such as heating
DOI: 10.1520/D1945-25.
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at www.astm.org/contact. For Annual Book of
3
ASTM Standards volume information, refer to the standard’s Document Summary The last approved version of this historical standard is referenced on
page on the ASTM website. www.astm.org.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

1
 D1945 − 25
TABLE 1 Natural Gas Components and Range of 6.2.2 Electronic or Computing Integrators—Proof of sepa-
Composition Covered ration and response equivalent to that for a recorder is required
Component Mol % for displays other than by chart recorder. Baseline tracking
Helium 0.01 to 10 with tangent skim peak detection is recommended.
Hydrogen 0.01 to 10
Oxygen 0.01 to 20 6.3 Attenuator—If the chromatogram is to be interpreted
Nitrogen 0.01 to 100 using manual methods, an attenuator must be used with the
Carbon dioxide 0.01 to 20
Methane 0.01 to 100 detector output signal to maintain maximum peaks within the
Ethane 0.01 to 100 recorder chart range. The attenuator must be accurate to within
Hydrogen sulfide 0.3 to 30 0.5 % between the attenuator range steps.
Propane 0.01 to 100
Isobutane 0.01 to 10 6.4 Sample Inlet System:
n-Butane 0.01 to 10
6.4.1 The sample inlet system shall be constructed of
Neopentane 0.01 to 2
Isopentane 0.01 to 2 materials that are inert and nonadsorptive with respect to the
n-Pentane 0.01 to 2 components in the sample. The preferred material of construc-
Hexane isomers 0.01 to 2
Heptanes+ 0.01 to 1
tion is stainless steel. Copper, brass, and other copper-bearing
alloys are unacceptable. The sample inlet system from the
cylinder valve to the GC column inlet must be maintained at a
temperature constant to 61 °C.
6.4.2 Provision must be made to introduce into the carrier
value and relative density, or for monitoring the concentrations gas ahead of the analyzing column a gas-phase sample that has
of one or more of the components in a mixture. been entrapped in a fixed volume loop or tubular section. The
fixed loop or section shall be so constructed that the total
6. Apparatus volume, including dead space, shall not normally exceed
6.1 Detector—The detector shall be a thermal-conductivity 0.5 mL at 100 kPag. If increased accuracy of the hexanes and
type, or its equivalent in sensitivity and stability. The thermal heavier portions of the analysis is required, a larger sample size
conductivity detector must be sufficiently sensitive to produce may be used (see Test Method D2597). The sample volume
a signal of at least 0.5 mV for 1 % by mole n-butane in a must be reproducible such that successive runs agree within
0.25-mL sample. 1 % on each component. A flowing sample inlet system is
6.2 Recording Instruments—Either strip-chart recorders or acceptable as long as viscosity effects are accounted for.
electronic integrators, or both, are used to display the separated NOTE 1—The sample size limitation of 0.5 mL or smaller is selected
components. Although a strip-chart recorder is not required relative to linearity of detector response, and efficiency of column
when using electronic integration, it is highly desirable for separation. Larger samples may be used to determine low-quantity
components to increase measurement accuracy.
evaluation of instrument performance.
6.2.1 The recorder shall be a strip-chart recorder with a 6.4.3 An optional manifold arrangement for entering
full-range scale of 5 mV or less (1 mV preferred). The width of vacuum samples is shown in Fig. 1.
the chart shall be not less than 150 mm. A maximum pen 6.5 Column Temperature Control:
response time of 2 s (1 s preferred) and a minimum chart speed 6.5.1 Isothermal—When isothermal operation is used,
of 10 mm/min shall be required. Faster speeds up to 100 mm- maintain the analyzer columns at a temperature constant to
⁄min are desirable if the chromatogram is to be interpreted 0.3 °C during the course of the sample run and corresponding
using manual methods to obtain areas. reference run.

FIG. 1 Suggested Manifold Arrangement for Entering Vacuum Samples

2
 D1945 − 25
6.5.2 Temperature Programming—Temperature program- 6.8.2.2 Partition Column—This column must separate eth-
ming may be used, as feasible. The oven temperature shall not ane through pentanes and carbon dioxide. If a recorder is used,
exceed the recommended temperature limit for the materials in the recorder pen must return to the base line between each peak
the column. for propane and succeeding peaks, and to base line within 2 %
6.6 Detector Temperature Control—Maintain the detector of full-scale deflection for components eluted ahead of
temperature at a temperature constant to 0.3 °C during the propane, with measurements being at the attenuation of the
course of the sample run and the corresponding reference run. peak. Separation of carbon dioxide must be sufficient so that a
The detector temperature shall be equal to or greater than the 0.25-mL sample containing 0.1-% by mole carbon dioxide will
maximum column temperature. produce a clearly measurable response. The resolution (R)
must be 1.5 or greater as calculated in the above equation. The
6.7 Carrier Gas Controls—The instrument shall be separation should be completed within 40 min, including
equipped with suitable facilities to provide a flow of carrier gas reversal of flow after n-pentane to yield a group response for
through the analyzer and detector at a flow rate that is constant hexanes and heavier components. Figs. 4-6 are examples of
to 1 % throughout the analysis of the sample and the reference chromatograms obtained on some of the suitable partition
standard. The purity of the carrier gas may be improved by columns.
flowing the carrier gas through selective filters prior to its entry 6.8.3 General—Other column packing materials that pro-
into the chromatograph. vide satisfactory separation of components of interest may be
6.8 Columns: used (see Fig. 7). In multicolumn applications, it is preferred to
6.8.1 The columns shall be constructed of materials that are use front-end backflush of the heavy ends.
inert and nonadsorptive with respect to the components in the
NOTE 3—The chromatograms in Figs. 3-8 are only illustrations of
sample. The preferred material of construction is stainless
typical separations. The operating conditions, including columns, are also
steel. Copper and copper-bearing alloys are unacceptable. typical and are subject to optimization by competent personnel.
6.8.2 An adsorption-type column and a partition-type col-
umn may be used to make the analysis. 6.9 Drier—Unless water is known not to interfere in the
analysis, a drier must be provided in the sample entering
NOTE 2—See Practice E260. system, ahead of the sample valve. The drier must remove
6.8.2.1 Adsorption Column—This column must completely moisture without removing selective components to be deter-
separate oxygen, nitrogen, and methane. A 13X molecular mined in the analysis.
sieve 80/100 mesh is recommended for direct injection. A 5A
NOTE 4—See A2.2 for preparation of a suitable drier.
column can be used if a pre-cut column is present to remove
interfering hydrocarbons. If a recorder is used, the recorder pen 6.10 Valves—Valves or sample splitters, or both, are re-
must return to the baseline between each successive peak. The quired to permit switching, backflushing, or for simultaneous
resolution (R) must be 1.5 or greater as calculated by Eq 1: analysis.
x2 2 x1 6.11 Vacuum Gauge—Any type of vacuum gauge may be
R ~ 1,2 ! 5 × 2, (1)
y 2 1y 1 used which has a resolution of 0.14 kPa or better and covers the
range of 0 to 120 kPa or larger.
where x1, x2 are the retention times and y1, y2 are the peak
widths. Fig. 2 illustrates the calculation for resolution. Fig. 3 is 6.12 Vacuum Pump—Must have the capability of producing
a chromatogram obtained with an adsorption column. a vacuum of 0.14 kPa absolute or less.

FIG. 2 Calculation for Resolution

3
 D1945 − 25

FIG. 3 Separation Column for Oxygen, Nitrogen, and Methane (See Annex A2)

FIG. 4 Chromatogram of Natural Gas (BMEE Column) (See Annex A2)

7. Preparation of Apparatus 7.1.1 The major component of interest (methane for natural
7.1 Linearity Check—To establish linearity of response for gas) is charged to the chromatograph by way of the fixed-size
the thermal conductivity detector, it is necessary to complete sample loop at partial pressure increments of 13 kPa from
the following procedure: 13 kPa to 100 kPa or the prevailing atmospheric pressure.

4
 D1945 − 25

FIG. 5 Chromatogram of Natural Gas (Silicone 200/500 Column) (See Annex A2)

FIG. 6 Chromatogram of Natural Gas (See Annex A2)

7.1.2 The integrated peak responses for the area generated at 7.1.3 The plotted results should yield a straight line. A
each of the pressure increments are plotted versus their partial perfectly linear response would display a straight line at a 45°
pressure (see Fig. 9). angle using the logarithmic values.

5
 D1945 − 25

FIG. 7 Chromatogram of Natural Gas (Multi-Column Application) (See Annex A2)

FIG. 8 Separation of Helium and Hydrogen

7.1.4 Any curved line indicates the fixed volume sample 7.1.5 The linearity over the range of interest must be known
loop is too large. A smaller loop size should replace the fixed for each component. It is useful to construct a table noting the
volume loop and 7.1.1 through 7.1.4 should be repeated (see response factor deviation in changing concentration. (See
Fig. 9). Table 2 and Table 3).

6
 D1945 − 25

FIG. 9 Linearity of Detector Response

TABLE 2 Linearity Evaluation of Methane TABLE 3 Linearity Evaluation for Nitrogen

S/B diff = (low mol % − high mol %) ⁄low mol % × 100 S/B diff = (low mol % − high mol %) ⁄low mol % × 100
S/B diff., % on low S/B diff., % on low
B area S mol % S/B mol % ⁄area B area S mol % S/B mol % ⁄area
value value
223 119 392 51 2.2858e-07 5 879 836 1 1.7007e-07
242 610 272 56 2.3082e-07 −0.98 29 137 066 5 1.7160e-07 −0.89
261 785 320 61 2.3302e-07 −0.95 57 452 364 10 1.7046e-07 −1.43
280 494 912 66 2.3530e-07 −0.98 84 953 192 15 1.7657e-07 −1.44
299 145 504 71 2.3734e-07 −0.87 111 491 232 20 1.7939e-07 −1.60
317 987 328 76 2.3900e-07 −0.70 137 268 784 25 1.8212e-07 −1.53
336 489 056 81 2.4072e-07 −0.72 162 852 288 30 1.8422e-07 −1.15
351 120 721 85 2.4208e-07 −0.57 187 232 496 35 1.8693e-07 −1.48

maximum pressure to which a given component can be blended

7.1.6 It should be noted that nitrogen, methane, and ethane
with nitrogen per Eq 2 and 3:
exhibit less than 1 % compressibility at atmospheric pressure.
Other natural gas components do exhibit a significant com- B 5 ~ 100 × V ! /i (2)
pressibility at pressures less than atmospheric. P 5 ~ i × M ! /100 (3)
7.1.7 Most components that have vapor pressures of less
than 100 kPa cannot be used as a pure gas for a linearity study where:
because they will not exhibit sufficient vapor pressure for a B = blend pressure, max, kPa;
vacuum gauge reading to 100 kPa. For these components, a V = vapor pressure, kPa;
mixture with nitrogen or methane can be used to establish a i = mol %;
partial pressure that can extend the total pressure to 100 kPa. P = partial pressure, kPa; and
M = vacuum gauge pressure, kPa.
Using Table 4 for vapor pressures at 38 °C, calculate the

7
 D1945 − 25
TABLE 4 Vapor Pressure at 38 °CA 8.2 Preparation—A reference standard may be prepared by
Component kPa absolute blending pure components. Diluted dry air is a suitable
Nitrogen >34 500 standard for oxygen and nitrogen (see 9.5.1).4,5
Methane >34 500
Carbon dioxide >5 520
Ethane >5 520 9. Procedure
Hydrogen sulfide 2 720
Propane 1 300 9.1 Instrument Preparation—Place the proper column(s) in
Isobutane 501 operation as needed for the desired run (as described in either
n-Butane 356 9.4, 9.5, or 9.6). Adjust the operating conditions and allow the
Isopentane 141
n-Pentane 108 chromatograph to stabilize.
n-Hexane 34.2 9.1.1 For hexanes and higher, heat the sample loop.
n-Heptane 11.2
A
The most recent data for the vapor pressures listed are available from the
NOTE 6—Most modern chromatographs have valve ovens that can be
Thermodynamics Research Center, Texas A&M University System, College temperature controlled. It is strongly recommended in the absence of
Station, TX 77843. valve ovens to mount the gas sampling valve in the chromatograph oven
and operate at the column temperature.
9.1.2 After the instrument has apparently stabilized, make
check runs on the reference standard to establish instrument
repeatability. Two consecutive checks must agree within the
7.2 Procedure for Linearity Check: repeatability limits for the % by mole amount present of each
7.2.1 Connect the pure-component source to the sample- component. Either the average of the two consecutive checks,
entry system. Evacuate the sample-entry system and observe or the latest check agreeing within the repeatability limits of
the vacuum gauge for leaks. (See Fig. 1 for a suggested the previous check on each component may be used as the
manifold arrangement.) The sample-entry system must be reference standard for all subsequent runs until there is a
vacuum tight. change in instrument operating conditions. Daily calibrations
7.2.2 Carefully open the needle valve to admit the pure are recommended.
component up to 13 kPa of partial pressure.
9.2 Sample Preparation—If desired, hydrogen sulfide may
7.2.3 Record the exact partial pressure and actuate the
be removed by at least two methods (see Annex A2.3).
sample valve to place the sample onto the column. Record the
9.2.1 Preparation and Introduction of Sample—Samples
peak area of the pure component.
must be equilibrated in the laboratory at 10 °C to 30 °C above
7.2.4 Repeat 7.2.3 for 26 kPa, 39 kPa, 52 kPa, 65 kPa,
the source temperature of the field sampling. The higher the
78 kPa, and 91 kPa on the vacuum gauge, recording the peak
temperature the shorter the equilibration time (approximately
area obtained for sample analysis at each of these pressures.
2 h for small sample containers of 300 mL or less). This
7.2.5 Plot the area data (x axis) versus the partial pressures
analysis method assumes field sampling methods have re-
(y axis) on a linear graph as shown in Fig. 9.
moved entrained liquids. If the hydrocarbon dewpoint of the
7.2.6 An alternative method is to obtain a blend of all the
sample is known to be lower than the lowest temperature to
components and charge the sample loop at partial pressure over
which the sample has been exposed, it is not necessary to heat
the range of interest. If a gas blender is available, the mixture
the sample.
can be diluted with methane thereby giving response curves for
9.2.2 Connections from the sample container to the sample
all the components. (Warning—If it is not possible to obtain
inlet of the instrument should be made with stainless steel or
information on the linearity of the available gas chromatograph
with short pieces of TFE-fluorocarbon. Copper, vinyl, or
detector for all of the test gas components, then as a minimum
rubber connections are not acceptable. Heated lines may be
requirement the linearity data must be obtained for any gas
necessary for high hydrocarbon content samples.
component that exceeds a concentration of 5 mol%. Chromato-
graphs are not truly linear over wide concentration ranges and 9.3 Sample Introduction—The size of the sample introduced
linearity should be established over the range of interest.) to the chromatographic columns shall not exceed 0.5 mL. (This
small sample size is necessary to obtain a linear detector
8. Reference Standards response for methane.) Sufficient accuracy can be obtained for
8.1 Moisture-free gas mixtures of known composition are the determination of all but the minor constituents by the use of
required for comparison with the test sample. They must this sample size. When increased response is required for the
contain known percents of the components, except oxygen determination of components present in concentrations not
(Note 5), that are to be determined in the unknown sample. All exceeding 5 mol %, it is permissible to use sample and
components in the reference standard must be homogenous in reference standard volumes not exceeding 5 mL. (Avoid
the vapor state at the time of use. The concentration of a introduction of liquids into the sample system.)
component in the reference standard gas should not be less than
one half nor more than twice the concentration of the corre-
sponding component in the test gas. 4
A suitable reference standard is available from Scott Specialty Gases Inc.,
Plumsteadville, PA.
NOTE 5—Unless the reference standard is stored in a container that has 5
A ten-component reference standard traceable to the National Institute of
been tested and proved for inertness to oxygen, it is preferable to calibrate Standards and Technology (NIST) is available from Institute of Gas Technology
for oxygen by an alternative method. (IGT), 3424 S. State St., Chicago, IL 60616.

8
 D1945 − 25
9.3.1 Purging Method—Open the outlet valve of the sample Enter a 1 to 5 mL sample and record the response for helium,
cylinder and purge the sample through the inlet system and followed by hydrogen, which will be just ahead of oxygen
sample loop or tube. The amount of purging required must be (Note 5). Obtain a corresponding response on a reference
established and verified for each instrument. The sample loop standard containing suitable concentrations of helium and
pressure should be near atmospheric. Close the cylinder valve hydrogen (see Fig. 8).
and allow the pressure of the sample in the loop or tube to
stabilize. Then immediately inject the contents of the loop or 10. Calculation
tube into the chromatographic column to avoid infiltration of 10.1 The number of significant digits retained for the
contaminants. quantitative value of each component shall be such that
9.3.2 Water Displacement—If the sample was obtained by accuracy is neither sacrificed or exaggerated. The expressed
water displacement, then water displacement may be used to numerical value of any component in the sample should not be
purge and fill the sample loop or tube. (Warning—Some presumed to be more accurate than the corresponding certified
components, such as carbon dioxide, hydrogen sulfide, and value of that component in the calibration standard.
hexanes and higher hydrocarbons, may be partially or com- 10.2 External Standard Method:
pletely removed by the water.) 10.2.1 Pentanes and Lighter Components—Measure the
9.3.3 Evacuation Method—Evacuate the charging system, height of each component peak for pentanes and lighter,
including the sample loop, and the sample line back to the convert to the same attenuation for corresponding components
valve on the sample cylinder, to less than 0.1 kPa absolute in the sample and reference standard, and calculate the con-
pressure. Close the valve to the vacuum source and carefully centration of each component in the sample per Eq 4:
meter the fuel-gas sample from the sample cylinder until the
C 5 S × ~ A/B ! (4)
sample loop is filled to the desired pressure, as indicated on the
vacuum gauge (see Fig. 1). Inject the sample into the chro- where:
matograph. C = component concentration in the sample, mol %;
9.4 Partition Column Run for Ethane and Heavier Hydro- A = peak height of component in the sample, mm;
carbons and Carbon Dioxide—This run is made using either B = peak height of component in the standard, mm; and
helium or hydrogen as the carrier gas; if other than a thermal S = component concentration in the reference standard,
conductivity detector is used, select a suitable carrier gas for mol %.
that detector. Select a sample size in accordance with 9.3. Enter 10.2.1.1 If air has been run at reduced pressure for oxygen
the sample, and backflush heavy components when appropri- or nitrogen calibration, or both, correct the equation for
ate. Obtain a corresponding response on the reference standard. pressure per Eq 5:
9.4.1 Methane may also be determined on this column if the C 5 S × ~ A/B ! × ~ P a /P b ! (5)
column will separate the methane from nitrogen and oxygen
(such as with silicone 200/500 as shown in Fig. 5), and the where:
sample size does not exceed 0.5 mL. Pa = pressure at which air is run and
Pb = true barometric pressure during the run, with both
9.5 Adsorption Column Run for Oxygen, Nitrogen, and pressures being expressed in the same units.
Methane—Make this run using helium or hydrogen as the
carrier gas. The sample size must not exceed 0.5 mL for the 10.2.1.2 Use composition values of 78.1 % nitrogen and
determination of methane. Enter the sample and obtain a 21.9 % oxygen for dry air, because argon elutes with oxygen
response through methane. Likewise, obtain a response on the on a molecular sieves column under the normal conditions of
reference standard for nitrogen and methane. Obtain a response this test method.
on dry air for nitrogen and oxygen, if desired. The air must be 10.2.2 Hexanes and Heavier Components—Measure the
either entered at an accurately measured reduced pressure, or areas of the hexanes portion and the heptanes and heavier
from a helium-diluted mixture. portion of the reverse-flow peak (see Annex A1, Fig. A1.1, and
X3.6). Also measure the areas of both pentane peaks on the
9.5.1 A mixture containing approximately 1 % of oxygen
sample chromatogram, and adjust all measured areas to the
can be prepared by pressurizing a container of dry air at
same attenuation basis.
atmospheric pressure to 2 MPa with pure helium. This pressure
10.2.3 Calculate corrected areas of the reverse flow peaks
need not be measured precisely, as the concentration of
per Eq 6 and 7:
nitrogen in the mixture thus prepared must be determined by
comparison to nitrogen in the reference standard. The percent Corrected C 6 area 5 72/86 × measured C 6 area (6)
nitrogen is multiplied by 0.268 to obtain the mole percent of Corrected C 7 and heavier area (7)
oxygen or by 0.280 to obtain the mole percent total of oxygen
and argon. Do not rely on oxygen standards that have been 5 72/A × measured C 7 and heavier area
prepared for more than a few days. It is permissible to use a where A = average molecular weight of the C7 and heavier
response factor for oxygen that is relative to a stable constitu- fraction.
ent.
NOTE 7—The value of 98 is usually sufficiently accurate for use as the
9.6 Adsorption Column Run for Helium and Hydrogen— C7 and heavier fraction average molecular weight; the small amount of C8
Make this run using either nitrogen or argon as the carrier gas. and heavier present is usually offset by the lighter methyl cyclopentane

9
 D1945 − 25
and cyclohexane that occur in this fraction. A more accurate value for the Component, mol % Reproducibility, mol %
molecular weight of C7 and heavier can be obtained as described in Annex
A1.3. 0 to 0.09 0.02
0.1 to 0.9 0.07
10.2.4 Calculate the concentration of the two fractions in 1.0 to 4.9 0.10
the sample per Eq 8 and 9: 5.0 to 10 0.12
Over 10 0.15
Mol % C6 5 ~ corrected C 6 area! (8)
11.2.3 Bias—Since there is no accepted reference material
× ~ mol % iC 5 1nC 5 ! / ~ iC 5 1nC 5 area! . for determining the bias, no statement on bias can be made.
Mol % C 7 1 5 ~ corrected C 7 1 area! (9)

× ~ mol % i C 5 1nC 5 ! / ~ iC 5 1nC 5 area! .

10.2.4.1 If the mole percent of iC5 + nC5 has been deter-

mined by a separate run with a smaller sized sample, this value
need not be redetermined.
10.2.5 The entire reverse flow area may be calculated in this
manner as C6 and heavier, or as C5 and heavier should the
carrier gas reversal be made after n-butane. The measured area
should be corrected by using the average molecular weights of
the entire reverse-flow components for the value of A. The
mole percent and area of the iC5 and nC5 reverse flow peak of
an identically sized sample of reference standard (free of C6
and heavier) shall then be used for calculating the final mole
percent value.
10.2.6 Normalize the mole percent values by multiplying
each value by 100 and dividing by the sum of the original
values. The sum of the original values should not differ from
100.0 % by more than 1.0 %.
10.2.7 See sample calculations in Appendix X2.

11. Precision and Bias

11.1 An ILS was conducted which included analyzing 2
standards with participation by 7 laboratories in 1996. The
detailed results are given in Research Report RR:D03-10086.
11.2 Precision—The precision of this test method, as deter-
mined by the statistical examination of the interlaboratory test
results, for gas samples of pipeline quality 38 MJ/m3 is as
follows:
11.2.1 Repeatability—The difference between two succes-
sive results obtained by the same operator with the same
apparatus under constant operating conditions on identical test
materials should be considered suspect if they differ by more
than the following amounts:
Component, mol % Repeatability, mol %

0 to 0.09 0.01
0.1 to 0.9 0.04
1.0 to 4.9 0.07
5.0 to 10 0.08
Over 10 0.10

11.2.2 Reproducibility—The difference between two results

obtained by different operators in different laboratories on
identical test materials should be considered suspect if they
differ by more than the following amounts:

6
Supporting data have been filed at ASTM International Headquarters and may
be obtained by requesting Research Report RR:D03-1008. Contact ASTM Customer
Service at www.astm.org/contact.

10
 D1945 − 25
12. Keywords
12.1 gas analysis; gas chromatograph; gas chromatography;
natural gas composition

ANNEXES

(Mandatory Information)

A1. SUPPLEMENTARY PROCEDURES

A1.1 Analysis for Only Propane and Heavier Components A1.2.2 Enter a 1 mL to 5 mL sample into the partition
A1.1.1 This determination can be made in 10 min to 15 min column and reverse the carrier gas flow after n-pentane is
run time by using column conditions to separate propane, separated. Obtain a corresponding chromatogram of the refer-
isobutane, n-butane, isopentane, n-pentane, hexanes, and ence standard. Measure the peak heights of ethane through
heptanes, and heavier, but disregarding separation on ethane n-pentane and the areas of the pentane peaks of the standard.
and lighter. Make calculations on ethane and heavier components in the
same manner as for the complete analysis method. Methane
A1.1.2 Use a 5 m bis-(2-(2-methoxyethoxy) ethyl)ether
and lighter may be expressed as the difference between 100 and
(BMEE) column at about 30 °C, or a suitable length of another
the sum of the determined components.
partition column that will separate propane through n-pentane
in about 5 min. Enter a 1 mL to 5 mL sample into the column A1.3 Special Analysis to Determine Hexanes and Heavier
and reverse the carrier gas flow after n-pentane is separated. Components
Obtain a corresponding chromatogram on the reference
standard, which can be accomplished in about 5 min run time, A1.3.1 A short partition column can be used advantageously
as there is no need to reverse the flow on the reference to separate heavy-end components and obtain a more detailed
standard. Make calculations in the same manner as for the breakdown on composition of the reverse-flow fractions. This
complete analysis method. information provides quality data and a basis for calculating
physical properties such as molecular weight on these frac-
A1.1.3 A determination of propane, isobutane, n-butane,
tions.
and pentanes and heavier can be made in about 5 min run time
by reversing the carrier-gas flow after n-butane. However, it is A1.3.2 Fig. A1.1 is a chromatogram that shows components
necessary to know the average molecular weight of the that are separated by a 2 m BMEE column in 20 min. To make
pentanes and heavier components. this determination, enter a 5 mL sample into the short column
and reverse the carrier gas after the separation of n-heptane.
A1.2 Single-Run Analysis for Ethane and Heavier Compo- Measure areas of all peaks eluted after n-pentane. Correct each
nents peak area to the mol basis by dividing each peak area by the
A1.2.1 In many cases, a single partition run using a sample molecular weight of the component. A value of 120 may be
size in the order of 1 mL to 5 mL will be adequate for used for the molecular weight of the octanes and heavier
determining all components except methane, which cannot be reverse-flow peak. Calculate the mole percent of the hexanes
determined accurately using this size sample with peak height and heavier components by adding the corrected areas and
measurements, because of its high concentration. dividing to make the total 100 %.

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 D1945 − 25

FIG. A1.1 Composition of Hexanes and Heavier Fraction

A2. PREPARATION OF COLUMNS AND DRIER

A2.1 Preparation of Columns—See Practice E260. sulfate in the line upstream of both the chromatograph and
drying tube. This procedure will remove small amounts of
A2.2 Preparation of Drier—Fill a 10 mm diameter by hydrogen sulfide while having but minimal effect on the carbon
100 mm length glass tube with granular phosphorus pentoxide dioxide in the sample.
or magnesium perchlorate, observing all proper safety precau-
tions. Mount as required to dry the sample. Replace the drying A2.4 Column Arrangement—For analyses in which
agent after about one half of the material has become spent. hexanes and heavier components are to be determined, Fig.
A2.1 shows an arrangement whereby columns can be quickly
A2.3 Removal of Hydrogen Sulfide:
and easily changed by the turn of a selector valve. Two
A2.3.1 For samples containing more than about 300 ppm by columns are necessary to determine all of the components
mass hydrogen sulfide, remove the hydrogen sulfide by con- covered in this test method. However, short and long partition
necting a tube of sodium hydrate absorbent (Ascarite) ahead of columns provide the flexibility of three partition column
the sample container during sampling, or ahead of the drying lengths, by using them either singly or in series. The connec-
tube when entering the sample into the chromatograph. This tion between V1 and V2 in Fig. A2.1 should be as short as
procedure also removes carbon dioxide, and the results ob- possible (20 mm is practical) to minimize dead space between
tained will be on the acid-gas free basis. the columns when used in series. If all columns are chosen to
A2.3.2 Hydrogen sulfide may also be removed by connect- operate at the same temperature, then stabilization time be-
ing a tube of pumice that has been impregnated with cupric tween changing columns will be minimized.

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 D1945 − 25

FIG. A2.1 Column Arrangement

APPENDIXES

(Nonmandatory Information)

X1. REFERENCE STANDARD MIXTURE

X1.1 Preparation Balance, 2000 g capacity, sensitivity of 10 mg.

X1.1.1 Gas mixtures of the following typical compositions Pure Components, methane through n-pentane, and carbon
will suffice for use as reference standards for most analytical dioxide. The pure components should be 99+ % pure. Methane
requirements (Note X1.1): should be in a 1 L cylinder at 10 MPa) pressure. Run a

TABLE X1.1 Composition of Typical Reference Standard

Component Lean Gas, mol % Rich Gas, mol %
Helium 1.0 0.5
Hydrogen 3.0 0.5
Nitrogen 4.0 0.5
Methane (maximum) 85 74
Ethane 6.0 10
Carbon dioxide 1.0 1.0
Propane 4.0 7.0
Isobutane 2.0 3.0
n-Butane 2.0 3.0
Neopentane 0.5 1.0
Isopentane 0.5 1.0
n-Pentane 0.5 1.0
Hexanes+ 0.1 0.2

NOTE X1.1—If the mixture is stored under pressure, take care to ensure chromatogram of each component to check on its given
that the partial pressure of any component does not exceed its vapor composition.
pressure at the temperature and pressure at which the sample is stored and
used. The lean mixture has a cricondentherm at 15.6 °C and the rich X1.1.2.2 Evacuate the 20 L cylinder for several hours.
mixture has a cricondentherm at 37.8 °C. Evacuate 100 mL Cylinder A, and obtain its true weight.
X1.1.2 A useful method for preparation of a reference Connect Cylinder A to a cylinder of pure n-pentane with a
standard by weight is as follows:4 metal connection of calculated length to contain approximately
X1.1.2.1 Obtain the following equipment and material: the amount of n-pentane to be added. Flush the connection with
Cylinder, 20 L the n-pentane by loosening the fitting at the valve on Cylinder
Pressure Cylinders, two 100 mL (A and B) A. Tighten the fitting. Close the n-pentane cylinder valve and

13
 D1945 − 25
open Cylinder A valve to admit the n-pentane from the X1.2.3 Switch to the partition column with helium carrier
connection and then close the valve on Cylinder A. Disconnect gas, and run the gas mixture at 70 kPa absolute pressure. Then
and weigh Cylinder A to obtain the weight of n-pentane added. admit samples of pure ethane and propane at 10 kPa absolute
X1.1.2.3 Similarly, add isopentane, n-butane, isobutane, pressure, and butanes, pentanes, and carbon dioxide at 5 kPa
propane, ethane, and carbon dioxide, in that order, as desired, absolute pressure.
in the reference standard. Weigh Cylinder A after each addition X1.2.4 Run the gas mixture at 70 kPa absolute pressure.
to obtain the weight of the component added. Connect Cylinder
A to the evacuated 20 L cylinder with as short a clean, X1.2.5 Calculate the composition of the prepared gas mix-
small-diameter connector as possible. Open the valve on the ture as follows:
20 L cylinder, then open the valve on Cylinder A. This will X1.2.5.1 Correct peak heights of all pure components and
result in the transfer of nearly all of the contents of Cylinder A the respective components in the blend to the same attenuation
into the 20 L cylinder. Close the cylinder valves, disconnect, (Note X1.2).
and weigh Cylinder A to determine the weight of mixture that X1.2.5.2 Calculate the concentration of each component per
was not transferred to the 20 L cylinder. Eq X1.1:
X1.1.2.4 Evacuate and weigh 100 mL Cylinder B. Then fill C 5 ~ 100V f !~ A/B !~ P b /P a ! (X1.1)
Cylinder B with helium and hydrogen respectively to the where:
pressures required to provide the desired concentrations of
these components in the final blend. (Helium and hydrogen are C = component concentration, mol;
A = peak height of component in blend;
prepared and measured separately from the other components
B = peak height of pure component;
to prevent their pressures, while in the 100-mL cylinder, from Pa = absolute pressure at which blend is run, kPa;
causing condensation of the higher hydrocarbons.) Weigh Pb = absolute pressure at which component is run, kPa; and
Cylinder B after each addition to obtain the weight of the Vf = volume fraction of pure component.
component added. Connect Cylinder B to the 20 L cylinder NOTE X1.3—Vf = 1.000 if the calibration component is free of impu-
with as short a clean, small-diameter connector as possible. rities.
Open the valve on the 20 L cylinder, then open the valve on X1.2.5.3 Normalize values to 100.0 %.
Cylinder B, which will result in the transfer of nearly all of the
contents of Cylinder B into the 20 L cylinder. Close the X1.3 Calibration using Relative Molar Response Values
cylinder valves, disconnect, and weigh Cylinder B to obtain the
weight of the mixture that was not transferred to the 20 L X1.3.1 Relative response ratios can be derived from linear-
cylinder. ity data and used for calculating response factors. This elimi-
X1.1.2.5 Weigh a 1 L cylinder containing pure methane at nates the need for a multicomponent standard for daily
about 10 MPa pressure. Transfer the methane to the 20 L calibration. The test method can be used on any gas chromato-
cylinder until the pressure equalizes. Weigh the 1 L cylinder to graph using a thermal conductivity or thermistor detector.
determine the weight of methane transferred. X1.3.2 Obtain a blend that brackets the expected concen-
X1.1.2.6 Thoroughly mix the contents of the 20 L cylinder tration the instrument will be analyzing. The major component
by heating at the bottom by a convenient means such as hot (methane) is used as the balance gas and may fall below the
water or a heat lamp, and leaving the cylinder in a vertical expected concentration. This component is present in the daily
position for at least 6 h. calibration standard and linearity is assured from previous
X1.1.2.7 Use the weights and purities of all components tests.
added to calculate the weight composition of the mixture. X1.3.3 Inject the sample at reduced pressures using the
Convert the weight percent to mole percent. apparatus in Fig. 1 or using a mechanical gas blender. Obtain
repeatable peak areas or height at 90 %, 75 %, 60 %, 45 %,
X1.2 Calibration with Pure Components 30 %, and 15 % of absolute pressure. For 100 kPa absolute
X1.2.1 Use helium carrier gas to admit a sample volume of pressure, the absolute pressures used are 90 kPa, 75 kPa, 60
0.25 mL to 0.5 mL into the adsorption column, providing kPa, 45 kPa, 30 kPa, 15 kPa.
methane at 50 kPa and nitrogen at 10 kPa absolute pressures. X1.3.4 Plot the area or height (attenuated at the same height
Run a sample of the standard mixture at 70 kPa absolute as the reference component) versus concentration and calculate
pressure and obtain peaks for methane and nitrogen. the slope of the line by the least squares method. Given the
NOTE X1.2—Each run made throughout this procedure should be equation of the line as Y = a0 + a1 X where Y represents the
repeated to ensure that peak heights are reproducible after correction for area or height points and X the concentration points. The line
pressure differences to within 1 mm or 1 % of the mean value. All peaks is assumed to intersect through the origin and a0 = 0. The slope
should be recorded at an instrument attenuation that gives the maximum a1 can be calculated per Eq X1.2:
measurable peak height.
X1.2.2 Change the carrier gas to argon or nitrogen and, after a1 5 ( XY (X1.2)
the base line has stabilized, enter a sample of pure helium at ~( Y!2
7 kPa absolute pressure, recording the peak at an attenuation X1.3.5 Ratio the slopes of the referenced components (i) to
that allows maximum peak height. Run a sample of the mixture the slopes of the reference components (r) present in the daily
at 70 kPa absolute pressure and obtain the helium peak. calibration standard. This gives the Relative Molar Response

14
 D1945 − 25
factor (RMRi) for component (i). The reference component X1.3.6 For daily calibration, a four-component standard is
must be present in the same instrumental sequence (except used containing nitrogen, methane, ethane, and propane. The
Hexanes+) as the referenced components. For instance, pro- fewer components eliminates dew point problems, reactivity, is
pane can be the reference component for the butanes and more accurate and can be blended at a higher pressure. The
pentanes if propane is separated on the same column in the referenced components’ response factors are calculated from
same sequence as the butanes and pentanes. Ethane can be the the current reference factor and the Relative Molar Response
reference component for carbon dioxide if it elutes in the same factor. Following is a description of the basic calculations, an
sequence as carbon dioxide. The hexanes + peak can be refer- example of deriving a Relative Molar Response factor (Fig.
enced to propane or calculated as mentioned in the body of the X1.1), and a table showing how response factors are calculated
standard. per Eq X1.3-X1.5 (Table X1.3).
TABLE X1.2 Least Square Calculation for Slope of Iso-Butane Mole %
Response Factor ~ R ! 5 , (X1.3)
Area
Area Mole %
Y X XY Y2 Mole % ~ i ! /Area~ i !
Relative Molar Response ~ RMRi ! 5 (X1.4)
984 515 1 984 515 9.693e + 11 Mole % ~ r ! /Area~ r !
900 410 0.9 810 369 8.107e + 11
758 917 0.75 569 187.75 5.670e + 11 R iC4 5 RMRic4 × R C 3 (X1.5)
611 488 0.6 366 892.8 3.739e + 11
466 037 0.45 209 716.65 2.172e + 11 X1.3.7 Periodic checks of the RMR relationship is recom-
314 649 0.3 94 394.7 9.900e + 10
159 303 0.15 23 895.45 2.538e + 10
mended. The relationship is independent of temperature,
sample size, and carrier gas flow rate. If changes occur in these
sum = 4 195 319 4.15 3 058 971.35 3.071 452e + 12 operating conditions, all of the components will be affected
slope = ^XY/^Y 2
9.9594e-07 equally and the calculated response factors will shift accord-
ingly. See Table X1.2 and Fig. X1.1 and Table X1.3.

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 D1945 − 25

FIG. X1.1 Example of Deriving a Relative Molar Response Factor

TABLE X1.3 Calculation of Response Factors Using Relative Molar Response Values
Mole % in Response of Response Factor Relative MolarA Response Factor
Reference Reference From Reference Response from of Referenced
Component Slopei/Ki
Standard Standard Standard Components
S B S/B,K RMRi (RMRi)x(Ki)
Nitrogen 5.08 2 685 885 1.8914E-6
Methane 82.15 36 642 384 2.2419E-6
Ethane 8.75 6 328 524 1.3826E-6
Propane 4.02 3 552 767 1.1315E-6
Carbon dioxide 1.116 07c2 1.5429E-6
Isobutane 0.729 58c3 9.9594E-7
n-Butane 0.693 10c3 9.1142E-7
Neopentane 0.682 71c3 8.9776E-7
Isopentane 0.638 74c3 8.3994E-7
n-Pentane 0.600 41c3 7.8953E-7
Hexanes+ 0.547 62c3 7.2012E-7
A
The Relative Molar Response is a constant that is calculated by dividing the slope of the referenced component by the component that is present in the reference
standard. For example:
RMRic4 5 s slopeic4 d / s Kc 3 d 5 9.9594E 2 7 1.1315E 2 6 5 0.729 58

X2. SAMPLE CALCULATIONS (SEE SECTION 10)

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 D1945 − 25
TABLE X2.1 Sample Calculations
Mol % in
Response of Reference Response Percent
Component Reference Response for Sample,A A Normalized, %
Standard, B Factor, S/B C = (S × A) ⁄B
Standard, S
Helium 0.50 41.1 0.0122 12.6 0.154 0.15
Hydrogen 0.74 90.2 0.0082 1.5 0.012 0.01
Oxygen 0.27 35.5 0.0076 2.1 0.016 0.02
Nitrogen 4.89 77.8 0.0629 75.6 4.755 4.75
Methane 70.27 76.4 0.9198 90.4 83.150 83.07
Ethane 9.07 96.5 0.0940 79.0 7.426 7.42
Carbon dioxide 0.98 57.5 0.0170 21.2 0.360 0.36
Propane 6.65 55.2 0.1205 20.6 2.482 2.48
Isobutane 2.88 73.2 0.0393 11.0 0.432 0.43
n-Butane 2.87 60.3 0.0476 15.0 0.714 0.71
Neopentane 0.59 10.4 0.0567 0.1 0.006 0.01
Isopentane 0.87 96.0 0.0091 24.0 0.218 0.22
n-Pentane 0.86 86.8 0.0099 20.5 0.203 0.20
Hexanes+B 72.1C 0.166D 0.17
100.094 % 100.00 %
A
The response for a constituent in the sample has been corrected to the same attenuation as for that constituent in the reference standard.
B
Average molecular weight of C6 + = 92.
C
Corrected C6 response = (original response of 92.1) × (72 ⁄92) = 72.1.
D
Mol % C6 + = (0.218 + 0.203) × (72.1) ⁄(96.0 + 86.8) = 0.166.
% iC5 % nC5 Areas iC + nC5

X3. PRECAUTIONS FOR AVOIDING COMMON CAUSES OF ERRORS

X3.1 Hexane and Heavier Content Change X3.3 Sample Dew Point
X3.1.1 The amounts of heavy-end compounds in natural gas X3.3.1 Nonrepresentative samples frequently occur because
are easily changed during handling and entering of samples to of condensation of liquid. Maintain all samples above the
give seriously erroneous low or high values. Concentration of hydrocarbon dew point. If cooled below this, heat 10 °C or
these components has been observed to occur in a number of more above the dew point for several hours before using. If the
cases because of collection of heavier components in the dew point is unknown, heat above the sampling temperature.
sample loop during purging of the system. The surface effect of
small diameter tubing acts as a separating column and must not X3.4 Sample Inlet System
be used in the sampling and entering system when components X3.4.1 Do not use rubber or plastic that may preferentially
heavier than pentanes are to be determined. An accumulation adsorb sample components. Keep the system short and the
of oily film in the sampling system greatly aggravates this drier small to minimize the purging required.
problem. Also, the richer the gas, the worse the problem.
X3.5 Sample Size Repeatability
Periodically, check C6 and heavier repeatability of the appara-
tus by making several check runs on the same sample. It is X3.5.1 Varying back pressures on the sample loop may
helpful to retain a sample containing some hexanes and heavier impair sample size repeatability.
for periodic checking. When enlargement of the heavy end X3.5.2 Make it a practice to make all reverse flow determi-
peaks is noted, thoroughly clean the sampling valve and loop nations in the same carrier gas flow direction. All single-peak
with acetone. This trouble has been experienced with some determinations and corresponding reference runs will then be
inlet systems even when clean and with the specified sample made in the same carrier gas flow direction.
loop size. This contamination can be minimized by such
techniques as purging with inert gas, heating the sample loop, X3.5.3 Be sure that the inlet drier is in good condition.
using a vacuum system, or other such effective means. Moisture on the column will enlarge the reverse flow peak.
X3.5.4 Be sure the column is clean by occasionally giving it
X3.2 Acid Gas Content Change several hours’ sweep of carrier gas in reverse flow direction. A
X3.2.1 The carbon dioxide and hydrogen sulfide contents of level baseline should be quickly attained in either flow direc-
gas are easily altered during sampling and handling. If samples tion if the column is clean.
containing carbon dioxide or hydrogen sulfide, or both, are to X3.5.5 When the reverse flow valve is turned, there is a
be taken, use completely dry sample cylinders, connections, reversal of pressure conditions at the column ends that upsets
and lines, as moisture will selectively absorb appreciable the carrier gas flow. This flow should quickly return to the same
amounts of the acid gases. If hydrogen is present, use flow rate and the baseline level out. If it does not, the cause
aluminum, stainless steel, or other materials inert to hydrogen may be a leak in the carrier gas system, faulty flow regulator,
sulfide for the cylinder, valves, lines, and connections. or an unbalanced condition of the column or plumbing.

17
 D1945 − 25
X3.6 Reference Standard same method. That is, use either geometric construction or
planimeter, but do not intermix. When a planimeter is used,
X3.6.1 Maintain the reference standard at +15 °C or a
carefully make several tracings and use the average. Check this
temperature that is above the hydrocarbon dew point. If the
average by a second group of tracings.
reference standard should be exposed to lower temperatures,
heat at the bottom for several hours before removing a sample. X3.8 Miscellaneous
If in doubt about the composition, check the n-pentane and
X3.8.1 Moisture in the carrier gas that would cause trouble
isopentane values with pure components by the procedure
on the reverse flow may be safeguarded against by installing a
prescribed in Annex A2.
cartridge of molecular sieves ahead of the instrument. Usually
1 m of 6 mm tubing packed with 30- to 60-mesh molecular
X3.7 Measurements sieves is adequate, if changed with each cylinder of carrier gas.
X3.7.1 The baseline and tops of peaks should be plainly X3.8.2 Check the carrier gas flow system periodically for
visible for making peak height measurements. Do not use a leaks with soap or leak detector solution.
fixed zero line as the baseline, but use the actual observed
baseline. On high sensitivity, this baseline may drift slightly X3.8.3 Use electrical contact cleaner on the attenuator if
without harm and it need not frequently be moved back to zero. noisy contacts are indicated.
A strip-chart recorder with an offset zero is desirable. The area X3.8.4 Peaks with square tops with omission of small peaks
of reverse flow peak may be measured by planimeter or can be caused by a sluggish recorder. If this condition cannot
geometric construction. The reverse flow area, and the pen- be remedied by adjustment of the gain, check the electronics in
tanes peaks used for comparison, should be measured by the the recorder.

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18