Method 21 is a gas detection method specification required by the United States Government under the EPA Title 40, Protection of Environment. DOD Technologies provides the XP-3160, one of very few gas leak detectors that meet this government enforced specification. The Method 21 specification is explained below as stated by the EPA (Official Document Here). For up-to-date information regarding Method 21, please visit the EPA website.
VOC – Volatile Organic Compound (See DOD VOC Products)
1.0 Scope and Application
Analyte CAS No.
Volatile Organic Compounds (VOC) No CAS number assigned.
1.2 Scope. This method is applicable for the determination of VOC leaks from process equipment. These
sources include, but are not limited to, valves, flanges and other connections, pumps and compressors,
pressure relief devices, process drains, open-ended valves, pump and compressor seal system degassing
vents, accumulator vessel vents, agitator seals, and access door seals.
1.3 Data Quality Objectives. Adherence to the requirements of this method will enhance the quality of
the data obtained from air pollutant sampling methods.
2.0 Summary of Method
2.1 A portable instrument is used to detect VOC leaks from individual sources. The instrument detector
type is not specified, but it must meet the specifications and performance criteria contained in Section 6.0.
A leak definition concentration based on a reference compound is specified in each applicable regulation.
This method is intended to locate and classify leaks only and is not to be used as a direct measure of mass
emission rate from individual sources.
3.1 Calibration gas means the VOC compound used to adjust the instrument meter reading to a known
value. The calibration gas is usually the reference compound at a known concentration approximately
equal to the leak definition concentration.
3.2 Calibration precision means the degree of agreement between measurements of the same known
value, expressed as the relative percentage of the average difference between the meter readings and the
known concentration to the known concentration.
3.3 Leak definition concentration means the local VOC concentration at the surface of a leak source that
indicates that a VOC emission (leak) is present. The leak definition is an instrument meter reading based
on a reference compound.
3.4 No detectable emission means a local VOC concentration at the surface of a leak source, adjusted for
local VOC ambient concentration, that is less than 2.5 percent of the specified leak definition
concentration. that indicates that a VOC emission (leak) is not present.
3.5 Reference compound means the VOC species selected as the instrument calibration basis for
specification of the leak definition concentration. (For example, if a leak definition concentration is
10,000 ppm as methane, then any source emission that results in a local concentration that yields a meter
reading of 10,000 on an instrument meter calibrated with methane would be classified as a leak. In this
example, the leak definition concentration is 10,000 ppm and the reference compound is methane.)
3.6 Response factor means the ratio of the known concentration of a VOC compound to the observed
meter reading when measured using an instrument calibrated with the reference compound specified in
the applicable regulation.
3.7 Response time means the time interval from a step change in VOC concentration at the input of the
sampling system to the time at which 90 percent of the corresponding final value is reached as displayed
on the instrument readout meter.
5.1 Disclaimer. This method may involve hazardous materials, operations, and equipment. This test
method may not address all of the safety problems associated with its use. It is the responsibility of the
user of this test method to establish appropriate safety and health practices and determine the applicability
of regulatory limitations prior to performing this test method.
5.2 Hazardous Pollutants. Several of the compounds, leaks of which may be determined by this method,
may be irritating or corrosive to tissues (e.g., heptane) or may be toxic (e.g., benzene, methyl alcohol).
Nearly all are fire hazards. Compounds in emissions should be determined through familiarity with the
source. Appropriate precautions can be found in reference documents, such as reference No. 4 in Section
6.5 The instrument shall be equipped with a probe or probe extension or sampling not to exceed 6.4 mm
(1/4in) in outside diameter, with a single end opening for admission of sample.
6.6 The instrument shall be intrinsically safe for operation in explosive atmospheres as defined by the
National Electrical Code by the National Fire Prevention Association or other applicable regulatory code
for operation in any explosive atmospheres that may be encountered in its use. The instrument shall, at a
minimum, be intrinsically safe for Class 1, Division 1 conditions, and/or Class 2, Division 1 conditions,
as appropriate, as defined by the example code. The instrument shall not be operated with any safety
device, such as an exhaust flame arrestor, removed.
7.0 Reagents and Standards
7.1 Two gas mixtures are required for instrument calibration and performance evaluation:
7.1.1 Zero Gas. Air, less than 10 parts per million by volume (ppmv) VOC.
7.1.2 Calibration Gas. For each organic species that is to be measured during individual source surveys,
obtain or prepare a known standard in air at a concentration approximately equal to the applicable leak
definition specified in the regulation.
7.2 Cylinder Gases. If cylinder calibration gas mixtures are used, they must be analyzed and certified by
the manufacturer to be within 2 percent accuracy, and a shelf life must be specified. Cylinder standards
must be either reanalyzed or replaced at the end of the specified shelf life.
7.3 Prepared Gases. Calibration gases may be prepared by the user according to any accepted gaseous
preparation procedure that will yield a mixture accurate to within 2 percent. Prepared standards must be
replaced each day of use unless it is demonstrated that degradation does not occur during storage.
7.4 Mixtures with non-Reference Compound Gases. Calibrations may be performed using a compound
other than the reference compound. In this case, a conversion factor must be determined for the
alternative compound such that the resulting meter readings during source surveys can be converted to
reference compound results.
8.0 Sample Collection, Preservation, Storage, and Transport
8.1 Instrument Performance Evaluation. Assemble and start up the instrument according to the
manufacturer’s instructions for the recommended warm-up period and preliminary adjustments.
8.1.1 Response Factor. A response factor must be determined for each compound that is to be measured,
either by testing or from reference sources. The response factor tests are required before placing the
analyzer into service but do not have to be repeated at subsequent intervals.
188.8.131.52 Calibrate the instrument with the reference compound as specified in the applicable regulation.
Introduce the calibration gas mixture to the analyzer and record the observed meter reading. Introduce
zero gas until a stable reading is obtained. Make a total of three measurements by alternating between the
calibration gas and zero gas. Calculate the response factor for each repetition and the average response
184.108.40.206 The instrument response factors for each of the individual VOC to be measured shall be less than
10 unless otherwise specified in the applicable regulation. When no instrument is available that meets this
specification when calibrated with the reference VOC specified in the applicable regulation, the available
instrument may be calibrated with one of the VOC to be measured, or any other VOC, so long as the
instrument then has a response factor of less than 10 for each of the individual VOC to be measured.
220.127.116.11 Alternatively, if response factors have been published for the compounds of interest for the
instrument or detector type, the response factor determination is not required, and existing results may be
referenced. Examples of published response factors for flame ionization and catalytic oxidation detectors
are included in References 1–3 of Section 17.0.
8.1.2 Calibration Precision. The calibration precision test must be completed prior to placing the analyzer
into service and at subsequent 3-month intervals or at the next use, whichever is later.
18.104.22.168 Make a total of three measurements by alternately using zero gas and the specified calibration
gas. Record the meter readings. Calculate the average algebraic difference between the meter readings
and the known value. Divide this average difference by the known calibration value and multiply by 100
to express the resulting calibration precision as a percentage.
22.214.171.124 The calibration precision shall be equal to or less than 10 percent of the calibration gas value.
8.1.3 Response Time. The response time test is required before placing the instrument into service. If a
modification to the sample pumping system or flow configuration is made that would change the response
time, a new test is required before further use.
126.96.36.199 Introduce zero gas into the instrument sample probe. When the meter reading has stabilized,
switch quickly to the specified calibration gas. After switching, measure the time required to attain 90
percent of the final stable reading. Perform this test sequence three times and record the results. Calculate
the average response time.
188.8.131.52 The instrument response time shall be equal to or less than 30 seconds. The instrument pump,
dilution probe (if any), sample probe, and probe filter that will be used during testing shall all be in place
during the response time determination.
8.2 Instrument Calibration. Calibrate the VOC monitoring instrument according to Section 10.0.
8.3 Individual Source Surveys.
8.3.1 Type I—Leak Definition Based on Concentration. Place the probe inlet at the surface of the
component interface where leakage could occur. Move the probe along the interface periphery while
observing the instrument readout. If an increased meter reading is observed, slowly sample the interface
where leakage is indicated until the maximum meter reading is obtained. Leave the probe inlet at this
maximum reading location for approximately two times the instrument response time. If the maximum
observed meter reading is greater than the leak definition in the applicable regulation, record and report
the results as specified in the regulation reporting requirements. Examples of the application of this
general technique to specific equipment types are:
184.108.40.206 Valves. The most common source of leaks from valves is the seal between the stem and housing.
Place the probe at the interface where the stem exits the packing gland and sample the stem
circumference. Also, place the probe at the interface of the packing gland take-up flange seat and sample
the periphery. In addition, survey valve housings of multipart assembly at the surface of all interfaces
where a leak could occur.
220.127.116.11 Flanges and Other Connections. For welded flanges, place the probe at the outer edge of the
flange-gasket interface and sample the circumference of the flange. Sample other types of nonpermanent
joints (such as threaded connections) with a similar traverse.
18.104.22.168 Pumps and Compressors. Conduct a circumferential traverse at the outer surface of the pump or
compressor shaft and seal interface. If the source is a rotating shaft, position the probe inlet within 1 cm
of the shaft-seal interface for the survey. If the housing configuration prevents a complete traverse of the
shaft periphery, sample all accessible portions. Sample all other joints on the pump or compressor
housing where leakage could occur.
22.214.171.124 Pressure Relief Devices. The configuration of most pressure relief devices prevents sampling at
the sealing seat interface. For those devices equipped with an enclosed extension or horn, place the probe
inlet at approximately the center of the exhaust area to the atmosphere.
126.96.36.199 Process Drains. For open drains, place the probe inlet at approximately the center of the area open
to the atmosphere. For covered drains, place the probe at the surface of the cover interface and conduct a
188.8.131.52 Open-ended Lines or Valves. Place the probe inlet at approximately the center of the opening to
184.108.40.206 Seal System Degassing Vents and Accumulator Vents. Place the probe inlet at approximately the
center of the opening to the atmosphere.
220.127.116.11 Access door seals. Place the probe inlet at the surface of the door seal interface and conduct a
8.3.2 Type II—“No Detectable Emission”. Determine the local ambient VOC concentration around the
source by moving the probe randomly upwind and downwind at a distance of one to two meters from the
source. If interference exists with this determination due to a nearby emission or leak, the local
ambient concentration may be determined at distances closer to the source, but in no case shall the
distance be less than 25 centimeters. Then move the probe inlet to the surface of the source and determine
the concentration as outlined in Section 8.3.1. The difference between these concentrations determines
whether there are no detectable emissions. Record and report the results as specified by the regulation.
For those cases where the regulation requires a specific device installation or that specified vents be
ducted or piped to a control device, the existence of these conditions shall be visually confirmed. When
the regulation also requires that no detectable emissions exist, visual observations and sampling surveys
are required. Examples of this technique are:
18.104.22.168 Pump or Compressor Seals. If applicable, determine the type of shaft seal. Perform a survey of
the local area ambient VOC concentration and determine if detectable emissions exist as described in
22.214.171.124 Seal System Degassing Vents, Accumulator Vessel Vents, Pressure Relief Devices. If applicable,
observe whether or not the applicable ducting or piping exists. Also, determine if any sources exist in the
ducting or piping where emissions could occur upstream of the control device. If the required ducting or
piping exists and there are no sources where the emissions could be vented to the atmosphere upstream of the
control device, then it is presumed that no detectable emissions are present. If there are sources in the
ducting or piping where emissions could be vented or sources where leaks could occur, the sampling
surveys described in Section 8.3.2 shall be used to determine if detectable emissions exist.
8.3.3 Alternative Screening Procedure.
126.96.36.199 A screening procedure based on the formation of bubbles in a soap solution that is sprayed on a
potential leak source may be used for those sources that do not have continuously moving parts, that do
not have surface temperatures greater than the boiling point or less than the freezing point of the soap
solution, that do not have open areas to the atmosphere that the soap solution cannot bridge, or that do not
exhibit evidence of liquid leakage. Sources that have these conditions present must be surveyed using the
instrument technique of Section 8.3.1 or 8.3.2.
188.8.131.52 Spray a soap solution over all potential leak sources. The soap solution may be a commercially
available leak detection solution or may be prepared using concentrated detergent and water. A pressure
sprayer or squeeze bottle may be used to dispense the solution. Observe the potential leak sites to
determine if any bubbles are formed. If no bubbles are observed, the source is presumed to have no
detectable emissions or leaks as applicable. If any bubbles are observed, the instrument techniques of
Section 8.3.1 or 8.3.2 shall be used to determine if a leak exists, or if the source has detectable emissions,
9.0 Quality Control
See Section 8.1.2 Instrument calibration precision check
10.0 Instrument calibration
10.0 Calibration and Standardization
10.1 Calibrate the VOC monitoring instrument as follows. After the appropriate warm-up period and zero
internal calibration procedure, introduce the calibration gas into the instrument sample probe. Adjust the
instrument meter readout to correspond to the calibration gas value.
Note: If the meter readout cannot be adjusted to the proper value, a malfunction of the analyzer is
indicated and corrective actions are necessary before use.
11.0 Analytical Procedures[Reserved]
12.0 Data Analyses and Calculations[Reserved]
13.0 Method Performance[Reserved]
14.0 Pollution Prevention[Reserved]
15.0 Waste Management[Reserved]
1. Dubose, D.A., and G.E. Harris. Response Factors of VOC Analyzers at a Meter Reading of 10,000
ppmv for Selected Organic Compounds. U.S. Environmental Protection Agency, Research Triangle Park,
NC. Publication No. EPA 600/2–81051. September 1981.
2. Brown, G.E., et al. Response Factors of VOC Analyzers Calibrated with Methane for Selected Organic
Compounds. U.S. Environmental Protection Agency, Research Triangle Park, NC. Publication No. EPA
600/2–81–022. May 1981.
3. DuBose, D.A. et al. Response of Portable VOC Analyzers to Chemical Mixtures. U.S. Environmental
Protection Agency, Research Triangle Park, NC. Publication No. EPA 600/2–81–110. September 1981.
4. Handbook of Hazardous Materials: Fire, Safety, Health. Alliance of American Insurers. Schaumberg,
17.0 Tables, Diagrams, Flowcharts, and Validation Data[Reserved]