Method
205 - Verification of Gas Dilution Systems for Field Instrument Calibrations
3.1 Laboratory
Evaluation (Optional).
3.2 Field Evaluation
(Required).
A gas dilution
system can provide known values of calibration gases through controlled
dilution of high-level calibration gases with an appropriate dilution gas. The
instrumental test methods in 40 CFR Part 60 -- e.g., Methods 3A, 6C, 7E, 10,
15, 16, 20, 25A and 25B -- require on-site, multi-point calibration using gases
of known concentrations. A gas dilution system that produces known low-level
calibration gases from high-level calibration gases, with a degree of
confidence similar to that for Protocol1 gases,
may be used for compliance tests in lieu of multiple calibration gases when the
gas dilution system is demonstrated to meet the requirements of this method.
The Administrator may also use a gas dilution system in order to produce a wide
range of Cylinder Gas Audit concentrations when conducting performance
specifications according to Appendix F, 40 CFR Part 60. As long as the
acceptance criteria of this method are met, this method is applicable to gas
dilution systems using any type of dilution technology, not solely the ones
mentioned in this method.
The gas dilution
system shall be evaluated on one analyzer once during each field test. A
precalibrated analyzer is chosen, at the discretion of the source owner or
operator, to demonstrate that the gas dilution system produces predictable gas
concentrations spanning a range of concentrations. After meeting the
requirements of this method, the remaining analyzers may be calibrated with the
dilution system in accordance to the requirements of the applicable method for
the duration of the field test. In Methods 15 and 16, 40 CFR Part 60, Appendix
A, reactive compounds may be lost in the gas dilution system. Also, in Methods
25A and 25B, 40 CFR Part 60, Appendix A, calibration with target compounds
other than propane is allowed. In these cases, a laboratory evaluation is
required once per year in order to assure the Administrator that the system
will dilute these reactive gases without significant loss. Note: The laboratory evaluation is required
only if the source owner or operator plans to utilize the dilution system to
prepare gases mentioned above as being reactive.
The gas
dilution system shall produce calibration gases whose measured values are
within +2 percent of the predicted values. The predicted values are calculated
based on the certified concentration of the supply gas (Protocol gases, when
available, are recommended for their accuracy) and the gas flow rates (or
dilution ratios) through the gas dilution system.
2.1.1 The gas dilution system shall be
recalibrated once per calendar year using NIST-traceable primary flow standards
with an uncertainty <0.25 percent. A label shall be affixed at all times to
the gas dilution system listing the date of the most recent calibration, the
due date for the next calibration, and the person or manufacturer who carried
out the calibration. Follow the manufacturer's instructions for the operation
and use of the gas dilution system. A copy of the manufacturer's instructions
for the operation of the instrument, as well as the most recent recalibration
documentation shall be made available for the Administrator's inspection upon
request.
2.1.2 Some manufacturers of mass flow
controllers recommend that flow rates below 10 percent of flow controller
capacity be avoided; check for this recommendation and follow the
manufacturer's instructions. One study has indicated that silicone oil from a
positive displacement pump produces an interference in SO2 analyzers utilizing ultraviolet
fluorescence; follow laboratory procedures similar to those outlined in Section
3.1 in order to demonstrate the significance of any resulting effect on
instrument performance.
An EPA
Protocol calibration gas is recommended, due to its accuracy, as the high-level
supply gas.
An EPA
Protocol gas shall be used as an independent check of the dilution system. The
concentration of the mid-level supply gas shall be within 10 percent of one of
the dilution levels tested in Section 3.2.
If the gas
dilution system is to be used to formulate calibration gases with reactive
compounds (Test Methods 15, 16, and 25A/25B (only if using a calibration gas
other than propane during the field test) in 40 CFR Part 60, Appendix A), a
laboratory certification must be conducted once per calendar year for each
reactive compound to be diluted. In the laboratory, carry out the procedures in
Section 3.2 on the analyzer required in each respective test method to be
laboratory certified (15, 16, or 25A and 25B for compounds other than propane).
For each compound in which the gas dilution system meets the requirements in
Section 3.2, the source must provide the laboratory certification data for the
field test and in the test report.
The gas
dilution system shall be evaluated at the test site with an analyzer or monitor
chosen by the source owner or operator. It is recommended that the source owner
or operator choose a precalibrated instrument with a high level of precision
and accuracy for the purposes of this test. This method is not meant to replace
the calibration requirements of test methods. In addition to the requirements
in this method, all the calibration requirements of the applicable test method
must also be met.
3.2.1 Prepare the gas dilution system according
to the manufacturer's instructions. Using the high-level supply gas, prepare,
at a minimum, two dilutions within the range of each dilution device utilized
in the dilution system (unless, as in critical orifice systems, each dilution
device is used to make only one dilution; in that case, prepare one dilution
for each dilution device). Dilution device in this method refers to each mass
flow controller, critical orifice, capillary tube, positive displacement pump,
or any other device which is used to achieve gas dilution.
3.2.2 Calculate the predicted concentration for
each of the dilutions based on the flow rates through the gas dilution system
(or the dilution ratios) and the certified concentration of the high-level
supply gas.
3.2.3 Introduce each of the dilutions from
Section 3.2.1 into the analyzer or monitor one at a time and determine the
instrument response for each of the dilutions.
3.2.4 Repeat the procedure in Section 3.2.3 two
times, i.e., until three injections are made at each dilution level. Calculate
the average instrument response for each triplicate injection at each dilution
level. No single injection shall differ by more than +2 percent from the
average instrument response for that dilution.
3.2.5 For each level of dilution, calculate the
difference between the average concentration output recorded by the analyzer
and the predicted concentration calculated in Section 3.2.2. The average
concentration output from the analyzer shall be within +2 percent of the
predicted value.
3.2.6 Introduce the mid-level supply gas
directly into the analyzer, bypassing the gas dilution system. Repeat the
procedure twice more, for a total of three mid-level supply gas injections.
Calculate the average analyzer output concentration for the mid-level supply
gas. The difference between the certified concentration of the mid-level supply
gas and the average instrument response shall be within +2 percent.
3.3 If the gas dilution system meets the
criteria listed in Section 3.2, the gas dilution system may be used throughout
that field test. If the gas dilution system fails any of the criteria listed in
Section 3.2, and the tester corrects the problem with the gas dilution system,
the procedure in Section 3.2 must be repeated in its entirety and all the
criteria in Section 3.2 must be met in order for the gas dilution system to be
utilized in the test.
1. "EPA Traceability Protocol for Assay
and Certification of Gaseous Calibration Standards," EPA-600/R93/224,
Revised September 1993.