On May 30, 2005, the final version of Method 205 was published by the U.S. Environmental Protection Agency (EPA) for inclusion in Appendix M of 40 CFR Part 51 under the title of 'Verification of Gas Dilution Systems for Field Instrument Calibrations.'
Gas calibration is a necessary process for Continuous Emissions Monitoring Systems (CEMS) during compliance audits (Relative Accuracy Test Audits (RATA’s)). For many years, the U.S. EPA enforced that the calibrations should all be performed by employing EPA Protocol 1 gases in an undiluted form to evaluate the analyzer performance. However, current use of dilution systems is now allowed, as Appendix M has changed the requirement.
Environics Gas Mixing System. Image Credit: Environics
As a result of many continuous emissions reference analyzers featuring multiple ranges, the personnel conducting the compliance audit must bring several EPA Protocol 1 gas cylinders to the audit site to cater to each range of each analyzer. Depending upon the number and type of analyzers, it can be estimated that the auditor might require 24 separate cylinders of expensive EPA Protocol gas.
Today, a computerized gas dilution calibration instrument satisfying the rigorous accuracy standards of EPA Method 205 Appendix M has been developed by a Connecticut-based instrument manufacturer to significantly reduce the costs of conducting compliance audits.
EPA Method 205
It is essential to present an overview of the EPA Method 205 to explain the cost and time savings that can be achieved by employing the method in the field. The aforementioned method requires that the gas dilution instrument that is utilized to calibrate the reference analyzers should generate a diluted gas standard that is within two percent of predicted values.
Additionally, this method specifies that the mass flow controllers in the dilution instrument must be annually calibrated against a National Institute of Standards and Technology (NIST) traceable standard.
Finally, this method makes it essential that the gas dilution instruments are field-checked against a pre-calibrated analyzer to determine that the dilution instrument satisfies the two percent accuracy level.
Following a check of the gas dilution instrument, one high concentration Protocol 1 gas cylinder of each gas species can be used to calibrate any point in all of the potential ranges of the analyzer being tested by an auditor. The key to achieving impressive savings in labor and gas costs is dependent upon the state, design and performance of the dilution calibration instrument.
Computerized Emissions Monitoring Calibration System (CEMCS)
The Computerized Emissions Monitoring Calibration System (CEMCS) operates on the principle of computerized control of gas flow on a mass basis while also incorporating three of the calibrated thermal mass flow controllers, gas control solenoids, a blending module and a 32-bit microprocessor.
Mass Flow Controllers (MFC’s) have typically been utilized to dynamically blend or dilute gases. However, currently-manufactured MFC’s are accurate to only one percent of full scale, which does not comply with the stringent demands of calibrating CEMCS. The microprocessor in the dilution calibration instrument electronically corrects the mass flow controller to improve their performance from a one percent full-scale accuracy of one percent of set point.
For instruments like the CEMCS, 'characterizing' each of its flow controllers allows for the mass flow controller performance to improve. This characterization process involves a 10-point comparison of the commanded flow against the actual flow when measured on a computerized NIST-traceable primary flow calibration standard that compensates for the ambient temperature.
The resulting table allows the commanded flow values to be compared with the actual flow values across the full operating range of the flow controller. The CEMCS RAM is a battery powered storage system that holds the table values and allows for future reference by the instrument to adjust the command voltage to its flow controllers as necessary. Values between the flow calibration points are obtained through linear interpolation.
The Environics Series 2020 uses this type of combination of pre-calibration and the continuous references to a stored 'lookup-table' during operation to maintain an accuracy of +/- 1% of set-point to ultimately surpass the specified +/- 2% of Method 205.
An important advantage of the CEMCS is through its ability to perform a broad range of dilution ratios. The concentrations of the calibration standards produced during the typical compliance audit cover a wide range, thereby allowing the instrument to hold a broad dynamic range to achieve the desired concentrations while simultaneously performing with a minimum number of high concentration cylinders.
A minimum of three flow controllers are required for the configuration of the CEMCS, however some situations will require up to five flow controllers, depending on the required dilution ratio. The instrument then chooses the appropriate pair of flow controllers to perform the dilution.
By using the CEMCS, the operator is able to cover the full range of concentrations for each gas species with only one or two source cylinders, thus avoiding the need of performing calibrations with separate cylinders for each concentration point and ultimately reduce the number of required cylinders for this process.
The construction materials used for the dilution calibration instrument for the gas handling system are particularly critical, especially when considering reactive gases like sulfur dioxide (SO2). If inappropriate materials are used during this process, the gas concentration produced by the instrument can be distorted as a result of desorption, absorption or any type of reaction of the gas with the wetted surfaces of the dilution instrument. The best way to prevent these types of adverse reactions between the instrument and the gases involves the use of inert elastomeric or metal seals in flow controllers, solenoid valves and electro-polished stainless steel for all gas wetted surfaces.
The system software is also a key element of the CEMCS, as it not only stores the mass flow controller calibration data but also allows the user to command the exact concentration needed for the delivery to the analyzer for each species of gas. Instrument memory allows the user to store a complete set of calibration routines for repetitive future use in compliance audits to ultimately avoid the need for the operator to re-enter concentration and flow commands for every individual test.
The calibration routine can be recalled for use by a mere keystroke following the routine for a given compliance audit to be loaded and stored in the instrument's battery backed-up RAM. This memory capability allows the user to easily check the pre-audit dilution systems, which is a mandated procedure according to the Method 205.
A back-lit LCD screen of 80 characters by 25 lines forms the interface with the user. The instrument's primary software routines can be accessed conveniently through the 'soft' keys located just below their on-screen labels. A parallel printer port and an RS-232 Serial Data Interface both enables the operator to produce a paper trail for every individual step.
When the dilution system is fitted to an analyzer-equipped van, each of the analyzers within the van can be automatically and immediately calibrated upon arrival at the audit site by commanding the dilution system and the analyzers with a PC and a program, such as a Windows-based software, Lab Tech Control or any other industrial control.
Computerized Gas Dilution Systems
Computerized gas dilution systems are a major aspect of today’s source sampling 'CEM Vans,’ as these systems save a considerable amount of space that is normally used for keeping a large number of gas cylinders. These systems help in the full automation of the calibration activities of the van for on-site testing.
The operator or consultant that is performing the compliance tests benefits the most from these reductions in the number of expensive EPA Protocol 1 gas cylinders that are needed for these procedures.
According to the number of targeted pollutants and calibration range, the use of the Method 205 allows for approximately 60% savings in the cost of raw materials as compared to traditional methods as a result of the raw material cost of the cylinders, related rentals, demurrages and any other costs associated with the labor needed to transport, handle, track and recertify the cylinders.
Method 205 allows the dilution of EPA Protocol 1 gases during compliance audits when the dilution system satisfies the rigorous accuracy criteria. Consultants following Method 205 can achieve a significant reduction in costs if a suitable calibration system is employed. The automated instrument should be accurate and capable of performing a broad range of dilutions to allow the consultant to achieve the maximum amount of potential savings.
The benefits of such a dilution system are particularly beneficial for source testing firms based in the United States, as well as by most users of gas analytical instruments in the international markets who depend on the automated, accurate and multi-point calibration of their gas analytical devices.
These markets cover the calibration of gas chromatographs and FTIRs for laboratory, environmental or industrial applications, as well as mass spectrometers, ambient pollution monitoring stations and automotive emission test benches, and finally for the generation of part-per-trillion gas standards for semi-conductor microchip fabrication.
This information has been sourced, reviewed and adapted from materials provided by Environics, Inc.
For more information on this source, please visit Environics, Inc.