By: Terrence P. Dunn, President of Environics
On May 30, 1995, the EPA published the final version of Method 205 for inclusion in Appendix M of 40 CFR Part 51 entitled, 'Verification of Gas Dilution Systems for Field Instrument Calibrations.'
Continuous emissions monitoring systems (CEMS) require calibration during compliance audits (Relative Accuracy Test Audits or RATA's). For a number of years, the U.S. Environmental Protection Agency had mandated that these calibrations be conducted using EPA Protocol 1 Gases in an undiluted form to check the analyzer performance. Appendix M has now changed this requirement and allows the use of dilution systems.
Most continuous emissions reference analyzers have multiple ranges. As a result, the personnel performing the compliance audit must bring to the audit site multiple protocol 1 Gas cylinders for each range of each analyzer. Depending on the type and number of analyzers, it is conceivable that the auditor might need as many as 24 separate cylinders of expensive EPA Protocol gas.
A Connecticut-based instrument manufacturer has developed a computerized gas dilution calibration instrument that meets the rigorous accuracy standards set forth in EPA Method 205, appendix M, greatly reducing the costs of performing compliance audits.
It is important to outline EPA Method 205 in order to detail the cost and time savings which can be realized by using the method in the field. The method specifies that the gas dilution instrument used to calibrate the reference analyzers must produce a diluted gas standard that is within two percent of predicted values and that the mass flow controllers in the dilution instrument must be calibrated yearly against a National Institute of Standards and Technology (NIST) traceable standard. In addition, Method 205 requires that the gas dilution instrument must be field-checked against a precalibrated analyzer to prove that the dilution instrument meets the two percent accuracy level.
Once the gas dilution instrument is checked, the auditor is able to use one high concentration Protocol 1 Gas cylinder of each gas specie to calibrate any point in all of the ranges of the analyzer being tested. The savings in gas costs and labor are impressive, however, the key to achieving the savings is in the design and performance of the dilution calibration instrument.
A computerized emissions monitoring calibration system (CEMCS) has been developed which operates on the principle of computerized control of gas flow on a mass basis and incorporates three of more calibrated thermal mass flow controllers, a 32 bit microprocessor, a blending module and gas control solenoids.
Mass flow controllers (MFCs) have been used to dynamically dilute or blend gases for years. However, MFCs as manufactured are accurate to one percent of full scale, which is not good enough for the exacting demands of calibrating CEMS. The microprocessor in the dilution calibration instrument corrects the mass flow controller electronically to enhance their performance from a one percent full-scale accuracy of one percent of set point.
In the CEMCS, mass flow controller performance is enhanced by 'characterizing' each of its flow controllers. This characterization consists of a 10-point comparison of the commanded flow versus the actual flow as measured on a computerized NIST-traceable primary flow calibration standard which compensates for ambient temperature. The resulting table compares the commanded versus actual flow values across the full operating range of the flow controller and this table is stored in the CEMCS' battery-backed-up RAM. The instrument the adjusts the command voltage to its flow controllers by referring to this table. Values between the flow calibration points are derived by linear interpolation. This combination of pre-calibration and then continuous reference to a stored 'lookup-table' during operation enables the Environics series 2020 to maintain an accuracy of +/- 1% of setpoint which exceeds the required +/- 2% specified in Method 205.
The extended dynamic range or the ability to perform a wide range of dilution ratios (turndown) is a crucial advantage of the CEMCS. Since the concentrations of the calibration standards generated during the typical compliance audit cover such a broad range, the instrument must have a very wide dynamic range to achieve the required concentrations and still work with an absolute minimum number of high concentration cylinders. The CEMCS is configured with a minimum of three flow controllers and, in some instances, as many as five. Based on the dilution ratio required, the instrument then selects the appropriate pair of flow controllers to perform the dilution.
With the CEMCS the operator has the ability to cover the full range of concentrations for each gas specie with one or perhaps two source cylinders instead of doing these calibrations with individual cylinders, one for each concentration point. Thus cylinder quantities can be significantly reduced.
The dilution calibration instrument's materials of construction for the gas handling system are also very important, particularly when reactive gases like S02 are considered. If the materials of construction are inappropriate, the gas concentration being generated by the instrument may be distorted by absorption, desorption or reaction of the gas with the wetted surfaces of the dilution instrument. The use of 316 electropolished stainless steel for all gas wetted surfaces and inert elastomeric or metal seals in solenoid valves and flow controllers is the best way to prevent any reaction between the gases and the instrument itself.
A key element of the CEMCS is system software. In addition to storing the mass flow controller calibration data, the software allows the user to command, for each gas specie, the precise concentration that the user wished to deliver to the analyzer. Instrument memory makes it possible for the user to store a complete set of calibration routines for repetitive use in compliance audits so that the operator is not obliged to reenter concentration and flow commands for each and every test. Once the calibration routine for a given compliance audit is loaded and stored in the instrument's battery-backed-up RAM, this calibration routine may be recalled for use with a single keystroke. This memory capability makes it easy for the user to do the pre-audit dilution systems checks required by Method 205.
The interface with the user is a back-lit LCD screen of 80 characters by 25 lines. All of the instrument's primary software routines are accessible through 'soft' keys immediately below their on-screen labels. The CEMCS includes an RS-232 Serial Data Interface and a parallel printer port allowing the operator to generate a paper trail for each and every step. If the dilution system is mounted in an analyzer-equipped van, all of the analyzers in the van may be automatically calibrated immediately upon arrival at the audit site by commanding the dilution system and the analyzers with a PC and a program such as Lab Tech Control or any other industrial control or windows-based software. A majority of the source sampling 'CEM Vans' Built today include computerized gas dilution systems. The dilution systems save space normally devoted to a high number of gas cylinders and they help to fully automate the calibration activities of the van for on-site testing.
The best news to the consultant or operator doing the compliance tests is the potential savings from the reduction in the number of expensive EPA Protocol 1 gas cylinders needed. In general, depending on the number of targeted pollutants and calibration range, the raw material-related cost savings realized by using Method 205 versus conventional methods is on the order of 60%. It is not just the raw material cost of the cylinders and the associated rental and demurrage, but it is also the significant savings realized in reduced labor to transport, handle, track, and recertify the cylinders.
Method 205 permits the dilution of EPA Protocol 1 Gases during compliance audits if the dilution system meets rigorous accuracy criteria. Consultants can achieve significant savings by following Method 205 provided a suitable calibration system is used. The instrument must be automated, accurate and have the ability to perform a wide range of dilutions if the consultant is to maximize the savings.
The benefits of such a dilution system are not only enjoyed by source testing firms in the U.S., but also by a large number of gas analytical instrument users in various markets world-wide who are dependent on the accurate, automated, multi-point calibration of their gas analytical devices.
These markets include the calibration of gas chromatographs, mass spectrometers, or FTIR's for laboratory, environmental or industrial applications, automotive emission test benches, ambient pollution monitoring stations and the generation of part-per-trillion gas standards for semi-conductor microchip fabrication.