Reducing Gas Calibration Costs

The final version of Method 205 was published by the EPA on May 30, 1005 for inclusion in Appendix M of 40 CFR Part 51 entitled, 'Verification of Gas Dilution Systems for Field Instrument Calibrations.'

Gas Calibration is necessary for continuous emissions monitoring systems (CEMS) during compliance audits (Relative Accuracy Test Audits (RATAs)). For many years, the U.S. Environmental Protection Agency had made it compulsory that the calibrations should be performed by employing EPA Protocol 1 Gases in an undiluted form in order to evaluate the analyzer performance. Now, the use of dilution systems is allowed as Appendix M has changed the requirement.

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Many continuous emissions reference analyzers feature multiple ranges. Due to this, the personnel conducting the compliance audit must bring several protocol 1 Gas cylinders to the audit site, in order to cater to each range of each analyzer. Depending on the number and type of analyzers, it can be estimated that the auditor might require 24 separate cylinders of expensive EPA Protocol gas.

Now, 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. This has significantly reduced the costs of conducting compliance audits.

EPA Method 205

It is essential to present an overview of the EPA Method 205 so as to explain the cost and time savings which can be achieved by employing the method in the field. The method requires that the gas dilution instrument, which is employed to calibrate the reference analyzers, should generate a diluted gas standard that is within two percent of predicted values. The method also 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. Moreover, the method makes it essential that the gas dilution instrument be field-checked against a pre-calibrated analyzer in order to show that the dilution instrument satisfies the two percent accuracy level.

After the gas dilution instrument is checked, one high concentration Protocol 1 Gas cylinder of each gas specie can be used by the auditor to calibrate any point in all of the ranges of the analyzer being tested. The key to achieving impressive savings in labor and gas costs lies in the design and performance of the dilution calibration instrument.

Computerized Emissions Monitoring Calibration System

Operating on the principle of computerized control of gas flow on a mass basis, a computerized emissions monitoring calibration system (CEMCS) incorporates three of more calibrated thermal mass flow controllers, gas control solenoids, a blending module, and a 32-bit microprocessor.

For many years, mass flow controllers (MFCs) have been employed to dynamically blend or dilute gases. However, the currently-manufactured MFCs are accurate to only one percent of full scale, which does not satisfy 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.

In the case of CEMCS, 'characterizing' each of its flow controllers enables improving the mass flow controller performance. This characterization involves a 10-point comparison of the commanded flow against the actual flow when measured on a computerized NIST-traceable primary flow calibration standard which compensates for ambient temperature. The resulting table enables the commanded flow values to be compared with the actual flow values across the full operating range of the flow controller. The CEMCS' battery-backed-up RAM is used to store the table. By referring to this table, the instrument then adjusts the command voltage to its flow controllers. Values between the flow calibration points are obtained through linear interpolation. The Environics series 2020 uses this combination of pre-calibration and the continuous reference to a stored 'lookup-table' during operation in order to maintain an accuracy of +/- 1% of setpoint, thus surpassing the specified +/- 2% of Method 205.

An important advantage of the CEMCS is the ability to perform a broad range of dilution ratios (turndown), i.e., an extended dynamic range. The concentrations of the calibration standards produced during the typical compliance audit cover a wide range. As a consequence, the instrument should have a very broad dynamic range to achieve the desired concentrations and still perform with a minimum number of high concentration cylinders. A minimum of three flow controllers are needed to configure the CEMCS, but in some instances, even five may also be required. Depending on the required dilution ratio, the instrument then chooses the appropriate pair of flow controllers in order to perform the dilution.

By using the CEMCS, the operator can cover the entire range of concentrations for each gas specie with one or two source cylinders, thus avoiding the calibrations to be performed with separate cylinders, one for each concentration point, and hence, significantly reduce the number of cylinders required.

The materials of construction for the dilution calibration instrument for the gas handling system are also critical, especially when considering reactive gases like SO₂. If inappropriate materials are used, the gas concentration being produced by the instrument may be distorted owing to desorption, absorption, or reaction of the gas with the wetted surfaces of the dilution instrument. The best way to prevent any reaction between the instrument and the gases is to employ inert elastomeric or metal seals in flow controllers and solenoid valves and 316 electropolished stainless steel for all gas wetted surfaces.

The system software is a key element of the CEMCS. The software not only stores the mass flow controller calibration data, but also allows the user to command, for each gas specie, the exact concentration that the user wanted to deliver to the analyzer. Instrument memory allows the user to store a complete set of calibration routines for repetitive use in compliance audits, thus avoiding 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, after the routine for a given compliance audit is loaded and stored in the instrument's battery-backed-up RAM. This memory capability enables the user to easily check the pre-audit dilution systems, as mandated by 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 conveniently accessed through the 'soft' keys located just below their on-screen labels. A parallel printer port and an RS-232 Serial Data Interface enable the operator to produce a paper trail for every individual step. When the dilution system is fitted to an analyzer-equipped van, all of the analyzers in the van can be automatically and immediately calibrated upon arrival at the audit site. This can be accomplished by commanding the dilution system and the analyzers with a PC and a program such as Windows-based software or Lab Tech Control or any other industrial control. Computerized gas dilution systems form a part of a majority of today’s source sampling 'CEM Vans.’ The dilution systems save space which is normally used for keeping a large number of gas cylinders, and the systems help in full automation of the calibration activities of the van for on-site testing.

The operator or consultant performing the compliance tests is best served by the potential savings from the reduction in the number of expensive EPA Protocol 1 gas cylinders needed. Generally, based on the number of targeted pollutants and calibration range, the raw material-related cost-savings achieved by employing Method 205 as compared to traditional methods is about 60%. The saving is because of the raw material cost of the cylinders and the related rental and demurrage as well as those achieved by reducing 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. Significant savings can be achieved by consultants by following Method 205 if a suitable calibration system is employed. The instrument should be accurate and automated, and must perform a broad range of dilutions, so that the consultant can achieve maximum savings.

The benefits of such a dilution system can be enjoyed by source testing firms based in the United States as well as by many gas analytical instrument users in the international markets who depend on the automated, accurate, multi-point calibration of their gas analytical devices.

These markets cover the calibration of gas chromatographs, FTIRs for laboratory, environmental or industrial applications, mass spectrometers, ambient pollution monitoring stations, and automotive emission test benches, and 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.