Instrument Span and Range Overview
According to the U.S. Environmental Protection Agency (EPA), when implementing section 2. 1. 1. set the range and for each parameter (SO2, NOx, CO2, O2, or flow rate) high enough to prevent full-scale exceedances from occurring yet allow enough to ensure good measurement accuracy and to maintain a high signal-to-noise ratio. To meet these objectives, select the range such that the majority of the readings obtained during typical unit operation are kept, to the extent practicable, between 20.0 and 80.0 percent of the full-scale range of the instrument.
Why are Span and Range Settings Important to My Facility?
Once you have reviewed your reports for the fourth quarter and the end of the year, it is time to look forward. Over time, the emissions levels at many facilities have declined due to pressure on reducing the plant’s overall emissions. Periodically reviewing historically recorded and measured data is a wise business practice to keep your monitoring systems in alignment with the characteristics of your emissions.
The Maximum Potential Concentration (MPC), Maximum Expected Concentration (MEC), and the Maximum Potential Flow (MPF) data sets are used to set the span and range of their respective measurement systems. These values are part of the Monitoring Plan records and are used as a missing data replacement value under specific absent data periods. For this reason alone, these values are the associated span and range settings and must be up-to-date.
How Do Span and Range Affect My Linearity Checks?
Minor adjustments typically require updated Monitoring Plan records, and usually, the same blend of gases for calibrations and linearity checks can still be used. However, sometimes a new combination of gas cylinders is required, triggering a successful passing of full linearity when the cylinders are received.
Where Can I Find Good Resources to Measure My Span and Range Settings?
In Appendix A, you will find several references to periodically reviewing the span and range settings for the SO2 (§2.1.1.5), NOx (§2.1.2.5), CO2 (§2.1.3.3), and flow (§2.1.4.3) systems. At a minimum, an annual evaluation is required. Review these sections of Appendix A carefully to fully understand what additional changes need to be made, such as updating the Monitoring Plan records triggered by changing the MPCs, the MEC, or spend and range settings.
In cases where a span adjustment is required, and the current calibration or linearity gases are unsuitable for use with the new span value, the owner or operator has up to 90 days after the end of the quarter in which the need to adjust the span is identified to implement the change. Reference Sections 2.1.1.5 and 2.1.2.5 of Appendix A. This allows time to purchase and received new gas cylinders blended to support the new span values—review P75 Policy Manual Question 9.23 for further guidance.
Span and Range Final Thoughts
Here are a few items to keep in mind when discussing span and range:
- Typically once every four operating quarters, you should be re-evaluating your span and range.
- Due to stricter limits imposed by regulations, span and range can also be affected by these changes. If you are not updating span and range accordingly, you could potentially hurt your facility’s reports using non-representative data.
- Diagnostic Range Evaluation Reports will break down the range of your monitor into ten perfect increments during the evaluation. This diagnostic report will break down where you are failing, and you can adjust accordingly based on this data (small or large adjustment).
- The Environmental Protection Agency (EPA) is particular with reports concerning your span and range ratio. For this reason, you must practice and prepare for your span and range evaluations so that all of your data is representative of your facility and will abide by all regulations required.
ESC Spectrum’s StackVision software follows all EPA requirements to evaluate span and range, guaranteeing that your report will be correct.
Interested in learning more about regulations and staying in compliance? Read our Definitive Guide to Air Emissions Regulations.
