A Method 21 reading is only as defensible as the calibration behind it. Here’s what the method requires (40 CFR Part 60, Appendix A-7) and what an auditor pulls first.
Daily calibration
Before monitoring, the analyzer is calibrated with a zero gas and a calibration (reference) gas at a concentration at or near the leak definition — using the reference compound specified in your applicable subpart. Most leak definitions are 500 or 10,000 ppm; a 10,000 ppm reading on a methane-calibrated instrument is a leak where that’s the definition.
The three numbers auditors check
- Calibration precision ≤ 10% of the calibration gas value.
- Response time ≤ 30 seconds.
- Response factor < 10 for the VOC measured, unless the applicable rule specifies otherwise.
What the auditor asks for
- Daily calibration and drift-check logs, in order, matching the monitoring dates
- Certificates for the calibration gases
- Evidence the reference compound matches the subpart
Why it matters
The number Method 21 produces is the number your leak definition is enforced against. If the calibration behind it won’t stand up, every reading downstream is in question. We keep the calibration record clean so the data holds — in the field and in the audit.
Frequently asked
What calibration gas do we use for Method 21?
The reference compound specified in your applicable subpart (often methane), at a concentration at or near the leak definition — commonly 500 or 10,000 ppm. The point is to calibrate against what the rule actually enforces.
What’s the calibration precision requirement?
Calibration precision must be equal to or less than 10% of the calibration gas value, with a response time of 30 seconds or less. These are baseline Method 21 requirements.
What if a compound’s response factor is over 10?
That means the instrument doesn’t respond reliably to it on your current reference gas. You recalibrate to a compatible reference compound (or one the rule permits) so leaks aren’t undercounted.
This is general information, not a regulatory applicability determination. How these requirements apply depends on your equipment, permit, and the subpart you’re subject to. Confirm your obligations with a qualified professional or your permitting authority.
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