---
title: Emission Correction to Reference Oxygen
url: https://www.sarveshanalytics.com/calculators/emission-o2-correction-calculator.html
description: Restate a measured emission concentration at a reference oxygen percentage, the standard correction used for boilers, furnaces and engines. Free calculator.
updated: 2026-11-02
source: html
---

Learn · Calculator

# Correct an emission value to a reference oxygen level

Enter the measured concentration, the oxygen measured at the same moment and the reference oxygen in your limit. The result is the concentration the limit compares against.

 Quick answer

Emission limits are stated at a reference oxygen level so that diluting the flue gas with extra air cannot hide pollution. The corrected value equals the measured value times (20.9 minus reference O2) divided by (20.9 minus measured O2). A reading of 100 mg/m3 at 6 % oxygen becomes 120 mg/m3 at a 3 % reference.

Updated 2 November 2026 · Figures from Sarvesh datasheets

Dry-basis oxygen and concentration are assumed. Use the reference oxygen your own consent or standard states; it differs between fuels and plant types.

## How it is calculated

Adding air to flue gas dilutes every pollutant in it. Without a correction, a plant could meet a limit by blowing more air through the stack. The correction scales the measured concentration back to what it would be at the reference oxygen level.

C_ref = C_measured x (20.9 - O2_ref) / (20.9 - O2_measured)

The constant 20.9 is the oxygen share of dry air. The same equation is used for any pollutant measured in the same sample, on a dry basis.

## Worked example

**Worked example**

A boiler's NOx reads 100 mg/m3 at 6 % oxygen. The limit is stated at 3 % oxygen. C_ref = 100 x (20.9 - 3) / (20.9 - 6) = 100 x 17.9 / 14.9 = 120.1 mg/m3.

The plant is over a 115 mg/m3 limit even though the raw reading looked compliant. The raw number was flattered by 6 % oxygen of dilution air.

This is also why a stack monitoring system needs an oxygen channel: the oxygen measurement is part of every concentration result.

## Limits

- Moisture: if the reading is wet-basis, convert to dry first.
- The correction is for combustion processes. Other processes, for example some chemical plants, report without oxygen correction.
- We make no claim of regulatory approval for any instrument or system; your consent or standard defines the method.

## Frequently asked questions

**Why correct emissions to a reference oxygen?**

So that dilution with extra air cannot lower the reported concentration. It lets limits compare like with like across plants running at different excess air.

**What reference oxygen should I use?**

The one in your consent, permit or applicable standard. Typical values differ by fuel, for example solid fuels and gas are often stated at different levels.

**Do I need the oxygen reading from the same sample?**

Yes, taken at the same time and basis (dry or wet) as the pollutant measurement.

**Is the 20.9 or 20.95 constant correct?**

Both appear in practice. Standards that give an equation usually say which to use; the difference is small.

## Related lessons

- [Emission monitoring (CEMS) basics: what a system does](https://www.sarveshanalytics.com/learn/emission-monitoring-cems-basics.html)
- [Combustion, excess air and flue gas oxygen explained](https://www.sarveshanalytics.com/learn/combustion-excess-air-and-flue-gas-oxygen.html)
- [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html)