--- title: Emission Monitoring (CEMS) Basics: What a System Does url: https://www.sarveshanalytics.com/learn/emission-monitoring-cems-basics.html description: What a continuous emission monitoring system measures, how extractive, hot-wet and in-situ systems differ, reference oxygen correction. updated: 2026-11-02 source: html --- Learn · Process primers # Continuous emission monitoring (CEMS) basics A CEMS is a measurement system with a legal job. Its design starts from the regulator's requirement and works down to the analyser. Intermediate 3 min read Lesson 10 of 13[Process primers](https://www.sarveshanalytics.com/learn/track-process-primers.html) Helpful first: [ppm to mg/Nm3: volume vs mass concentration](https://www.sarveshanalytics.com/learn/volume-mass-and-mg-per-nm3-concentration-units.html), [Combustion, excess air and flue gas oxygen explained](https://www.sarveshanalytics.com/learn/combustion-excess-air-and-flue-gas-oxygen.html) Quick answer A continuous emission monitoring system measures pollutants and oxygen in stack gas and reports concentrations corrected to a reference oxygen and dry basis. Most extractive systems take a sample through a heated probe and line to a conditioner and analysers; in-situ systems measure in the stack. Daily zero and span checks keep it accurate. We make no claim of statutory approval; your regulator defines what is accepted. Updated 2 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - List what a CEMS measures and the parts of an extractive system - Explain reference oxygen and dry-basis reporting - Compare extractive, hot-wet and in-situ approaches - Understand why calibration checks are routine ## What it measures Common parameters: NO and NO2 (NOx), SO2, CO, CO2 and O2, together with flow, temperature and pressure, and dust (opacity or particulates, with other instruments). Oxygen is essential even if it is not itself a limit, because other concentrations are corrected to a reference oxygen: [Emission correction to reference oxygen](https://www.sarveshanalytics.com/calculators/emission-o2-correction-calculator.html). ## Extractive system parts 1. **Probe** with a filter and blowback at the stack. 2. **Heated line** keeping the sample above its dew point. 3. **Conditioner** (a Peltier cooler or dilution) that dries the sample to a fixed dew point. 4. **Analysers** for each gas. 5. **Calibration gas** switching, for zero and span checks. 6. **Data system** that corrects, averages and records. SGI-320 combines the probe with auto-blowback, heated line, Peltier conditioner and analysers for NO, SO2, CO, CO2 and O2 as one system. It suits samples with SO2, NO and CO below 1000 ppm and excludes HCl, HF and H2S: [What samples an analyser cannot take: know the limits](https://www.sarveshanalytics.com/learn/what-samples-an-analyser-cannot-take.html). ## Hot-wet, cold-dry and in-situ | Approach | How | Good for | Watch | |---|---|---|---| | Cold-dry extractive | Cool and dry the sample, then analyse | Most gases, simple analysers | Soluble gases such as SO2 and NO2 can be lost in condensate | | Hot-wet extractive | Keep the sample hot all the way to a heated analyser | Soluble and reactive gases | Heated everything; more cost and maintenance | | In-situ | Measure in the stack | Oxygen, some gases, no sample line | Fewer gases; harder to check with test gas | ## Dry basis and reference oxygen Regulators state limits dry and at a reference oxygen. The system must therefore either measure moisture, or dry the sample to a known state, and measure oxygen, to restate each reading. See [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html) and [ppm to mg/Nm3: volume vs mass concentration](https://www.sarveshanalytics.com/learn/volume-mass-and-mg-per-nm3-concentration-units.html). ## Calibration, quality and approval - Daily automatic zero and span checks with certified gas, and periodic full calibration, are normal practice: [Auto-Calibration and validation checks for gas analysers](https://www.sarveshanalytics.com/learn/auto-calibration-and-validation-checks.html). - Quality assurance schemes require tests against a reference method. - Regulatory approval requirements differ by country and sector. We make no claim of statutory approval for our systems; tell us your regulator's requirement and we will scope the system around it. **In short** - A CEMS measures pollutants and oxygen, then corrects to dry basis and reference oxygen. - Extractive systems use probe, heated line, conditioner and analysers; SGI-320 packages these. - Hot-wet keeps soluble gases; cold-dry is simpler. - Regular calibration checks are part of the system; approvals are the regulator's call. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **Why does a CEMS measure oxygen even when oxygen is not a limit?** To correct other concentrations to the reference oxygen. **Why use a heated line?** To keep the sample above its dew point until the conditioner. **What gases does SGI-320 exclude?** HCl, HF and H2S. ## Key terms - [CEMS](https://www.sarveshanalytics.com/learn/glossary.html#t-cems) - [Dry basis](https://www.sarveshanalytics.com/learn/glossary.html#t-dry-basis) - [Peltier cooler](https://www.sarveshanalytics.com/learn/glossary.html#t-peltier-cooler) - [Reference conditions](https://www.sarveshanalytics.com/learn/glossary.html#t-reference-conditions) ## Frequently asked questions **What is a CEMS?** A continuous emission monitoring system that measures stack gases and reports them corrected to regulatory conditions. **Is SGI-320 approved by regulators?** We make no statutory approval claim; send your regulatory requirement. **What is the difference between hot-wet and cold-dry?** Hot-wet keeps the sample hot and wet to the analyser; cold-dry dries it first. **Do I need to measure oxygen?** Yes, normally, for reference oxygen correction. ## Related products [ ### SGI-300 / SGI-320 Peltier sample cooler with integrated pump, filter and condensate detector; SGI-320 for emission monitoring. View SGI-300 / SGI-320 ](https://www.sarveshanalytics.com/sample-coolers.html)[ ### SP and SRP sample probes Heated and non-heated probes with blowback, and retractable probes, in SS316, Inconel or ceramic. View SP and SRP sample probes ](https://www.sarveshanalytics.com/sample-probes.html)[ ### i-Cons Smart Peltier sample conditioner with diagnostics, auto blowback and Modbus, in a compact 19 inch cabinet. View i-Cons ](https://www.sarveshanalytics.com/smart-systems-for-gas-analyser.html) ## Where this applies - [Stack and emission monitoring](https://www.sarveshanalytics.com/applications/stack-and-emission-monitoring-system.html) - [i-Cons and SGI-320](https://www.sarveshanalytics.com/smart-systems-for-gas-analyser.html)