--- title: Dry Basis vs Wet Basis in Gas Measurement url: https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html description: What dry basis and wet basis mean, how water vapour changes every other reading, how to convert between them and why in-situ and extractive analysers differ. updated: 2026-11-03 source: html --- Learn · Foundations # Dry basis vs wet basis: why water changes every reading Water vapour is the hidden component of nearly every process gas. Whether it is counted or removed changes the number on the screen. Intermediate 3 min read Lesson 6 of 9[Foundations](https://www.sarveshanalytics.com/learn/track-foundations.html) Helpful first: [ppm, percent and ppb explained for gas analysis](https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html) Quick answer A wet-basis reading counts water vapour as part of the gas; a dry-basis reading removes it. Because water takes up part of the volume, a wet-basis oxygen value is lower than the dry-basis value for the same gas. In-situ analysers read on a wet basis, extractive analysers that dry the sample read on a dry basis, and limits are usually dry. Updated 3 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - Explain dry and wet basis and when each applies - Convert an oxygen reading between wet and dry - State which analyser types read on which basis ## Water is a gas component too Flue gas, biogas and fermentation off-gas all carry water vapour, often 5 to 20 % by volume. When a sensor measures oxygen as a share of all molecules in the gas, water is part of the denominator. If you remove the water first, the denominator shrinks and the oxygen share rises. ## Converting between them dry value = wet value / (1 - water fraction) **Worked example** A hot flue gas reads 2.7 % oxygen in situ and holds 10 % water vapour. The dry-basis oxygen is 2.7 / (1 - 0.10) = 3.0 %. The gas is the same; only the counting basis changed. This is why a boiler operator who compares an in-situ zirconia reading with a dry extractive reading from the same duct sees a difference even when both instruments are right. ## Which analysers read which basis - **In-situ analysers** such as zirconia read wet: they see the gas as it is, with its water. - **Extractive analysers** that cool the sample to a low dew point before measuring read dry, to the extent that the water is removed. A Peltier cooler holds the gas at a fixed dew point rather than removing all water. - **Hot-wet extractive systems** keep the sample hot and measure wet, then correct if a dry value is needed. Check which basis a specification means, and which basis your instrument reports. Environmental limits are normally stated dry. ## Why it matters for control and compliance - Combustion trim uses oxygen; changing the water content of the fuel changes a wet reading even when the burner air is unchanged. - Excess-air formulas assume dry oxygen: [Excess air from flue gas oxygen calculator](https://www.sarveshanalytics.com/calculators/excess-air-calculator.html). - Emission values are corrected to dry and to a reference oxygen: [Emission correction to reference oxygen](https://www.sarveshanalytics.com/calculators/emission-o2-correction-calculator.html). Wet value = dry value x (1 - water fraction). The same flue gas reads 3 % dry or 2.55 % wet at 15 % water. **In short** - Water vapour is part of the gas volume; dry basis removes it. - Wet-basis oxygen is lower than dry-basis oxygen for the same gas. - dry = wet / (1 - water fraction). - In-situ reads wet; a cooled extractive sample reads dry. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **A flue gas has 8 % water vapour and reads 4.6 % oxygen wet. What is the dry value?** 4.6 / 0.92 = 5.0 %. **Why do in-situ and extractive oxygen readings on the same duct differ?** In-situ reads on a wet basis and the extractive instrument reads after drying, so the water share changes the value. **What basis do emission limits normally use?** Dry basis, and a stated reference oxygen. ## Key terms - [Dry basis](https://www.sarveshanalytics.com/learn/glossary.html#t-dry-basis) - [Wet basis](https://www.sarveshanalytics.com/learn/glossary.html#t-wet-basis) - [Dew point](https://www.sarveshanalytics.com/learn/glossary.html#t-dew-point) - [Peltier cooler](https://www.sarveshanalytics.com/learn/glossary.html#t-peltier-cooler) ## Frequently asked questions **Is oxygen higher or lower when measured wet?** Lower, because the water vapour takes up part of the volume. **How much water is in flue gas?** Commonly 5 to 20 % by volume depending on the fuel. Gas-fired plants carry more than coal-fired. **Do dry-gas analysers see water?** A cooled sample still holds water at the cooler's dew point, which is usually a few percent or less. **Can I measure moisture in the same sample?** Only if the sample is kept above its dew point, which is a different sample system from a cooled dry one. ## Related products [ ### ACCURRA-Zr Zirconia percent O2 analyser for furnaces and boilers, single-point air calibration, options to 1400 C. View ACCURRA-Zr ](https://www.sarveshanalytics.com/flue-gas-analysers.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 - [Boiler and furnace oxygen (flue gas)](https://www.sarveshanalytics.com/applications/boiler-and-furnace-oxygen-flue-gas.html)