Learn & Understand

Extractive vs. In-Situ Gas Analyzers: Why the Conversion Direction Matters

Whether you need to convert a reading from dry to wet basis or the other way around often comes down to which type of analyzer took the measurement in the first place - the two dominant CEMS analyzer designs handle moisture completely differently by their basic physical design.

Extractive Analyzers: Pull, Condition, Then Measure

Extractive continuous emissions monitoring systems physically pull a sample of flue gas out of the stack or duct, route it through a sample conditioning system (typically including a chiller or dryer that condenses out water vapor), and only then pass the now largely moisture-free sample into the actual gas analyzer. Because the sample has already had its water vapor physically removed before analysis, the reading these systems produce is naturally a dry-basis measurement - there's no wet-basis reading to convert from at all, since the moisture was removed before measurement rather than mathematically corrected after.

In-Situ Analyzers: Measure the Gas Exactly As It Is

In-situ analyzers instead measure the flue gas directly inside the duct or stack itself, in its natural, moisture-containing (wet) state, typically using optical techniques like infrared or ultraviolet absorption that can operate through the actual hot, wet gas stream without needing to extract and condition a sample first. This approach avoids the complexity, cost, and potential for sample-line contamination or condensation problems inherent in extractive systems - but it means the raw measurement comes out on a wet basis, and needs exactly the dry-to-wet or wet-to-dry conversion covered in this category to be compared against a dry-basis regulatory limit or another wet-basis measurement point.

Why a Facility Might Choose One Design Over the Other

Why Knowing Your Analyzer Type Matters Before Applying a Conversion

Applying a dry-to-wet conversion to a reading that already came from an extractive system (already dry basis) would introduce an unnecessary and incorrect adjustment, just as failing to convert an in-situ analyzer's genuinely wet-basis reading before comparing it to a dry-basis limit would produce a misleadingly low apparent pollutant concentration. Confirming which analyzer design produced a given reading is the essential first step before deciding whether - and in which direction - a moisture basis conversion is actually needed.

Applying This in Practice

When cross-checking readings from two different emissions monitoring points on the same combustion system, verify each point's analyzer type first - mixing an extractive system's naturally dry reading with an in-situ system's naturally wet reading, without applying this conversion correctly to bring both onto the same basis, is a common source of confusing or seemingly contradictory emissions data across a facility.