Gas Moisture (H2O) Measurement: Water Vapor Analyzers
Water vapor (H₂O) measurement in flue gas and high-temperature processes; direct in-line moisture monitoring without sampling, using a TDLAS laser analyzer.
Measurement methods: TDLAS Laser Absorption
Why is water vapor measured in gas?
Water vapor (H₂O) in gas streams is a parameter that must be measured in many industrial applications. In flue gas, moisture is used for conversion between wet and dry basis and for bringing emission values to reference conditions. In combustion and furnace processes, water vapor concentration is an indicator of combustion products; in drying, calcination and process gas lines, it affects product quality and efficiency. In natural gas and hydrocarbon lines, water is a risky component because of corrosion and hydrate formation.
Where is moisture measured?
- Stacks and ducts: Moisture correction for emission calculations.
- High-temperature furnaces and boilers: Combustion control and optimization.
- Drying and process gas lines: Moisture content control.
The method used for moisture in gas
In hot, humid gas streams, water vapor condenses during cooling in extractive systems, which affects measurement. The TDLAS approach eliminates this problem: a tunable diode laser measures water vapor's characteristic absorption line within the gas, on the duct. Because no sample is taken, condensation, line lag and filter maintenance do not arise. Some laser analyzers also monitor O₂, CO and CH₄ over the same optical path, providing several parameters on one instrument for combustion control.
Choosing a moisture analyzer
- Process temperature and pressure; optical path length and mounting point
- Dust load and window contamination
- Hazardous area suitability
- Output signal and connection to the process control system
Gas moisture measurement with Tetra
Tetra supplies NEO Monitors LaserGas iQ2 and LaserGas II SP laser analyzers. It carries out mounting, alignment, commissioning and calibration verification, and provides periodic maintenance and spare parts support.
Frequently asked questions
What unit is moisture in flue gas expressed in?
Gas moisture is generally given in percent by volume (% vol) or ppmv; at low levels, dew point temperature is also used. In emission calculations, the percent by volume value is used directly to convert a wet-basis concentration to a dry basis.
How is an emission value measured on a wet basis converted to a dry basis?
The dry-basis concentration is found by dividing the wet-basis concentration by (1 − water vapor volume fraction). For example, if the gas contains 10% water vapor by volume, the wet value is divided by 0.9. An error in moisture measurement is therefore directly reflected in the corrected emission result.
How are a TDLAS moisture analyzer's calibration and maintenance carried out?
TDLAS analyzers are calibrated at the factory, and periodic checks on site are generally made with a reference gas or verification cell. The main part of maintenance is keeping the optical windows clean and maintaining a continuous purge air supply. In very dusty ducts, light transmission can fall, so monitoring the signal level is recommended.
Which methods are used for moisture measurement at low temperature or in natural gas?
For clean gases near ambient temperature, capacitive humidity sensors and dew point meters are common. For trace water in lines such as natural gas, laser-based analyzers or dew point analyzers are preferred. Expected moisture range, pressure and interference from other components in the gas determine the choice of method.
Products measuring this parameter
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