Simple explanation
Wet basis includes water vapor in the gas. Dry basis removes water vapor. If moisture increases, wet-basis concentrations can look lower due to dilution.
Field reality: If your sample conditioning changes (dryer/chiller problems),
your reported values can shift even if the stack emissions did not change.
When wet vs dry matters most
- Stack gas with high moisture (boilers, SRU, cement)
- During chiller/dryer issues
- When probe heaters or line heaters fail
- When moisture measurement or correction is used
Common technician mistakes
- Comparing a wet-basis handheld reading with a dry-basis CEMS value
- Ignoring chiller temperature drift or drain blockages
- Not documenting basis in reports (wet/dry not stated)
- Using incorrect moisture correction factor or wrong DAHS tag
Quick checks
- Is the sample conditioning stable (chiller temp, dryer health)?
- Any condensation in lines, filters, or manifolds?
- Is moisture measurement configured correctly in DAHS?
- Do you know if reporting is wet or dry basis?