A drying-plant dew-point sensor measures its sample stream. That stream may have been diluted, cooled or delayed between the active part and the sensor. Without the path and pressure basis, a low reading can be more reassuring than the process evidence warrants. [1]
Audit the route to the sensor
Identify the source point: active-part outlet, chamber, return line or mixed exhaust.
Mark fresh-gas or other dilution points before the sample branch.
Identify line low points and cold sections where condensation can alter the stream.
Record sensor pressure and whether the result is reported on that pressure basis.
Estimate or measure transport and instrument response lag for the defined sampling arrangement.
Pressure and condensation change interpretation
Dew point is tied to water-vapour partial pressure. Expanding a gas sample to a different total pressure can change its pressure dew point even if its water fraction is unchanged. State whether the reported reading has been converted and how.
If water condenses upstream, the gas reaching the sensor can lose moisture relative to the source. Evaluate materials, line temperature and flushing so that the sample remains representative. [1]
For a swept line, volume and volumetric flow must share the same pressure and temperature basis; continuity then gives its nominal gas residence time. [2]
Put the sample delay on its pressure basis
Assumptions: a well-swept 2 L sampling line carries 0.5 L/min, both at line pressure and temperature, with plug flow and negligible instrument lag. Its residence time is 2/0.5 = 4 min. A programme changes at 12:00; a 12:02 analyser reading still describes gas admitted at 11:58 and cannot verify the new stage. Hold that endpoint interpretation. Establish the real response, including mixing, adsorption and sensor dynamics, and retain the pressure dew-point convention. [1] [2]
Match time before comparing process stages
For a simple swept line, residence time scales roughly with contained volume divided by volumetric flow at the relevant pressure. Dead legs, adsorption and sensor dynamics can add delay. A sudden process change may therefore appear later at the analyser.
References
- S. Bell, The Beginner’s Guide to Humidity Measurement, National Physical Laboratory, Measurement Good Practice Guide 124, 2013, sections 4.3, 4.4 and 9.
- U.S. Department of Energy, DOE-HDBK-1012/3-92, Thermodynamics, Heat Transfer, and Fluid Flow, Volume 3, 1992.