A dew-point result is meaningful only with its pressure and phase convention. We explain how to compare moisture specifications and investigate conflicting readings.
Why two correct readings can disagree
Dew point describes the temperature at which water begins to condense at the measurement pressure. Below freezing, a reported frost point uses equilibrium over ice; some instruments or specifications use a different convention. Record the convention as well as the temperature. Water concentration in ppmv is a mole fraction, while ppm by mass is a different quantity. None of these labels should be silently interchanged.
For an ideal-gas screening calculation, water mole fraction is approximately saturation vapor pressure at the dew or frost point divided by the sample’s absolute pressure. Use a suitable saturation correlation and the correct liquid-water or ice basis. This approximation is not a substitute for validated calculations in dense or non-ideal CO₂.
A pressure example, not a moisture limit
Suppose a hypothetical frost-point calculation gives a water partial pressure of 1 Pa. At a sample pressure of 100,000 Pa absolute, the corresponding ideal mole fraction is 1 / 100,000 × 1,000,000 = 10 ppmv. At 1,000,000 Pa absolute, it is 1 ppmv. The same reported frost-point temperature at these two pressures does not mean the same moisture concentration.
Conversely, reducing the pressure of a sample without adding or removing water changes its dew point while its mole fraction remains approximately unchanged. That statement stops being valid if condensation, adsorption, leaks or contamination alter the sample.
Write a moisture requirement that can be tested
| Record | Reason |
|---|---|
| Outlet location and gas/liquid phase | A vapor-space sample may not represent liquid product. |
| Dew or frost point, pressure and units | Atmospheric and pressure dew points are different. |
| Analyzer range, calibration and sample arrangement | A display alone does not establish traceability. |
| Limit and release response | The customer specification determines acceptance, not the example above. |
For a cold-end duty, we connect the moisture requirement to the downstream process conditions and selected equipment. A dryer regeneration cycle is not evidence that its outlet is dry enough; outlet verification and a response to breakthrough are separate requirements.
When a laboratory and online analyzer disagree
First compare sample locations, times, phase, pressure and reporting units. Then examine sample transport, stabilization and calibration records. A wet sampling line can create a high reading after the process has dried; a sampling arrangement that removes water can create an apparently favorable result. Preserve the observations and investigate the sample system before changing the process limit or forcing one instrument to match the other.
Sources and scope
- Vaisala DMT152 manual — absolute pressure in moisture conversion
- Swagelok — avoiding fractionation during sample vaporization
We use these sources for the physical principles or guidance identified in this article. Our examples and decision tables are explanatory calculations, not measured plant results. Industry guidance does not replace the edition, customer specification and local requirements agreed for your project. Publication listings are cited for their public scope; we do not claim to reproduce their full standards.
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