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Plant Engineering Calculator

Updated Sep 18, 2026

Compressed Air Pressure Dew Point Converter — PDP to Atmospheric Dew Point

Convert a pressure dew point at one absolute pressure to the equivalent dew point after expansion or compression using water-vapor partial-pressure scaling.

Live calculationWorking shownPrivate in browser

Inputs

Results

Known vapor partial pressure 0.1258 kPa
Target vapor partial pressure 0.0157 kPa
Equivalent target dew point -41.79 °C
Dew-point shift -21.79 °C
More actions

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Formula

e1=Psat(T1); e2=e1(P2/P1); T2=MagnusInverse(e2).

Your calculation, step by step

These steps use the values currently entered above so the result can be checked independently.

  1. 1.Magnus saturation pressure at -20.00 °C: Psat = 0.61094 × exp[17.625T/(T + 243.04)] = 0.12578 kPa.
  2. 2.Absolute-pressure ratio = P₂ ÷ P₁ = 1.000 ÷ 8.000 = 0.12500.
  3. 3.Target vapor partial pressure e₂ = 0.12578 × 0.12500 = 0.01572 kPa.
  4. 4.Magnus inverse of 0.01572 kPa gives target dew point -41.790 °C.
  5. 5.Dew-point shift = -41.790 − -20.000 = -21.790 °C.

How to use this calculator

  1. Enter measured or design values using the units shown beside each field.
  2. Review the live results and the substituted calculation steps before using the estimate.
  3. Check the assumptions below, then continue to a related calculator when the next engineering decision needs another model.

Worked example

A -20°C PDP at 8 bar absolute expands to a much lower vapor partial pressure at 1 bar absolute, producing a lower atmospheric dew point.

Assumptions and limits

  • Pressures are absolute, not gauge.
  • Uses a Magnus saturation-pressure approximation; verify very low dew points with dryer/OEM or standards-based property data.

How to interpret the result

Treat the primary result as an engineering screening value, then use the secondary outputs to understand scale, energy or operating impact. Confirm site measurements, equipment limits and applicable standards before making a design or control change.

Questions engineers ask

Why must the two pressures be absolute?

The conversion scales water-vapor partial pressure by the ratio of total absolute pressures. Gauge pressure omits atmospheric pressure and therefore gives the wrong ratio, especially when converting line pressure dew point to an atmospheric basis.

Is pressure dew point the same as atmospheric dew point?

No. They describe the same moisture content at different total pressures. Expanding compressed air lowers the water-vapor partial pressure, so the equivalent atmospheric dew point is normally colder than the pressure dew point measured at line pressure.

Can this replace an ISO 8573-1 dryer or moisture specification?

No. This tool converts a dew-point basis using a Magnus vapor-pressure approximation. Dryer selection, ISO 8573-1 purity class, instrument accuracy and very low dew-point work should use the applicable standard, validated property method and supplier data.

Continue the calculation

Use the next calculator in the same engineering workflow instead of restarting from search.

compressed air moisturedew pointcompressed air dryer purge loss
Methodology. MAXScanner shows the governing formula, entered values and calculation outputs so the estimate is auditable. The calculator is a screening aid; assumptions and equipment-specific limits remain part of the engineering review.