Compressed Air Dew Point Converter (PDP ↔ ADP) #

Q: How do I convert pressure dew point to atmospheric dew point?

Find saturation vapor pressure at the pressure dew point with the Magnus equation, scale by P_atm / P_abs (constant mole-fraction approximation), then invert Magnus to get atmospheric dew point. Example: +3°C PDP at 7 bar(g) ≈ −23°C ADP.

Q: What is the difference between PDP and ADP for compressed air?

Pressure dew point (PDP) is the condensation temperature while air is still under line pressure. Atmospheric dew point (ADP) is the equivalent dew point if that same moisture content were expanded to atmospheric pressure. Dryer specs and ISO 8573 water classes use PDP.

Q: Is this the same as an HVAC dew point calculator?

No. This tool converts compressed-air pressure dew point using line pressure. HVAC psychrometric calculators solve moist air properties for rooms and ducts at near-atmospheric pressure.

Convert compressed air pressure dew point (PDP) to atmospheric dew point (ADP) and back. Uses a Magnus vapor-pressure approximation at line pressure—not an HVAC room psychrometric calculator. Next: size the dryer with the air dryer sizing calculator.

Conversion Inputs

Direction

Pressure Dew Point (PDP) (°C)

Working Pressure (bar gauge)

Temperature Units

Advanced calculator

Absolute pressure = gauge + atmospheric. Default sea-level atmosphere ≈ 1.01325 bar.

Pressure Units

Atmospheric Pressure (bar abs)Change for altitude

About this calculator

Engineering approximation: e_s from Magnus at the known dew point, then e₂ = e₁ × (P₂ / P₁) with constant mole fraction, then invert for the other dew point. ISO 8573-1 water class uses pressure dew point. For dryer rated SCFM, open the air dryer sizing calculator. Browse all pneumatic air treatment tools.

Results

Converted Dew Point

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ISO 8573 Water Class (from PDP)

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Absolute Pressure Used

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Continue Your Calculation

After converting PDP and ADP, size the dryer for your target pressure dew point.

Size dryer →

Quick Reference: PDP vs ISO Water Class #

ISO 8573-1 water	Max PDP	Typical dryer
Class 4	≤ +3°C	Refrigerated
Class 3	≤ −20°C	Desiccant
Class 2	≤ −40°C	Desiccant
Class 1	≤ −70°C	Desiccant (deep dry)

Formulas and Method #

Magnus saturation vapor pressure

e_s(T) = 6.112 × exp(17.62·T / (243.12 + T))   [hPa, T in °C]

Pressure scaling (engineering approximation)

e_atm = e_p × (P_atm / P_abs)  ;  P_abs = P_gauge + P_atm

Invert Magnus to recover the other dew point. Accuracy depends on ideal-gas assumptions and how closely mole fraction is conserved.

Assumptions and Limitations

Planning estimate only. Gauge vs absolute pressure must be consistent. Not for instrument calibration, and not for HVAC psychrometric processes (use the psychrometric calculator for room air).

Worked example #

PDP +3°C @ 7 bar(g) (P_abs ≈ 8.013 bar, sea level) → ADP ≈ −23°C. Reverse the same inputs to close the loop near +3°C PDP. Then size a refrigerated dryer for Class 4 water with the dryer sizing calculator.

Frequently Asked Questions #

How do I convert pressure dew point to atmospheric dew point?

Compute saturation vapor pressure at the PDP with Magnus, scale by P_atm / P_abs, then invert for ADP. At +3°C PDP and 7 bar(g), ADP is about −23°C with this approximation.

What is the difference between PDP and ADP for compressed air?

PDP is condensation temperature under line pressure (what dryer and ISO specs use). ADP is the dew point of the same moisture content expanded to atmosphere—useful for comparing sensors or charts, not for replacing PDP in dryer specs.

Is this the same as an HVAC dew point calculator?

No. This page is for compressed-air pressure dew point at working pressure. HVAC psychrometrics solve moist air at near-atmospheric conditions for rooms and ducts.