Formula
Pw_line × P_atm = Pw_atm × P_line (partial pressure of water scales with total pressure)Dryer ratings list pressure dewpoint (PDP) at line pressure. Instrument and outdoor air specs are often quoted as atmospheric dewpoint (ADP) at 0 psig. ADP is always colder than PDP at the same line.
Atmospheric dewpoint (at 0 psig)
-4.6°F
-20.3 °C
Reference points
- Refrigerated dryer at 100 psig, 38°F PDP ≈ 1°F ADP.
- Desiccant dryer at 100 psig, –40°F PDP ≈ –68°F ADP.
- Desiccant –100°F PDP ≈ –123°F ADP — meets ISO 8573 Class 2 humidity.
How to use this calculator
Convert pressure dew point to atmospheric dew point and check dryer performance.
- Enter the measured dew point and note whether it is at line pressure or atmospheric.
- Enter the line pressure.
- Read the converted dew point at the other reference condition.
- Compare against the dryer's rated dew point and the coldest ambient the piping sees.
Frequently asked questions
- What is pressure dew point?
- It is the temperature at which moisture condenses out of the air while it is still at line pressure. It is always higher than the equivalent atmospheric dew point.
- What dew point should a refrigerated dryer produce?
- A refrigerated dryer typically holds a 35–40 °F pressure dew point; desiccant dryers reach −40 °F or lower for outdoor or instrument air.
- Why do I still get water after the dryer?
- Common causes are a dew point above the coldest pipe temperature, an overloaded or high-inlet-temperature dryer, a failed drain, or condensation in uninsulated outdoor runs.