# Pressure Dew Point (PDP)

> Source: https://rodlesspneumatic.com/resource-center/pneumatic-glossary/pressure-dew-point/
> Language: en-US
> Category: Air preparation

## Definition

Pressure dew point is the temperature at which water vapour in compressed air begins to condense at the stated operating pressure. A lower pressure dew point indicates drier air. The dryer target should stay below the lowest temperature encountered by the pressurized air, with a practical margin for changing load and ambient conditions.

## Key facts

- **Meaning:** Condensation temperature at stated pressure
- **Lower value:** Drier compressed air
- **Selection rule:** Target below the coldest pressurized-air temperature

## Pressure dew point is not ambient relative humidity

Compression changes the partial pressure of water vapour, and cooling compressed air can create liquid condensate even when the intake air did not appear unusually humid. A dew-point value is incomplete unless its pressure basis and measurement location are stated.

## Match the dryer to the coldest part of the system

Indoor plant air may tolerate a refrigerated dryer target, while outdoor winter pipework, instrument air, painting, electronics, or sensitive processes can require a much lower pressure dew point. Measure or specify at a point representative of the delivered air after treatment and distribution losses.

## Dryness is only one part of air quality

ISO 8573-1 purity notation separately addresses particles, water, and oil. A low pressure dew point does not prove low particle or oil content, and a fine filter does not dry the air. State all required purity positions and the measurement point.

## What to verify before selection

- Lowest ambient and pipe temperature
- Operating pressure at the stated dew point
- Peak flow and dryer inlet condition
- Required ISO 8573-1 water class
- Measurement point and sensor method
- Pressure loss, purge demand, and maintenance

## Common mistakes

- Comparing pressure dew point with atmospheric dew point without conversion
- Specifying dryer type without peak inlet conditions
- Treating a water-class result as a complete air-purity specification

## Frequently asked questions

### Is a lower pressure dew point always better?

It means drier air, but achieving it can cost more energy and maintenance. Specify the level the process and coldest system temperature actually require.

### Where should pressure dew point be measured?

At a stated pressure and a representative point for the delivered air. The dryer outlet and the point of use can show different conditions.

## Technical references

- [ISO 8573-1:2010 — Compressed-air purity classes](https://www.iso.org/standard/46418.html): Defines purity classes for particles, water, and oil.
- [ISO 8573-3:1999 — Humidity measurement](https://www.iso.org/standard/25282.html): Guidance on humidity measurement methods, sampling, evaluation, uncertainty, and reporting.