Why two dew points exist
Dew point is the temperature at which water vapour starts to condense, and it depends on the partial pressure of that vapour. Compress air and you pack the same water molecules into less volume, raising the partial pressure and therefore the dew point.
So the same air has two dew points depending on where you measure it. Pressure dew point is at line pressure — the condition the air is actually in, and the number that determines whether water forms in your instrument tubing. Atmospheric dew point is that air expanded to 1 atmosphere, which always gives a much lower and much more flattering figure.
The gap is large. A pressure dew point of +3 °C at 7 barg corresponds to an atmospheric dew point of about −23 °C. Quote the second number and the air sounds bone dry; quote the first and you know it will condense on any surface below 3 °C.
Worked example
A dryer outlet reads +3 °C pressure dew point on a 7 barg header:
- Saturation vapour pressure at 3 °C = 7.58 hPa
- Scale to atmospheric: 7.58 × 1.013 ÷ 8.013 = 0.958 hPa
- Inverting the Magnus relation gives −23.1 °C atmospheric dew point
- That is ISO 8573-1 class 4 — adequate for general plant air, not for instrument air
ISO 8573-1 humidity classes
| Class | Pressure dew point | Typical use |
|---|---|---|
| 1 | −70 °C | pharmaceutical, electronics |
| 2 | −40 °C | instrument air standard |
| 3 | −20 °C | general plant, indoor |
| 4 | +3 °C | shop air, heated areas |
| 5 | +7 °C | low-duty utility air |
| 6 | +10 °C | non-critical |
Why instrument air is specified at −40 °C
The governing rule is simple: the pressure dew point must sit at least 10 °C below the coldest temperature the pipework will ever reach. Air lines run outdoors, through unheated racks and across the plant on winter nights. Once condensate forms it collects in low points, and if the temperature keeps falling it freezes — blocking the line to a positioner or filling a valve actuator with ice.
Class 2 at −40 °C gives comfortable margin almost anywhere, which is why it became the default specification rather than because instruments themselves demand that dryness. Water in instrument air also washes lubricant out of positioners, corrodes actuator internals, and carries rust particles into I/P converters, whose orifices are small enough to block.
Field notes
- Measure at the point of use, not just at the dryer. A dryer performing perfectly cannot help if a long run of uninsulated pipe picks up moisture from a failed after-cooler downstream.
- Refrigerant dryers cannot reach class 2. They bottom out around +3 °C pressure dew point because the coil would ice up. Desiccant dryers are needed for −40 °C.
- Below 0 °C it is frost point, strictly. Water sublimes rather than condenses, and vapour pressure over ice differs slightly from over supercooled water. This calculator uses the over-ice relation below 0 °C.
- Pressure changes the answer, so state it. A dew point figure without its pressure is meaningless — the same air is class 4 at 7 barg and much drier at 2 barg.
Frequently asked questions
What is the difference between pressure dew point and atmospheric dew point?
Pressure dew point is measured at line pressure and is the figure that matters for a compressed air system, because that is the condition the air is actually in. Atmospheric dew point is the same air expanded to 1 atmosphere, which is always a much lower number. A +3 °C pressure dew point at 7 barg equals about −23 °C atmospheric.
What dew point does instrument air need?
ISO 8573-1 class 2, a pressure dew point of −40 °C, is the usual instrument air specification. The governing rule is that the dew point must sit at least 10 °C below the lowest ambient temperature the pipework will ever see, so outdoor runs in cold climates drive the requirement.
Why does compressing air raise the dew point?
Compression packs the same water molecules into a smaller volume, raising the partial pressure of water vapour. Since dew point tracks that partial pressure, it climbs — which is why air that seemed dry at atmospheric conditions condenses freely once compressed.
What are the ISO 8573-1 humidity classes?
Class 1 is −70 °C, class 2 is −40 °C, class 3 is −20 °C, class 4 is +3 °C, class 5 is +7 °C and class 6 is +10 °C, all expressed as pressure dew point. Lower class numbers mean drier air.
Provided for reference and education, using the Magnus relation for saturation vapour pressure. See our disclaimer.