Pressure Dew Point Calculator & ISO 8573-1 Class Checker
Convert between pressure dew point and atmospheric dew point, get ppmV, g/Nm³ and %RH, and check instantly which ISO 8573-1 humidity class your compressed air meets.
*Water carried past the dryer in 1,000 Nm³/hr of air, litres per 24 h.
Is your dryer actually holding this?
A dryer’s rating is not a measurement. WiseAir WADS dew point sensors monitor pressure dew point continuously down to −80 °C, so you find a failing desiccant bed before your product does.
See WADS Dew Point Sensors Talk to an Air Quality EngineerWhy pressure dew point and atmospheric dew point are not the same number
Dew point is the temperature at which water vapour starts to condense. Compress air and you squeeze the same water molecules into a smaller volume — the vapour partial pressure rises, and so does the temperature at which it condenses. Pressure dew point is always higher (wetter) than the atmospheric dew point of the same air.
At 7 bar(g) the gap is roughly 17–25 °C, widening as the air gets wetter. Air at −40 °C pressure dew point has an atmospheric dew point of about −57 °C; air at +3 °C PDP corresponds to about −21 °C at atmosphere. This is why a sensor reading is meaningless unless you state the pressure it was measured at, and why moving a sensor from downstream of a pressure regulator to upstream of it changes the number completely.
The practical consequence: if your specification says “−40 °C dew point” without saying whether it is at pressure or at atmosphere, you and your supplier may be 20 °C apart on what you actually agreed. In compressed air, PDP is almost always what is meant — but say it explicitly.
ISO 8573-1:2010 humidity and liquid water classes
| Class | Pressure dew point | Typically used for |
|---|---|---|
| Class 0 | Better than Class 1, as agreed between user and supplier | Specified critical processes |
| Class 1 | ≤ −70 °C | Ultra-dry process air, some semiconductor use |
| Class 2 | ≤ −40 °C | Pharma, food contact, instrument air, electronics, PET |
| Class 3 | ≤ −20 °C | Outdoor lines, cold stores, freeze-risk piping |
| Class 4 | ≤ +3 °C | General plant air, tools, paint spray, textile looms |
| Class 5 | ≤ +7 °C | Non-critical general purpose |
| Class 6 | ≤ +10 °C | Non-critical, warm indoor use only |
| Classes 7–9 | Liquid water: ≤0.5, 0.5–5, 5–10 g/m³ | Where a dryer is not used at all |
An air quality specification is written as three numbers — for example ISO 8573-1:2010 [1:2:1] meaning particle Class 1, humidity Class 2, oil Class 1. This calculator addresses the middle number only. The application guidance above reflects common industry practice; always confirm against your own customer specification, GMP documentation or standard.
The formulas we use
Saturation vapour pressure uses the Magnus–Sonntag formulation, with separate coefficients over water and over ice — which matters, because every compressed air dew point below 0 °C is a frost point over ice, and using the water equation there introduces several degrees of error.
Ideal gas behaviour and an enhancement factor of 1.0 are assumed. Across normal compressed air pressures this introduces well under 1 °C of dew point error — smaller than the accuracy of any field sensor. Cross-checked against published values: at 1 atm, a −40 °C dew point gives 127 ppmV and a −70 °C dew point gives 2.6 ppmV.
Frequently asked questions
What dew point do I need for pharmaceutical compressed air?
Where compressed air contacts product or product-contact surfaces, ISO 8573-1 humidity Class 2 (−40 °C PDP) is the usual specification, generally alongside particle Class 1 and oil Class 1. GMP does not name a class itself — it requires you to justify and monitor your own specification, which is precisely why continuous dew point monitoring rather than annual sampling is the defensible position in an audit.
My refrigerant dryer is rated +3 °C. Why does my sensor read +12 °C?
Almost always one of: ambient temperature above the dryer’s design condition, air inlet temperature above rating, flow above the dryer’s capacity, a fouled condenser, or a failed drain leaving condensate in the heat exchanger. A refrigerant dryer’s rating is a design-point figure and real conditions rarely match it — which is the case for measuring rather than assuming.
Can a refrigerant dryer reach −40 °C?
No. Refrigerant dryers are physically limited to around +3 °C because water freezes on the evaporator below that. Reaching −40 °C or −70 °C requires a desiccant (adsorption) dryer. If your specification says −40 °C and you have a refrigerant dryer, the specification cannot be met by that equipment.
Where should I install the dew point sensor?
Downstream of the dryer and its after-filter, at full line pressure, in a properly designed sampling point with continuous flow past the sensor. Dead-legs are the most common cause of readings that respond in hours rather than seconds. For desiccant dryers, install downstream of the outlet so you catch a bed breakthrough at the moment it happens.
How often should dew point sensors be calibrated?
Annually is the common interval, and it is what most auditors expect to see documented. Sensors used continuously in the −40 °C and drier range drift more than those sitting near ambient, so verify rather than assume.
Dew points below 0 °C are frost points over ice. Results are calculated for planning and comparison purposes; for GMP, contractual or regulatory documentation, use a calibrated instrument with a traceable certificate. Related tools: Leak Cost Calculator · Flow Unit Converter.
