Humidity Calculator & Dew Point Calculator

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. Accurate pressure measurement is also important when interpreting dew point readings; see our pressure sensor solutions.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.

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.

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.For a broader assessment of compressed-air quality and system performance, see our compressed air audit services.

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 continuous pressure dew point monitoring, you can also explore the WADS 205 dew point sensor.

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. For applications requiring continuous measurement and monitoring, the WADS 207/208 dew point meter provides a practical solution.