Pressure dew point conversion calculator

What the pressure dew point conversion calculator calculates

The pressure dew point conversion calculator converts a dew point at line pressure into the dew point the same air has after expanding to atmospheric pressure, or the reverse. From the dew point, line pressure (bar g), atmospheric pressure (bar a) and pipe air temperature, it returns the converted dew point, water content, relative humidity in the pipe and ISO 8573-1 humidity class.

Plant engineers use it to compare dryer offers quoted on different bases, to check a hygrometer reading taken after a sampling valve, and to decide between a refrigerated and a desiccant dryer. Quality teams use it when writing air specifications for paint shops, pharmaceutical packing, food lines and instrument air.

What pressure dew point and atmospheric dew point mean

Pressure dew point is the temperature at which water vapour in compressed air starts to condense while the air is still at line pressure. Atmospheric dew point is the temperature at which the same air, expanded to atmospheric pressure, would start to condense. The water content is identical; only the stated pressure differs.

Compression raises the partial pressure of the water vapour in proportion to absolute pressure, so the air saturates at a higher temperature. At any line pressure above atmospheric, the pressure dew point is therefore warmer than the atmospheric dew point, and the gap widens with line pressure: a +3 °C PDP equals −20.8 °C ADP at 7 bar g and −24.0 °C ADP at 10 bar g.

Pressure dew point is the figure that matters for the system, because pipes, valves and instruments sit at line pressure. If any pipe is colder than the pressure dew point, liquid water forms in it. Atmospheric dew point matters where air is released and when a hygrometer measures expanded air.

Frost point, ISO 8573-1 classes and the dew point you need

A frost point is the temperature at which water vapour deposits as ice rather than condensing as liquid water, and below 0 °C the calculator reads the dew point as a frost point by default. Many dew point instruments report frost point below freezing, and the basis matters: a −40 °C pressure dew point at 7 bar g converts to −57.0 °C ADP as a frost point, but −58.1 °C if both values are read over water.

ISO 8573-1 grades compressed air humidity by pressure dew point, not atmospheric dew point.

The dryer column is a general guide, not a guarantee: a refrigerated dryer reaches class 4 only at its rated inlet temperature, flow and pressure. The dew point you need is set by the process and by the coldest pipe. Refrigerated dryers deliver a pressure dew point a few degrees above freezing; desiccant dryers are used when lines see freezing temperatures or the process needs class 1, 2 or 3.

Method and formulas

The calculator uses the Magnus saturation vapour pressure equation and the fact that the water vapour fraction of air does not change on expansion.

• Saturation vapour pressure over water, in hPa: es = 6.1094 × exp(17.625 × t ÷ (t + 243.04)), where t is the temperature in °C.
• Saturation vapour pressure over ice (frost point), in hPa: es = 6.1121 × exp(22.587 × t ÷ (t + 273.86)).
• Absolute pressures: Pline = line pressure in bar g + Patm, where Patm is the atmospheric pressure in bar a.
• Conversion: es(ADP) = es(PDP) × Patm ÷ Pline. For ADP to PDP, es(PDP) = es(ADP) × Pline ÷ Patm.
• Inversion to a temperature: over water t = 243.04 × L ÷ (17.625 − L) with L = ln(e ÷ 6.1094); over ice t = 273.86 × L ÷ (22.587 − L) with L = ln(e ÷ 6.1121).
• Water vapour mixing ratio: x = 0.622 × e ÷ (P − e), in kg of water per kg of dry air, where e is the vapour pressure at line pressure and P is Pline, both in hPa (1 bar = 1,000 hPa).
• Water content at normal conditions: x × 1.2922 kg/Nm³, multiplied by 1,000 for g/Nm³.
• Relative humidity in the pipe: RH = e ÷ es(pipe air temperature), using the vapour pressure at line pressure.
• Warning: if the pressure dew point is equal to or higher than the pipe air temperature, liquid water condenses in the pipe.

The enhancement factor, a small non-ideal correction, is ignored; accuracy is about ±1 °C, ample for dryer selection.

Pressure dew point to atmospheric dew point at 7 and 10 bar g

At 7 bar g the six ISO 8573-1 class limits, +10 °C to −70 °C pressure dew point, equal atmospheric dew points of −15.7 °C to −83.0 °C; at 10 bar g they equal −19.1 °C to −84.9 °C. Frost points are used below 0 °C and atmospheric pressure is 1.013 bar a.

The same pressure dew point at 10 bar g instead of 7 bar g means drier air, so the atmospheric dew point is 1.9 °C to 3.4 °C lower across this range.

Worked example 1: refrigerated dryer at +3 °C pressure dew point and 7 bar g

This is the calculator’s default case: a refrigerated dryer gives +3 °C pressure dew point at 7 bar g, atmospheric pressure is 1.013 bar a, frost point is used below 0 °C and the pipe air is at 30 °C.

1. Saturation vapour pressure at +3 °C over water: es = 6.1094 × exp(17.625 × 3 ÷ 246.04) = 7.574 hPa.
2. Absolute line pressure: Pline = 7 + 1.013 = 8.013 bar a.
3. Vapour pressure after expansion: 7.574 × 1.013 ÷ 8.013 = 0.9575 hPa.
4. Inverting the ice formula: L = ln(0.9575 ÷ 6.1121) = −1.8537, so t = 273.86 × (−1.8537) ÷ (22.587 + 1.8537) = −20.8 °C, the atmospheric frost point.
5. Water vapour mixing ratio, with 8.013 bar a = 8,013 hPa: x = 0.622 × 7.574 ÷ (8,013 − 7.574) = 0.000588 kg/kg, which is 0.588 g/kg of dry air.
6. Water content at normal conditions: 0.588 × 1.2922 = 0.76 g/Nm³.
7. Saturation vapour pressure at the 30 °C pipe temperature: 6.1094 × exp(17.625 × 30 ÷ 273.04) = 42.37 hPa, so RH in the pipe = 7.574 ÷ 42.37 = 17.9%.
8. A pressure dew point of +3 °C meets the class 4 limit of ≤ +3 °C, so the air is ISO 8573-1 class 4.

The air is comfortably dry in a 30 °C pipe. In a pipe at 2 °C, such as a line in an air-conditioned room or outdoors on a winter night, saturation vapour pressure is only 7.05 hPa, below the 7.574 hPa in the air: relative humidity reaches 100%, liquid water condenses and the calculator shows a warning.

Worked example 2: desiccant dryer at −40 °C pressure dew point and 7 bar g

A desiccant dryer delivers a −40 °C pressure dew point at 7 bar g with atmospheric pressure at 1.013 bar a, read as a frost point.

1. Saturation vapour pressure over ice at −40 °C: es = 6.1121 × exp(22.587 × (−40) ÷ 233.86) = 0.1283 hPa.
2. Absolute line pressure: 7 + 1.013 = 8.013 bar a.
3. Vapour pressure after expansion: 0.1283 × 1.013 ÷ 8.013 = 0.01622 hPa.
4. Inverting the ice formula: L = ln(0.01622 ÷ 6.1121) = −5.9318, so t = 273.86 × (−5.9318) ÷ (22.587 + 5.9318) = −57.0 °C, the atmospheric frost point.
5. Water vapour mixing ratio: 0.622 × 0.1283 ÷ (8,013 − 0.1283) = 0.00000996 kg/kg, about 0.010 g/kg of dry air against 0.588 g/kg in example 1.
6. A −40 °C pressure dew point meets the class 2 limit of ≤ −40 °C, so the air is ISO 8573-1 class 2.
7. With both dew points read over water instead of ice, the same calculation gives an ADP of −58.1 °C.
8. Reverse check: a hygrometer showing −40 °C at atmospheric pressure means a vapour pressure of 0.1283 hPa; multiplied by 8.013 ÷ 1.013 this becomes 1.015 hPa at line pressure, a pressure dew point of only −20.2 °C at 7 bar g.

The reverse check is the key lesson: a “−40 °C dew point” quoted without its pressure may mean only −20.2 °C at line pressure, class 3 rather than class 2.

Indian site conditions that change the answer

Site conditions in Hyderabad, Telangana and Andhra Pradesh change the dryer load and the coldest pipe temperature more than the conversion itself, which depends only on dew point, line pressure and atmospheric pressure.

• Altitude: Hyderabad lies about 500–550 m above sea level, where atmospheric pressure is about 0.95 bar a. At 7 bar g the line is then 7.95 bar a, and a +3 °C PDP gives 7.574 × 0.95 ÷ 7.95 = 0.9051 hPa, a frost point of about −21.4 °C instead of −20.8 °C at 1.013 bar a.
• Summer heat: intake air at 40–45 °C can hold more water vapour and leaves the aftercooler hotter, so a refrigerated dryer sized for milder conditions may deliver a higher pressure dew point.
• Monsoon humidity: near-saturated intake air raises the load on dryers and drains; check drains before the season.
• Hot compressor rooms: poor ventilation raises dryer inlet and condenser temperatures, pushing the dew point up.
• Air-conditioned rooms: lines through cooled spaces can sit below a refrigerated dryer’s pressure dew point, as in example 1.
• Power cuts: if the compressor restarts before the dryer, wet air enters the network; interlock the two.
• Dusty sites: dust clogs air-cooled dryer condensers, raising the dew point, and blocks pre-filters, raising pressure drop.

How to interpret the result, and its limits

The result is an engineering conversion for selecting and comparing dryers, not a calibration method. Its accuracy of about ±1 °C is smaller than the 3 °C to 30 °C spacing between ISO 8573-1 class limits but cannot settle a dispute over an instrument reading.

• It assumes the vapour fraction stays constant; if condensate forms or drains between two points, the conversion no longer applies.
• A hygrometer that samples air after a pressure-reducing valve sees expanded air; state whether the reading is at line pressure or atmospheric pressure before converting.
• Pipe relative humidity is only as good as the pipe temperature entered; use the coldest point in the system.
• A dryer rated exactly at a class limit, such as +3 °C for class 4, has no margin; allow for the ±1 °C accuracy and for real operating conditions.
• The ISO 8573-1 class shown covers humidity only. Particle and oil classes need separate filtration and testing.

How to check a dew point specification step by step

1. Write down the dew point figure and confirm whether it is a pressure dew point or an atmospheric dew point.
2. Record the line pressure in bar g at which it applies and the local atmospheric pressure in bar a.
3. Confirm whether a value below 0 °C is a frost point or a dew point over water.
4. Convert it to pressure dew point at your own working pressure with the calculator.
5. Check that the pressure dew point is below the coldest temperature any pipe, hose or instrument will see.
6. Check the ISO 8573-1 class against what the process needs, and choose a refrigerated or desiccant dryer accordingly.
7. After installation, measure the dew point at line pressure, or state the pressure with every reading.

How Reddy Compressors helps

Reddy Compressors is an independent multi-brand compressor specialist operating since 2002 from its office at Tarnaka, Secunderabad, supplying new and used or refurbished compressors, spare parts, service and rentals. Our service covers all major brands across Telangana and Andhra Pradesh, and we supply equipment across India. Rental machines are available from 50 to 2,000 cfm at 2 to 30 bar, with special high-pressure DTH and borewell units up to 35 bar. Send us your dew point requirement and working pressure, and we will help match the air treatment to your process.

What to send us

Send us these details so we can recommend the right air treatment.

• The required dew point, and whether it is a pressure or atmospheric dew point.
• Working pressure in bar g and the compressor’s free air delivery in m³/min or cfm.
• The coldest temperature any pipe or instrument line will see.
• The process or application and any ISO 8573-1 class it must meet.
• Details of the existing aftercooler, dryer, filters and drains.
• Recent dew point readings, with the pressure at which each was taken.

Frequently asked questions

How do I convert pressure dew point to atmospheric dew point?

Convert the pressure dew point to a saturation vapour pressure, multiply it by atmospheric pressure divided by absolute line pressure, then convert that vapour pressure back to a temperature. The vapour fraction is unchanged by expansion. For example, a +3 °C pressure dew point at 7 bar g gives 7.574 × 1.013 ÷ 8.013 = 0.9575 hPa, which is an atmospheric frost point of −20.8 °C.

What is the difference between PDP and ADP?

Pressure dew point (PDP) is the condensation temperature of compressed air at line pressure, while atmospheric dew point (ADP) is the condensation temperature of the same air after it expands to atmospheric pressure. Both describe the same water content, but compression raises the vapour pressure, so above atmospheric pressure PDP is always the warmer figure. At 7 bar g a −40 °C PDP equals a −57.0 °C ADP. PDP decides whether water condenses in your pipes.

What is the atmospheric dew point of a refrigerated dryer at 7 bar g?

A refrigerated dryer delivering a +3 °C pressure dew point at 7 bar g produces air with an atmospheric dew point of about −20.8 °C, with atmospheric pressure at 1.013 bar a. At 10 bar g it corresponds to −24.0 °C. The air holds about 0.588 g of water per kg of dry air, or 0.76 g/Nm³, and meets ISO 8573-1 humidity class 4.

What dew point do I need for compressed air?

You need a pressure dew point below the coldest temperature that any pipe, hose or instrument in the system will reach. Warm indoor lines are normally served by a refrigerated dryer giving a pressure dew point a few degrees above freezing, ISO 8573-1 class 4 to 6. Lines exposed to freezing temperatures, or processes needing class 1 to 3, require a desiccant dryer, with class 2 corresponding to a pressure dew point of −40 °C or lower.

Why is a dew point meaningless without its pressure?

A dew point quoted without its pressure is meaningless because the same air has a different dew point at every pressure. Compression raises the vapour pressure, so the dew point rises with line pressure. An atmospheric dew point of −40 °C equals only −20.2 °C pressure dew point at 7 bar g, which is class 3, while a −40 °C pressure dew point at 7 bar g is class 2. Always state the pressure with the reading.

Should a dew point below 0 °C be read as frost point or dew point?

Read it as a frost point unless your instrument states otherwise, because many dew point instruments report frost point below 0 °C and the calculator uses frost point by default. Saturation vapour pressure over ice is lower than over water. The choice shifts the answer by about a degree: a −40 °C pressure dew point at 7 bar g converts to −57.0 °C as a frost point but −58.1 °C over water.

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