Compressed Air Dew Point Calculator Australia

Author

Byron Raal, CAS Founder-Editor About the author

Date last checked

13 September 2026

Convert a pressure dew or frost point to its atmospheric equivalent, compare the entered temperature with ISO 8573-1 water-class thresholds, and estimate moisture concentration. State both the pressure and the water or ice reference. This calculator provides an indicative conversion; the required air quality and dryer selection need a separate process assessment.

1How do I convert pressure dew point to atmospheric dew point?

Dew point describes saturation over liquid water; frost point describes saturation over ice. Pressure dew point (PDP) is quoted at line pressure, while an atmospheric value uses a stated atmospheric pressure. This calculator uses 1.01325 bar absolute and switches to the ice reference below freezing. A sub-zero liquid-water dew-point value is different from a frost-point value. See Vaisala’s explanation of dew and frost point.

2What This Calculator Does

A moisture figure needs its reference conditions. Enter the pressure dew or frost point and line pressure to estimate the atmospheric dew or frost point and water-vapour concentration. The density output is referenced to the entered dew or frost temperature, not to the actual operating temperature of the pipe. The tool does not calculate condensate drainage, total dryer moisture load or the temperature of a discharge jet.

Inputs are a pressure dew or frost point from −80 to +20 °C and pressure from 0 to 16 bar gauge. Outputs use a reference atmosphere of 101.325 kPa absolute. Below freezing, enter a value referenced to ice. The ISO 8573-1:2010 catalogue describes the particles, water and oil classification framework. The initial +3 °C and 7 bar g are example values, not a recommendation for your application.

3ISO 8573-1:2010 Water Classes (Classes 1 to 6)

ISO 8573-1 provides a common way to specify compressed-air purity, with separate particle, water and oil classes. For example, the water component of a 1:2:1 specification is Class 2. This tool compares only the entered temperature with the water pressure-dew-point thresholds. It does not assess particles, oil, microorganisms or suitability for medical, food or pharmaceutical use.

The pressure-dew-point thresholds for water Classes 1–6 are listed below. Water Classes 7–9 concern liquid-water concentration, which cannot be determined from a dew-point temperature alone. The tool reports a threshold band, not a measured purity certificate.

• Class 1: PDP ≤ -70 °C
• Class 2: PDP ≤ -40 °C
• Class 3: PDP ≤ -20 °C
• Class 4: PDP ≤ +3 °C
• Class 5: PDP ≤ +7 °C
• Class 6: PDP ≤ +10 °C

For a temperature-only comparison, −45 °C falls in the Class 2 band and −38 °C in the Class 3 band. Actual classification requires the appropriate measurement method, pressure and operating conditions. A single entered number cannot establish conformity. Particle and oil requirements are separate; see the compressed air filtration guide.

4Pressure Dew Point Versus Atmospheric Dew Point

Reducing pressure lowers water-vapour partial pressure when the vapour mole fraction stays unchanged. Its corresponding saturation temperature also falls. This ideal-gas calculation assumes no condensation, water addition or leakage during the pressure change. It does not model a real nozzle’s temperature history. At zero gauge pressure the input and atmospheric result coincide because the pressure references are equal.

For the default +3 °C at 7 bar gauge, the absolute line pressure is 8.01325 bar. This model gives an atmospheric frost point of approximately −20.8 °C over ice. A calculation extended over supercooled liquid water gives a different below-zero dew point, approximately −23 °C, as shown in the illustration above. Those are different phase references, not interchangeable measurements.

Worked example: +3 °C at 7 bar gauge. The water saturation pressure is approximately 758.03 Pa. Multiply by 101325 / 801325 to obtain approximately 95.85 Pa at the reference atmosphere. Inverting the ice saturation equation gives approximately −20.77 °C frost point. Water-vapour mole fraction is approximately 946 ppmv on a wet basis. At the entered +3 °C reference temperature, water-vapour density is approximately 5.948 g/m³; it will differ at the actual line temperature.

The saturation equations are the ASHRAE water and ice equations documented in PsychroLib. The calculator uses the triple-point transition at 0.01 °C and numerically inverts the relevant equation. It does not include high-pressure enhancement factors or claim an accuracy suitable for certification. Confirm the instrument’s phase reference before entering a below-zero reading.

5Dew Point Calculator

6How This Calculator Works

The calculation separates saturation pressure, pressure conversion and temperature inversion. It follows the SI saturation equations documented by PsychroLib, with the open-source attribution retained in the calculator code. The pressure conversion is an ideal-gas approximation, not a dryer performance simulation.

Step 1: Saturation pressure. Convert the entered temperature to kelvin and evaluate the ASHRAE saturation-pressure equation over ice at or below 0.01 °C, or liquid water above it. The result is in pascals. The input range is narrower than the source equation range to keep the pressure-converted result within the supported inversion range.

Step 2: Pressure conversion. Calculate absolute line pressure as 101325 + 100000 × gauge pressure in bar. Atmospheric water-vapour partial pressure equals the input saturation pressure multiplied by 101325 / absolute line pressure. This assumes unchanged water-vapour mole fraction and excludes high-pressure enhancement effects.

Step 3: Atmospheric dew or frost point. Find the temperature whose saturation pressure equals the converted partial pressure. A result at or below 0.01 °C is identified as frost point over ice. A liquid-water dew-point calculation below freezing would return a different temperature.

Step 4: Moisture concentration. Wet-basis ppmv equals saturation pressure / absolute line pressure × 1,000,000. Reference vapour density equals saturation pressure / (461.5 × temperature in kelvin), converted from kg/m³ to g/m³. That density is at the entered dew or frost temperature; actual operating temperature is not an input, so actual line-temperature density is not calculated.

Step 5: Threshold comparison. Compare the entered temperature with −70, −40, −20, +3, +7 and +10 °C for the Class 1–6 PDP bands. This comparison does not determine the process requirement or prove measured conformity. Above +10 °C, the tool reports that the temperature is outside those bands; it cannot infer a liquid-water class or the presence of liquid water.

7Refrigerated Dryers and Lower Dew-Point Requirements

Refrigerated dryers cool compressed air so that moisture condenses and can be separated and drained. Conventional units commonly quote a positive pressure dew point near +3 °C under declared rating conditions. ISO 7183 addresses dryer performance data and test methods, including dew point, flow, pressure loss, air loss and power.

A +3 °C rating is not a guarantee at every inlet temperature, pressure or flow. Check the actual model’s rated conditions and correction factors. Ice formation is a design constraint for conventional refrigerated drying; do not infer a universal evaporator-temperature limit or a particular purity result from the technology name alone.

Desiccant and membrane dryers can provide lower dew or frost points, depending on the model, flow and conditions. Heatless desiccant is not limited to Class 2, and membrane technology is not universally limited to −20 °C: CAGI’s treatment handbook discusses desiccant performance, while Parker’s membrane literature gives product-specific examples. Compare rated air loss and energy demand. Further reading: the US Department of Energy sourcebook and our dryers and air quality guide.

8Practical Guidance for Class Selection

Begin with the process requirement, then check equipment performance at the intended operating conditions. The calculator cannot decide which water class your process requires. Use numeric examples only to explore the conversion, and prepare the following information for a supplier review.

• Required quality. Identify the point of use, contaminants and process requirement. The water target is only one part of the air-quality specification.
• Operating envelope. State flow and its reference basis, pressure range, dryer inlet temperature and cooling conditions. Include part-load behaviour and the lowest downstream temperature.
• Application review. Pharmaceutical and electronics processes need their own quality assessment. A sector label does not select a universal class or regeneration technology.
• Model evidence. Ask for rated dew point, flow, pressure loss, purge or air loss, energy and maintenance information at the intended conditions. CAGI resources provide background; the actual supplier rating and local application still need review.

9Australian Climate and Inlet Conditions

The compressor intake’s moisture content, pressure and temperature affect the water entering the system. The aftercooler, separator and drains then affect the conditions reaching the dryer. Plant-room temperature can differ from outdoor weather. Record the actual dryer inlet conditions and cooling environment; see compressed air in Australian heat.

Use a documented site design envelope and the manufacturer’s correction factors, including credible combinations of peak flow, inlet temperature, ambient or cooling-water temperature and minimum pressure. Annual averages alone may hide demanding conditions. There is no universal January, February or 99th-percentile rule for every site and process.

Local atmospheric pressure and elevation affect pressure references and intake calculations. This tool fixes atmosphere at 101.325 kPa; it does not adjust for site elevation or weather. Where that difference matters, use site conditions in a more complete calculation and have the supplier confirm the rating basis.

10Common Errors in Dew Point Specification

Check these three points before comparing a quotation or measurement with a calculator output.

• Missing references. Record whether the figure is at line or atmospheric pressure, the absolute pressure basis, and whether a sub-zero value is over water or ice.
• Assuming a class proves suitability. A purity classification does not by itself establish the process requirement or compliance for a particular application. Confirm the applicable specification and assessment method separately.
• Using the wrong measurement setup. Check instrument range, calibration, pressure rating, sample arrangement and stabilisation. See Vaisala’s measurement guidance.

11What Happens When Dew Point Is Under-Specified

Corrosion and drainage. Liquid water can contribute to internal corrosion and maintenance problems. Check low points, separators, drains and the suitability of the distribution system. Pipe material alone does not control moisture; aluminium piping still needs an appropriate system design and operating plan.

Product contact. Moisture can be one part of a wider contamination assessment. Define and validate the actual process requirements rather than adopting a calculator preset. Relevant reading includes our pharmaceutical compressed-air guide and food-contact compressed-air guide. This page does not assign a universal moisture limit or financial loss to either sector.

Instruments and controls. Check the air-quality limits specified for the actual instruments and valves. If performance changes, investigate the supply, treatment, distribution and measurement conditions; moisture is one possible cause, not a diagnosis from this calculator.

Cold surfaces and freezing. Water may condense when local conditions reach saturation, and liquid water can freeze on sufficiently cold surfaces. Establish the lowest credible downstream temperature and a suitable margin for the process. A universal Class 3 minimum cannot be selected from the site name or an outdoor installation alone.

Equipment condition. Follow the equipment’s stated air-quality and maintenance requirements. There is no universal seal-life multiplier for wet air. Moisture control, pressure equipment inspection and other maintenance tasks are separate parts of the site plan.

12How to Use This Calculator

Enter a temperature and pressure or choose a numeric example. Confirm the water or ice reference before entering a sub-zero value. Read the atmospheric phase label, reference density, threshold band and wet-basis ppmv together. Blank or out-of-range values show no result and are not silently changed.

The threshold band is a comparison of your input, not a target selected for your process. No dryer family is recommended automatically. For further questions to take to a supplier, see the refrigerated versus desiccant comparison, energy audit guide and filtration guide.

Include the required dew or frost point, pressure reference, flow, inlet conditions and location in your enquiry. CAS can review the brief before discussing whether a provider introduction is appropriate.

13Frequently asked questions

What is the difference between pressure dew point and atmospheric dew point?

Pressure dew point is quoted at line pressure; an atmospheric value is quoted at a stated atmospheric pressure. Also distinguish dew point over liquid water from frost point over ice. For +3 °C at 7 bar gauge, this ideal-gas model gives about −20.8 °C atmospheric frost point at 1.01325 bar absolute. A below-zero liquid-water dew-point calculation gives a different temperature. State the pressure and phase reference when comparing results.

Which ISO 8573-1:2010 water class can a refrigerated dryer actually achieve?

Conventional refrigerated dryers commonly quote about +3 °C PDP, corresponding to the Class 4 threshold, at declared rating conditions. Actual performance depends on the model, flow, pressure and inlet and cooling conditions. For lower dew or frost points, compare suitable desiccant or membrane products using their performance data. The calculator neither selects a dryer nor certifies a class.

Does ISO 8573-1:2010 govern Australian medical, surgical, or pharmaceutical compressed air?

ISO 8573-1 describes compressed-air purity classes. A class designation alone does not establish suitability or compliance for a medical, surgical or pharmaceutical application. Identify the specific gas use and applicable requirements with the responsible qualified personnel, including the measurement method, moisture reference and other contaminants. This general calculator is not a medical-gas or pharmaceutical validation tool.

How does ambient temperature affect compressed air dew point?

Intake humidity and temperature affect incoming moisture, while the aftercooler, separator and dryer affect the air reaching the distribution system. Dryer performance also depends on flow, pressure, inlet temperature and its cooling environment. Use measured site conditions and the supplier’s rating corrections. Ambient temperature alone does not determine compressed-air PDP.

How do I measure pressure dew point on an existing system?

Use an instrument with the appropriate dew or frost-point range, pressure rating and calibration, installed or sampled according to its instructions. Record the measurement pressure, sample flow, operating conditions and water or ice reference, and allow adequate stabilisation. Pressure reduction can change the reading. Choose test locations and conditions appropriate to the intended assessment; investigate a warmer reading rather than assuming a single dryer fault.

14Related reading

• ISO 8573-1:2010 classification explained: Full reading of the three-number classification framework including particles, water, and oil with worked examples
• Dryers and air quality guide: Technology comparison across refrigerated, desiccant, and membrane dryers with TCO and efficiency data
• Refrigerated vs desiccant dryer comparison: Side-by-side cost and performance for the two most common dryer types
• Compressed air filtration: Particle and oil class selection to complete the ISO 8573-1:2010 specification
• Leak cost calculator: Quantify the electricity cost of compressed air leaks in your system
• TGA pharmaceutical compressed air: Compliance framework for PIC/S PE 009-17 Annex 1 sterile-product contact
• Medical and dental compressed air: Define the actual gas use and seek the applicable specialist assessment
• Surgical tool air: Review the required quality and measurement basis for the intended use
• Pharmaceutical industry guide: Air quality requirements for Australian pharma manufacturing
• Electronics and semiconductor industry guide: Ultra-clean air specifications for cleanroom and assembly operations
• Tools and calculators hub: All CAS engineering calculators

Calculator disclaimer. Results are indicative and depend on the stated pressure and water/ice reference, ideal-gas assumptions and input range. The tool does not measure air quality, select equipment, predict actual line-temperature density or establish suitability for a regulated application. Use the appropriate measurement method and a site-specific assessment before specification or purchase.