KEVOS® MAKEOS · Engineering
Moist Air Dew Point and Surface Condensation Check (Humidity Ratio, Enthalpy, Dew-Point Margin)
At this air temperature, relative humidity and pressure, what are the dew point, humidity ratio and enthalpy of the air, and will a surface at this temperature stay dry with the margin you need?
Moist-air properties from the dry-bulb temperature, relative humidity and pressure (vapour pressure, humidity ratio, enthalpy, specific volume and dew or frost point), and a check of whether a surface at a given temperature stays above the dew point with the margin you need.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Air dry-bulb temperature | °C | Air temperature from a thermometer or the hygrometer’s temperature reading. |
| Relative humidity of the air | % | Relative humidity of the air from a calibrated hygrometer, relative to saturation over liquid water. |
| Total (barometric) pressure | kPa | Barometric (total) pressure: 101.325 kPa at sea level, lower at altitude; local weather data gives it. |
| Surface | ||
| Surface temperature | °C | Temperature of the coldest surface the air touches: a contact thermometer, an infrared reading corrected for emissivity, or a heat-transfer calculation. |
| Margin required above the dew point | K | How far above the dew point the surface must stay to allow for sensor error and changes in conditions; your own specification or practice. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Air dry-bulb temperature
- What it is
- The dry-bulb temperature of the air around the surface, as an ordinary shielded thermometer reads it.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0 °C to 90 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- At a fixed relative humidity, warmer air holds more moisture, so the dew point rises.
- Get it right
- Measure the air near the surface but not touching it; the tool accepts 0 to 90 °C.
What this means: Relative humidity of the air
- What it is
- How much water vapour the air holds as a share of the most it could hold at its temperature, relative to saturation over liquid water.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher humidity raises the vapour pressure and the dew point, reducing the margin.
- Get it right
- Many hygrometers drift by several per cent; a recent calibration matters when the margin is small.
What this means: Total (barometric) pressure
- What it is
- The total pressure of the moist air, the barometric pressure at the site, used to turn vapour pressure into a humidity ratio.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 50 kPa to 110 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.325 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Changes the humidity ratio, enthalpy and specific volume but not the dew point.
- Get it right
- Use about 101.325 kPa at sea level and roughly 1.2 kPa less per 100 m of altitude near the ground.
Surface
What this means: Surface temperature
- What it is
- The temperature of the coldest surface in contact with the air, where condensation or frost would form first.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -40 °C to 90 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 22 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raising it increases the margin above the dew point and the chance of staying dry.
- Get it right
- Infrared readings of shiny metal are badly wrong; use a contact probe or tape of known emissivity.
What this means: Margin required above the dew point
- What it is
- The least temperature difference you accept between the surface and the dew point, covering measurement error and changes in the weather.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0 K to 30 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the line between a dry result and a margin-too-small warning.
- Get it right
- A few kelvin is common practice; use more where conditions swing or sensors are uncertain.
Method and formulas
- Saturation pressure of water vapour from the Hyland–Wexler equations (ASHRAE Fundamentals, chapter 1): over liquid water at or above 0 °C, over ice below; they match the steam-table values in Çengel’s Table A-4 to within 0.02 % between 0.01 and 100 °C.
- Vapour pressure p_v = φ p_sat(T); humidity ratio ω = 0.622 p_v ÷ (P − p_v); enthalpy h = 1.005 T + ω (2500.9 + 1.82 T) kJ/kg of dry air; specific volume v = 0.287 (T + 273.15) ÷ (P − p_v).
- Dew point: the temperature at which p_sat equals p_v, solved by bisection; below 0 °C this is the frost point over ice.
- Margin = surface temperature − dew point: at or above the required margin the surface stays dry; between zero and the margin the margin is too small; at or below zero water (or frost) forms.
- Example: air at 25 °C and 60 % at sea-level pressure has a dew point of 16.7 °C and 11.896 g/kg of moisture; a surface at 22 °C stays 5.3 K above it, more than the 3 K required.
Limitations
- Uniform air and steady conditions; real condensation often happens where air is trapped against a cold surface, at night, or when warm humid air enters a cool enclosure.
- Air temperatures from 0 to 90 °C; relative humidity is taken over liquid water.
- Hygrometers are often only accurate to a few per cent of relative humidity, which moves the dew point by about a degree; the margin should cover this.
- Example values are illustrations. Replace them with values for your actual site.
Typical uses
- Check whether cold pipes, ducts, tanks or walls will sweat
- Check whether an electrical enclosure, machine or store needs heating or dehumidifying
- Find the dew point and humidity ratio of air for psychrometric work
Who it is for
- Mechanical services and maintenance engineers
- Electrical and controls engineers
- Building and facility managers
What you get
- Dew point (frost point below 0 °C) and margin of the surface above it
- Relative humidity the air would have at the surface temperature
- Humidity ratio, enthalpy, specific volume, vapour and saturation pressure
Interpreting the result
- High relative humidity at the surface, short of 100 %, can already support mould growth and corrosion on dusty or salty surfaces before liquid water appears.
- The dew point depends only on the moisture in the air; heating the air does not change it, but heating the surface does.
Before relying on the result
- Log temperature and humidity over a full day and night: the worst case is usually not when you visit.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 14 (sections 14-1 to 14-3: dry and atmospheric air, specific and relative humidity, dew-point temperature). Method reference; no tables, figures or examples are reproduced.
- ASHRAE Handbook—Fundamentals (SI), chapter 1 Psychrometrics: Hyland–Wexler saturation pressure equations over ice and liquid water.
- National Construction Code (Australian Building Codes Board), condensation management provisions for buildings.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Explains the concept · KEVOS® original book
Water in air: vapour pressure, humidity and dew point
In Condensation Control by the Numbers
What it covers: What vapour pressure is, why it depends only on temperature, how relative humidity, dew point and absolute humidity follow from it, and why dew point is the number to watch.
Explains the concept · Library collection
Psychrometrics for HVAC and process air: humidity, dew point and the loads you cannot feel
In Fluids, HVAC and Building Services
What it covers: A practical guide to moist air: dry-bulb, wet-bulb, dew point, humidity ratio and enthalpy, the main processes on the psychrometric chart, sensible and latent loads, and condensation.
Method explained here · KEVOS® original book
Preparing surfaces for bonding
In Adhesive Bonding by the Numbers
What it covers: The order of degreasing, abrading and cleaning, choosing a solvent that will not attack a plastic part, primers and conversion coatings, and keeping prepared parts above the dew point.
Method explained here · KEVOS® original book
Moist air and the wet-bulb temperature
In Cooling Towers by the Numbers
What it covers: Work out the air a tower uses: humidity ratio, enthalpy, wet-bulb and dew point from temperature and humidity, why enthalpy drives a tower, and catch a 3 pm temperature paired with 9 am humidity.
Method explained here · KEVOS® original book
Moist air in numbers
In Humidity Control by the Numbers
What it covers: The properties an air-handling engineer works with: humidity ratio, enthalpy, specific volume, dew point and relative humidity, on a dry-air basis, from temperature, humidity and pressure.
Method explained here · KEVOS® original book
Condensation and the gutters that carry it
What it covers: Calculate the room's dew point, predict the inner surface temperatures of RL-24's glass and bars on a winter night, and size the condensation channels and weeps that carry the water outside.
Method explained here · KEVOS® original book
Evaporative cooling, blush and condensation
In Solvent Vapours by the Numbers
What it covers: How evaporating solvent cools a surface, the limiting temperature of a wet film, how much a real part cools, and why humid air condenses on fresh coatings and makes them blush.
Learning pathway
Check whether a cold surface will sweat
Learn how vapour pressure sets the dew point, then check your own surface against it with a margin.
1 Learn
Read Water in air: vapour pressure, humidity and dew point in Condensation Control by the Numbers.
What vapour pressure is, why it depends only on temperature, how relative humidity, dew point and absolute humidity follow from it, and why dew point is the number to watch.
2 Apply
Open Moist Air Dew Point and Surface Condensation Check (Humidity Ratio, Enthalpy, Dew-Point Margin) and enter your own figures.
At this air temperature, relative humidity and pressure, what are the dew point, humidity ratio and enthalpy of the air, and will a surface at this temperature stay dry with the margin you need?
3 Verify
Read the tool’s feasibility and verification checks under the result, and test your answer against its stated limitations:
- Uniform air and steady conditions; real condensation often happens where air is trapped against a cold surface, at night, or when warm humid air enters a cool enclosure.
- Air temperatures from 0 to 90 °C; relative humidity is taken over liquid water.
- Hygrometers are often only accurate to a few per cent of relative humidity, which moves the dew point by about a degree; the margin should cover this.
- Example values are illustrations. Replace them with values for your actual site.
4 Improve
Download the decision report or copy the link with your inputs to keep a record, then change one assumption at a time and compare.
Next reading: Cold pipes, sweating and insulation in Condensation Control by the Numbers.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.