Project notes
Understand this estimate
Dew point is estimated from air temperature and relative humidity. Comparing it with a separately measured surface temperature gives a temperature margin, not a universal coating approval.
How to use the calculator
Measure air temperature and humidity
Use current local air readings. Relative humidity must be above zero for this approximation.
Inputs Air temperature · Relative humidity
Measure the coating surface
The surface reading is separate from air temperature and is required for the margin.
Inputs Surface temperature
Enter a product requirement only if stated
The optional threshold is compared with the estimate; no universal safe margin is supplied.
Inputs Product-required margin
Formula and variables
Convert air temperature to °C. Let b = 7.5 ln(10), γ = ln(RH/100) + bT/(237.3+T), and dew point °C = 237.3γ/(b−γ). Convert dew point to °F and subtract it from the entered surface temperature.
- TAir temperature
- Converted to Celsius inside the dew-point approximation.°F
- RHRelative humidity
- Relative humidity fraction used in the approximation.%
- MSurface margin
- Surface temperature minus estimated dew point.°F
What the result means
Result interpretation
A negative margin means the surface is below the estimated dew point. A positive margin is not automatically acceptable; compare it with the current coating instructions and consider changing conditions.
Limits and assumptions
The dew point is approximate. A negative margin means the surface is below the calculated dew point. No universal coating-safe margin or application approval is supplied; compare only with current product instructions.
Worked example
Compare a 72°F surface with 77°F air at 60% RH
Assumptions
- Air temperature and humidity are representative at measurement time.
- The entered 10°F requirement comes from the product instructions.
Calculation
- The Magnus/Tetens approximation gives a dew point of about 62.05°F.
- 72 − 62.05 = 9.95°F surface margin, below the entered 10°F requirement.
| Approximate dew point | 62.05 °F |
|---|---|
| Surface-to-dew-point margin | 9.948 °F |
Common questions
- Does a positive margin mean coating conditions are approved?
- No. This estimates one condition only. Use the current product requirements and site measurements.
- Why measure the surface separately?
- Surface temperature can differ from the surrounding air and determines the calculated margin.